optical coherence tomography ( oct ) is a non - invasive imaging technique using low - coherence interferometry to produce depth profiles of a sample .oct has found many biomedical applications ; prominent examples include the diagnosis of ocular diseases or detection of early - stage cancer .axial resolution in oct is ultimately limited by the coherence length of the light source and can be less than 1  m for very broadband sources .this resolution is hindered by material dispersion which both broadens the features in the interferograms and reduces the contrast .exciting developments in quantum interferometry led to the proposal and demonstration of quantum - optical coherence tomography ( q - oct ) .this technique replaces white - light interferometry ( wli ) with hong - ou - mandel ( hom ) interferometry utilizing frequency - entangled photon pairs .this device automatically cancels all even orders of dispersion in the resulting interferogram , including group - velocity dispersion , the most significant contribution . dispersion cancellation in hom interferenceis `` blind '' , requiring no _ a priori _ knowledge of the material properties , in contrast with dispersion compensation methods ( see and references therein ) .in addition to dispersion cancellation , the hom interferometer is phase insensitive , has better resolution than wli with the same bandwidth , and the interference visibility is insensitive to unbalanced loss .unfortunately , this technique is based on entangled photon pairs and the costs , in terms of speed and expense , have limited its widespread adoption .other techniques for blind dispersion compensation without entanglement have been proposed or demonstrated , but they require unavailable technology or significant numerical post - processing and do not have the other advantageous properties of q - oct .we have recently demonstrated a completely classical technique , based on oppositely - chirped laser pulses , for producing an interferogram with _ all _ the advantages of hom interference with vastly higher signal  .  in the present work ,we use chirped - pulse interferometry ( cpi ) for axial profiling a sample with two optical interfaces .q - oct interferograms have been shown to contain artifacts , signals that do not correspond to real features of the sample . due to the strong analogy , these artifacts also appear in cpi - based axial imaging .we experimentally demonstrate a straightforward method for controlling these artifacts .although it is possible to control these artifacts in q - oct as well  , it is technically challenging and has not been demonstrated .our interferometer is shown in fig . 1 and described in the caption .it relies on pairs of oppositely - chirped laser pulses that have been stretched to several hundred times their initial , transform - limited , pulse duration . in this large - chirp limit , at any given time the two frequency - anticorrelated pulses have frequencies and , respectively . here , is the average of the instantaneous frequencies of these pulses . if the chirped pulses are coincident at the input beam splitter , is equal to the centre frequency of the laser , but it can be tuned by changing the relative delay between the pulses .we refer to as the _ operating frequency _ to distinguish it from the centre frequency of the laser . following the theoretical framework from , we assume that the effect of the sample is modelled by a linear transfer function , .the reference arm contains an adjustable path delay , .after propagation in each arm , the beams undergo sum - frequency generation ( sfg ) in a nonlinear medium .we detect sfg light in a very narrow frequency band near ensuring that the output signal is almost exclusively due to cross - correlations between the chirped and anti - chirped pulses . under these conditions ,the signal integrated over all frequencies in the chirped pulses and measured by a square - law detector , , is ,\end{aligned}\ ] ] where is the intensity spectrum of both laser pulses .the cpi signal is identical to that in q - oct when is equal to the spectrum of the entangled photons ( see , eqns .( 6)-(8 ) ) .  a mode - locked ti : sapphire laser ( center wavelength 790 nm , average power 2.7 w , repetition rate 80 mhz ) was used to create a pair of horizontally - polarized beams of oppositely - chirped pulses using a grating - based stretcher and compressor .these stretched the initial pulses from 100 fs to 54 ps ( 48 ps ) with 11 nm ( 10 nm ) bandwidth for the chirped ( anti - chirped ) pulses .note that the difference in the pulse duration is due to slightly different bandwidths , not different chirp rates .details on the stretcher and compressor can be found in .we used a borosilicate microscope coverglass as the sample .the cpi ( wli ) data was taken by recording the signal of detector d1 ( d2 ) over the delay . in each scan datawas accumulated for 0.5 s over a range of 0.5 mm .the data without and with the calcite blocks are shown in fig .2(a ) and fig .2(b ) , respectively , where the upper ( lower ) plots are cpi ( wli ) scans . without additional dispersion ( fig. 2(a ) ) , the cpi interference dips have widths of 20.1.3  m and 20.6.4  m fwhm and corresponding visibilities of 39.0% and 40.9% for the front and the back surface of the coverglass , respectively .the wli patterns have widths 30.4.3 m and 29.9.3 m fwhm and corresponding visibilities of 12.9% and 14.0% .the errors in these measurements are mainly due to positioning uncertainty of the motor . even without the addition of calcite blocks, we see that the cpi signal has enhanced resolution by a factor of 1.5 compared to wli .this enhancement factor is slightly larger than the theoretically expected value of ( assuming gaussian spectra ) , due to uncompensated dispersion caused by the additional pbs in the sample arm .  the average optical power in the wli scans in figs .2(a ) & 2(b ) was . in the cpi scans ,the normalized intensity of 1.0 corresponds to 0.3  of measured optical power .this corresponds to approximately  photons / s .it is difficult to directly compare this rate with the performance of q - oct demonstrated in since only normalized rates are presented ; however , the highest measured coincidence - counting rate from an entangled photon source to date is photons / s . given a total sample reflectivity of our measured power is 7 orders of magnitude larger than what could be achieved in q - oct with the best available technology .the path delay between the two cpi interference dips is 286.1.4  m .the operating frequency was measured by taking the spectrum of the light after the first beam splitter ; it is the frequency at which the chirped and antichirped pulses interfere .the observed delay can be converted to the thickness of the coverslip by dividing by the group index , , of borosilicate glass at the operating wavelength  nm ,  . the optical measurement of the coverglass thickness is m which is in good agreement with a direct measurement , using a micrometer , yielding  m .to investigate the effects of material dispersion on the interference , we added a pair of calcite beam displacers into the setup .the sum of their lengths is 80.58.01 mm and the light propagates through them with ordinary polarization .the widths of the cpi dips are unchanged at 19.4.6  m and 20.7.4  m fwhm ; the wli envelopes are significantly broadened by 74% to 53.0.3 m and 53.4.3 m . under these conditions , cpi has a factor of 2.65 better resolution than wli .both cpi and q - oct signals contain artifacts , additional features in the interferograms that do not correspond to real interfaces .these can be seen in the data in fig . 2 in between the interference dips .to illuminate the origin of these features in cpi we measured the full sfg spectrum as a function of delay using a high - resolution spectrometer ( acton sp-2758 ) .the results are shown in fig .3 . on this scale ,the only features visible are due to cross - correlations ; the autocorrelations form a weak , broadband background .when the paths are unbalanced , the signal contains two doublets of narrow spectral lines .one of these doublets is due to the chirped pulse traversing the sample arm and the anti - chirped pulse traversing the reference .the two peaks of the doublet are separated in frequency due to the difference in optical delay reflecting from the front and back surface of the sample .when the chirped pulse is reflected from the front surface the frequency of the cross - correlation will be slightly higher than upon reflection from the back surface .the other doublet can be understood by swapping the roles of the chirped and anti - chirped pulses .changing the path delay changes the spacing between the two doublets .an interference dip occurs when the delay in the reference path is equal to a delay in the sample path .the interference results from two different processes ( chirp in one of the two arms and anti - chirp in the other ) each producing light at the same frequency , but out of phase . in fig . 3 the four lines , formed by the two doublets for varying path delay , cross at four distinct points .two of these points occur at the same wavelength but at different path delays ; they correspond to the dips in the cpi scans indicating real features of the sample .the other two crossing points occur at the same path delay but different wavelengths ; these give rise to the artifacts .  as in q - oct ,the interference giving rise to the artifacts can be constructive or destructive whereas for real features it is always destructive . if the interference is constructive , as in fig . 2 ,the artifact is easy to identify . if instead it is destructive , artifactscan easily be confused with real features . in q - octit was predicted that this could be adjusted by changing the sum - frequency of the entangled photon pair  . in practice , however , this is difficult as most uv narrow - band pump sources for spdc are not tunable .cpi has the intrinsic advantage that the operating frequency is tunable by changing the relative delay between the chirped and anti - chirped pulses at the input beam splitter . in figs .4(a ) and 4(b ) , we show two examples of cpi interferograms taken at different operating wavelengths 792.10 nm and 791.54 nm illustrating constructive and destructive interference in the artifact , respectively .the operating frequency is half the sfg frequency measured near zero delay . in these scansthe path delay was not varied continuously but in discrete steps , accounting for fewer data points .we employ the model transfer function for the coverslip , , where ( ) is the reflection amplitude from the front ( back ) surface , is the wavevector in the glass , and is the thickness . inserting this expression into eq .( 1 ) , one finds that the term describing the artifact is modulated by . if the operating frequency changes from to , then and the expected change in wavelength required to flip the sign of the artifact is .figure 4(c ) shows the visibility of the artifact as a function of the operating wavelength .visibility is defined as , where and are the intensities at the centre of the dip and at the shoulder , respectively .a fit to this data yields a period of ( 1.13.02 ) nm in good agreement with the theoretical prediction of 1.09 nm .changing the operating wavelength in this straightforward way allows identification and removal of artifacts from axial scans .there is a difference between q - oct and cpi . even in the absence of dispersion ,the resolution in q - oct is a factor of better than in wli  while , assuming gaussian spectra , cpi has a factor of better resolution than wli .the difference originates from the effective bandwidths used in the comparison . assuming a source of entangled photons with spectrum ( the modulus squared of in eq .( 5 ) in ) , the effective bandwidths for hom interference ( see , eq .( 8) ) and wli ( see , eq .( 4 ) ) are the same because their spectra , , are identical . for chirped pulses with spectra , the bandwidth for cpiis determined by ( see eq .( 1 ) ) while that for wli is determined by . for gaussianspectra the effective bandwidth for cpi is narrower than that for the wli , accounting for the resolution advantage of q - oct .is this a fundamental feature of entanglement ?surprisingly no .the same difference in resolution could be achieved using purely classical correlations . in cpi ,time - correlations , but not intensity correlations are created between anticorrelated frequencies . in the same setup ,one replaces the chirped pulses with two cw lasers tuned to frequencies and . during the integration time of the detectionthe laser frequencies are swept in an anticorrelated way according to the distribution . in this case , the effective spectrum in eq .( 1 ) is , while the wli bandwidth is determined by the marginal .the effective bandwidths are identical in both cases and thus this classical scheme achieves the same factor of higher resolution as q - oct . in practice , however , chirped pulses offer dramatically higher nonlinear conversion efficiency compared to cw lasers far outweighing this rather small difference .we have shown that chirped - pulse interferometry accrues the benefits of quantum - oct , but with a dramatic increase in signal , direct optical detection , and a straightforward means of identifying artifacts .we have experimentally demonstrated improved resolution in cpi over wli with and without mismatched dispersion by up to a factor of 2.65 .increasing the bandwidth of the light source is required for the resolution in cpi - based oct to compete with established techniques .broadband ( 148 nm ) three - wave mixing of anticorrelated frequencies has been demonstrated in 1.5mm - thick nonlinear materials .for safe _ in vivo _ imaging , the optical power can be attenuated before the sample without loss of signal visibility .we have demonstrated sfg detection of reflected signals as low as 5mw . based on the results of , efficient sfg should be measureable with much lower reflected powers .incorporating better nonlinear materials , such as ppktp , and more sensitive detectors will further allow operation at lower power levels .cpi achieves the benefits of quantum interferometry at macroscopic power levels and represents a powerful new technique for optical imaging .more generally , we have clarified the role of entanglement versus correlation in axial imaging .we acknowledge financial support from nserc and cfi . we thank g. weihs and c. couteau for the use of their spectrometer and technical assistance .we thank d. biggerstaff , k. bizheva , d. strickland , and g. weihs for valuable discussions .j.l . acknowledges financial support from the bell family fund and r.k . acknowledges financial support from iqc and ordcf .j. g. fujimoto , m. e. brezinski , g. j. tearney , s. a. boppart , b. bouma , m. r. hee , j. f. southern , and e. a. swanson , `` optical biopsy and imaging using optical coherence tomography , '' nature med .* 1 * , 970 - 972 ( 1995 ) .if the bandwidth in hom interference is determined by a pair of gaussian bandpass filters in front of the detectors , rather than by the source , then the hom dip is narrower than the wli by only a factor of .s. carrasco , m. b. nasr , a. v. sergienko , b. e. saleh , m. c. teich , j. p. torres , and l. torner , `` broadband light generation by noncollinear parametric downconversion '' opt . lett . * 31 * , 253 - 255 ( 2006 ) .a. peer , y. bromberg , b. dayan , y. silberberg , and a. a. friesem , `` broadband sum - frequency generation as an efficient two - photon detector for optical tomography '' opt .express * 15 * , 8760 - 8769 ( 2007 ) .

Abstract: quantum - optical coherence tomography ( q - oct ) is an interferometric technique for axial imaging offering several advantages over conventional methods . chirped - pulse interferometry ( cpi ) was recently demonstrated to exhibit all of the benefits of the quantum interferometer upon which q - oct is based . here we use cpi to measure axial interferograms to profile a sample accruing the important benefits of q - oct , including automatic dispersion cancellation , but with 10 million times higher signal . our technique solves the artifact problem in q - oct and highlights the power of classical correlation in optical imaging .


many physical phenomena can be described as a linear superposition of some elementary processes . in acoustics , for example , recordings made by a array of microphonescan be adequately described as a linear superposition of sounds that originate from different sources .the problem of recovering these individual sources from their mixtures is a longstanding problem .a hallmark for this is the _ cocktail party problem _ , in which a person tries to isolate a voice that is hidden in the ambient noise .blind source separation ( bss ) is a recent and powerful framework for recovering the original sources from their mixtures using the least prior knowledge on the sources and on their mixing process .  why blind separation ?if some properties of the sources are known beforehand , then this prior information should be incorporated in the extraction and a bayesian approach is appropriate . likewise ,if the sources are known to interact non - linearly , then nonlinear techniques should be considered , e.g. ref .quite frequently , however , the sources are not known beforehand , and so an unsupervised analysis is required before we may consider a model that incorporates more information . in the last decade , bss has become a very active field of research with applications in disciplines such as acoustics , computer vision , astrophysics , multispectral imaging , and many more . in plasma physics , many observations based on multichannel diagnostics are good candidates for doing bss and yet , very few applications have bee reported so far .this article aims at giving a short introduction to bss in the context of multichannel or multipoint plasma diagnostics .two examples are given : the first one is about coherent electron temperature fluctuations in a tokamak , and the second one on solar spectral imaging in the extreme ultraviolet ( euv ) . in both examples ,we consider multichannel data and show how these can be adequately modelled as a linear superposition of individual sources . some comments and a conclusion follow in the last sectionthere exist several approaches for doing bss , depending on the assumptions one is making on the sources and on their mixing coefficients .let us assume that we have a bivariate data set  ] ) .these data are most conveniently stored in a matrix .bss consists in decomposing the data into a separable set of modes of time and space = \sum_{k=1}^n \phi_k[t ] \ ; \psi_k[x ] + \epsilon[x , t ] \ , \label{eq : bss1}\ ] ] where  ] are called _ sources _ and the temporal ones are called the _ mixing coefficients _ , or conversely .equation [ eq : bss1 ] is heavily underdetermined and so the temporal and spatial modes need to be constrained .the choice of the constraint leads to different variants of the bss .a convenient constraint is orthonormality , i.e. \phi_l[t ]  =  \sum_x \psi_k[x ] \psi_l[x ] = \left\ { \begin{array}{lll } 0 & \textrm{if } & k \neq l \\ 1 & \textrm{if } & k = l \end{array } \right . \ .\ ] ] this requires the definition of weights , finally to obtain = \sum_{k=1}^n a_k \ ; \phi_k[t ] \ ; \psi_k[x ] + \epsilon[x , t ] \ .\label{eq : svd}\ ] ] this decomposition is unique and has numerous interesting properties . in plasma physics, it goes under the name of principal component analysis , singular value decomposition ( svd ) , or biorthogonal decomposition .many applications to the extraction of coherent fluctuations from multichannel plasma measurements have been reported , see for example refs . .the simplicity of the svd is also one of its major shortcomings .indeed , the modes  ] are nothing but the eigenvalues of the sample covariance matrix and so they rely on second order statistical properties only .for that reason , the svd can not properly distinguish random variables with a gaussian probability density from those that are non - gaussian .quite often , this departure from gaussianity is precisely one of the properties that gives deeper insight into the underlying physics and so a different approach is required .the svd was the classical technique for doing bss until the mid-90 s , when new algorithms opened the way for source separation based on higher order statistical moments .an interesting generalisation of the svd is _ independent component analysis _ ( ica ) , which consists in applying the model of eq .[ eq : bss1 ] while constraining the modes to be independent rather than orthonormal .independence implies where stands for the probability density .independence is a more stringent and often a more meaningful constraint than orthogonality . for gaussian data ,the ica and the svd give similar results .non - gaussianity , however , provides an additional leverage that is precisely used by ica .interestingly , ica can be considered as a way to go against the central limit theorem : while a mixture of random variables tends to be more gaussian than its individual constituents , ica tries to recover the constituents that are maximally non - gaussian .the computation of the svd is straightforward whereas that of the ica requires the maximisation of a measure of non - gaussianity .different algorithms have been developed , such as jade and fastica .although many applications have been reported ( see for example for a list of them ) , there have been only very few applications to plasmas so far .one of the crucial issues with bss is the determination of the number of sources in eq .[ eq : bss1 ] . until recently, it was widely believed that this number could not exceed the number of observables , otherwise the problem would be underdetermined .multiresolution ( i.e. wavelet ) techniques have opened the way for new concepts that allow to overcome this limitation .the key concept here is sparsity , which we shall come back to in the last section .  with the svd ,the number of sources equals the smallest dimension of the data matrix . for multichannel data, this number usually corresponds to the number of channels .the salient features are captured the modes that have large weights all one has to do is identify the number of outstanding weights . in ica , on the contrary , the number of sources must be specified beforehand .for that reason , careful validation is crucial .the independence constraint in the ica is not always an appropriate condition for separating sources .there are instances where the positivity of the sources and their mixing coefficients also needs to be imposed .consider for example soft x - ray measurements in a plasma column made by a pinhole camera .each line - integrated measurement is a linear combination of positive contributions ( there are no negative sources ) that originate from different regions , with positive mixing coefficients ( contributions can only be additive ) .  positive blind source separation ( positive - bss ) has become a very active field of research since the advent of efficient numerical schemes .a powerful method for doing positive - bss is bayesian positive source separation ( bpss ) , which is the method we shall use below .there exists , however , a variety of other approaches that differ in the way the positivity constraint is applied . although the convergence properties of positive - bss algorithms have received a lot of attention , careful validation is important to avoid ending up with local solutions .  by using the bpss, we assume that the data can be decomposed as in eq .[ eq : bss1 ] with in addition \ge 0 ] .the modes are stored in random matrices whose a posteriori distribution is given by bayes theorem we assume that the distribution functions of and are zero for negative values , which can be enforced by taking for example a gamma distribution .for more details , see ref . .let us first consider temporal mixtures first with electron temperature fluctuations in the tore supra tokamak , as measured by electron cyclotron emission .we focus on the stationary regime of an ohmic discharge with the injection of a small hydrogen pellet .the temperature is recorded at 12 positions , radially distributed from to , where m is the minor radius .the dynamics of the plasma reveals two types of phenomena : the ubiquitous sawtooth instability with its periodic oscillation , and the sharp temperature drop caused by the pellet , followed by a slow relaxation .the recovery of the radial profile of the diffusive and convective transport processes from the dynamical response of the plasma is an important issue here but it is hampered by the simultaneity of the two perturbations .therefore , it is important to find a means for disentangling the two perturbations .the fluctuation levels are small ( ) so the dynamic response of the plasma may reasonably be assumed to be linear .this response can then be expressed as linear superposition of eigenfunctions of the transport operator .we know that the two types of perturbations are independent in time , so in the following we try to separate them by using bss .no positivity of the modes is enforced in this first example because we are interested in small fluctuations around a mean value .a proper normalisation is required because the residual error to be minimised is scaling - dependent . for fluctuations , two reasonable normalisationsare \longrightarrow \frac{i[t ] - \langle i \rangle}{\langle i \rangle } , \qquad i[t ] \longrightarrow \frac{i[t ] - \langle i \rangle}{\sigma_i } \ , \ ] ] where is the standard deviation of  ] obtained by ica ( left column ) and their corresponding spatial modes  ] ) whereas the response of the plasma to the pellet can be obtained by summing over the second and third modes only .note , however , that the separation is not total as modes  ] still carry a weak signature of the pellet injection . nevertheless , the quality of the reconstruction is largely sufficient to carry out a perturbative transport analysis .concerning the choice of the number of sources , we find that modes 1 to 5 remain unaffected when or more sources are selected , or when standardised data are used .modes 6 and beyond capture mostly incoherent fluctuations that can not be ascribed to known physical processes , see fig .2 . these are strong indications that sources are required to properly describe the dynamics of the electron temperature .more sources might be needed with a lower noise level or with more ece channels .in this second example , we consider spatial mixtures with spectroscopic data from the sun and use positive - bss .the solar spectral irradiance in the euv results from the superposition of emissions that are caused by different physical processes in the highly complex and dynamic solar atmosphere .there is strong observational evidence for the spectral variability to arise from a linear combination of a few elementary spectra with different time evolutions .this remarkable spatial coherency of the spectral variability is rooted in the strong structuring of the solar atmosphere by the intense magnetic field . in studied the spectral unmixing of solar euv spectra . herewe concentrate on the spatial unmixing of solar images taken in the euv .figure [ fig3 ] shows a series of 2d images of the solar limb , taken by the coronal diagnostic spectrometer ( cds ) onboard the soho satellite . in this example , cds was used to measure the intensity of 14 spectral lines , with a spectral resolution better than 0.06 nm .the intensity of each spectral line depends on various plasma parameters , and in particular on the temperature . a quantitative picture of the temperature distribution , however ,can only be obtained by time - consuming comparisons with simulations from radiation transfer models , and at the price of several assumptions .there is a clear need here for finding new and more empirical means to rapidly infer pertinent physical properties from such data cubes .a conspicuous feature in fig . [ fig3 ] is the similarity between the different images .there are two reasons for this .first , the plasma can be multithermal so that the same line of sight ( i.e. pixel ) has contributions in different spectral lines .second , the temperature response associated with each spectral line is generally wide , and sometimes even multimodal .one can therefore assume that the intensity of each pixel to be a linear mixture of a few elementary sources , whose structure we ll now extract .each cds image is pixels in size .we unfold the data cube into a matrix by lexicographically ordering each image . in doing so, we implicitly assume that the pixel intensities  ] and the mixing coefficients \}$ ] are random matrices whose elements are independent and distributed according to gamma probability density functions .in what follows , each image is normalised to its mean intensity and the sources are normalised to have unit norm .an inspection by svd suggests that there are 2 to 6 significant sources out of 14 . from the analysis of the rootmean squared error of the difference between the original data and the intensities reconstructed from the positive sources , this number should be between 3 and 5 .an inspection of the sources for different solar events , however , shows that when more than 3 sources are selected , the 3 first ones remain almost unchanged , whereas the subsequent ones are smaller in intensity ( when the mixing coefficients are normalised to unit norm ) and are event - dependent .these results suggest that sources should be considered , and that additional terms should rather be considered as minor corrections .figure [ fig4 ] shows the 3 sources obtained by bpss and their mixing coefficients . in contrast to the svd and the ica , the bpss reveals structures that can be readily linked to known processes in the solar atmosphere .interestingly , the mixing coefficient reveal a clear temperature ordering as the 3 sources capture emissions that respectively originate from cool ( 1 ) , medium hot ( 2 ) and hot ( 3 ) plasmas . to support this ,we have plotted in fig .[ fig4b ] the amplitude of the mixing coefficients versus the characteristic temperature at which the emission of each spectral line peaks .the correspondence between a spectral line and a given temperature is of course very approximate but this plot nevertheless shows a clear temperature structuring in the mixing coefficients .a quantitative comparison with the more meaningful differential emission measure is under way .    , plotted versus the characteristic emission temperature of each spectral line . ]  in fig .[ fig4 ] , the coolest source is associated with the chromosphere , in which the quiet sun stands out as a bright disk .two groups of magnetic loops appear at the limb ; note in particular how they are entangled .the key result is that the emission in all 14 spectral lines , in spite of the complexity of the underlying atomic physics , can be expressed in terms of 3 temperature bands only , whose properties can be inferred from the data without imposing a physical model .these results are robust , in the sense that the same temperature ordering is obtained for other solar regions or for different events , provided that all three temperature bands are properly represented in the observations .we can now make a compact representation of the solar corona by compressing all 14 images into one single colour plot .we do so by assigning the hot , medium hot and cool sources respectively to the red , green and blue channels .the result is displayed in fig .[ fig5 ] . also shown is the same picture obtained by assigning the coronal fe xiii and si x lines , and the chromospheric he i line respectively to the red , green and blue channels .although the two pictures look similar , the former has a sharper contrast because of the better separation of the temperature .this allows the loop structure to be better seen ; furthermore , the whitish pixels can now be interpreted as multithermal regions .such multichannel representations are useful as quick - looks but also be used as a guide for supervised classification .they are particularly useful for visualising the large stream of high resolution ( 4k ) images from the recently launched sdo satellite , which continuously monitors the sun in up to 10 spectral lines , several times per minute . since the sources are recovered simply by linear combination of the observed images , using coefficients that are computed once for ever , the decomposition can be carried out in real - time .the two examples we have discussed ( electron temperature fluctuations in a tokamak and solar spectral imaging ) both show that bss , even though it is an empirical method , can give better insight into the physics by separating different processes .let us stress again , however , that bss makes sense only if we know that the dynamics can be described by a linear superposition of various contributions , so that the salient features can be captured by a few sources .the instantaneity of the mixture is a condition that can be partly relaxed , and some recent bss techniques can handle convolutive mixtures . in the first example , the mixture actually was not instantaneous since the temperature perturbation diffuses through the plasma column .the propagation , however , was sufficiently fast to allow for its description in terms of a few sources .the instantaneity constraint becomes a major obstacle when dealing with wavefields consisting of structures that move randomly or that are convected . for such wavefields , the separable model eq .[ eq : bss1 ] is not appropriate and indeed none of the bss methods will properly work . if the wavefield is homogeneous , then the svd will simply yield fourier modes .the only exception occurs with travelling waves , which can be described as spatio - temporal symmetries .bss can not only be used for physical interpretation but also for prepocessing .a typical example is the tomographic reconstruction of density profiles in a plasma column .most tomographic techniques involve a regularisation step based on concepts such as entropy maximisation , which require positive definite data .the processing of a full discharge then requires as many tomographic reconstructions as there are observations , which can become very time - consuming .the computational burden can be considerably alleviated by first estimating the positive sources , then applying the tomographic reconstruction to these few sources only , and finally combining these inverted sources to obtain the full data set .careful validation is a crucial issue with bss .one should always compute the solutions for different numbers of sources and investigate the sources and their mixing coefficients before making decisions .preprocessing also matters since these techniques are scaling dependent . in practice, one should consider a normalisation that gives equal weight to all channels or , better , that gives the same noise level to all channels .the information about the noise can actually be incorporated in the bayesian priors , and so for better performance the techniques ( in particular bpss ) should be fine - tuned to the data under consideration .bss is a rapidly evolving field and although recent techniques such as ica and bpss have already found their way into applications , much more is to come .a key concept is sparsity , which means that the data , when projected on a suitable set of basis functions ( typically wavelets ) , require a limited amount of information to describe them .this implies that sources with some quantitatively measurable diversity should have differing signatures when projected on such bases , which then provide additional leverage for separating them .recent techniques such as morphological component analysis take advantage of this sparsity and of the different morphological properties of the sources to do bss .one of their assets is their ability to extract more sources than there are observables .i gratefully acknowledge discussions with s. moussaoui , j. aboudarham , p .- o .amblard , f. auchre , g. cessateur , m. kretzschmar and j. lilensten as well as the international space science institute ( issi , bern ) for hospitality .special thanks are due to the instrumental teams that provided the data : tore supra ( cea , cadarache ) and soho / cds .this study received funding from the european community s seventh framework programme ( fp7/2007 - 2013 ) under the grant agreement nr .218816 ( soteria project , www.soteria-space.eu ) .leach , j.f .cardoso , c. baccigalupi , r.b .barreiro , m. betoule , j. bobin , a. bonaldi , j. delabrouille , g. de zotti , c. dickinson , h.k .eriksen , j. gonzlez - nuevo , f.k .hansen , d. herranz , m. le jeune , m. lpez - caniego , e. martnez - gonzlez , m. massardi , j.b .melin , m.a .miville - deschnes , g. patanchon , s. prunet , s. ricciardi , e. salerno , j.l .sanz , j.l .starck , f. stivoli , v. stolyarov , r. stompor , and p. vielva , * 491*(2 ) , 597615 ( 2008 ) .cardoso , the three easy routes to independent component analysis ; contrasts and geometry , in : proc . of the ica2001 workshop , edited by t. lee , t. jung , s. makeig , and t. sejnowski ( san diego , december 2001 ) .g. verdoolaege , g. telesca , and g. van oost , reconstruction of zeff profiles at the textor tokamak through bayesian source separation , in : 24th international workshop on bayesian inference and maximum entropy methods in science and engineering , edited by r. fischer , r. preuss , & u. v. toussaint , american institute of physics conference series vol.735 ( november 2004 ) , pp.344351 .

Abstract: multipoint or multichannel observations in plasmas can frequently be modelled as an instantaneous mixture of contributions ( waves , emissions , ... ) of different origins . recovering the individual sources from their mixture then becomes one of the key objectives . however , unless the underlying mixing processes are well known , these situations lead to heavily underdetermined problems . blind source separation aims at disentangling such mixtures with the least possible prior information on the sources and their mixing processes . several powerful approaches have recently been developed , which can often provide new or deeper insight into the underlying physics .   this tutorial paper briefly discusses some possible applications of blind source separation to the field of plasma physics , in which this concept is still barely known . two examples are given . the first shows how concurrent processes in the dynamical response of the electron temperature in a tokamak can be separated . the second example deals with solar spectral imaging in the extreme uv and shows how empirical temperature maps can be built .


a recent frontier in physics is to investigate organic and living matter , trying to formulate models able to capture their main features .this investigation has the double mark of being in itself interesting due to the lack of knowledge in the subject and of exploiting wide potentialities in the production of a new generation of electronic devices . as a matter of fact, some specific tasks of sensing protein can be reproduced in artificial devices to improve their performances .it is the case of nano - biosensors which aim to mimic _ in vitro _ the peculiar sensing abilities shown _ in vivo _ by proteins and organic materials .accordingly , much attention has been devoted to transmembrane proteins ( tmps ) , which interface external and internal cell environments .specifically , tmps are proteins consisting of one or several helices crossing the cell membrane and able to modify their structure when capturing a specific molecule or a small protein ( ligand ) . among available experiments , here we report on those performed on two different seven - helix tmps , bacteriorhodopsin ( br ) and proteorhodopsin ( pr ) , proteins able to transform energy coming by light in energy useful for the cell survival .therefore , they appear very interesting to work out bio - solar - cells and/or bio - diodes . to this purpose ,recent measurements on the electrical properties of br and pr have revealed the possibility to use them _ in vitro _ like organic transistors . in particular , they are almost insulators at the lowest values of the applied bias , and their conductance suddenly increases for orders of magnitude , above some threshold value where they enter a sharp super - ohmic regime . furthermore , in the presence of green light , i.e. after a conformational change , the conductance exhibits a further increase above the dark value , thus confirming the hypothesis of a structure - mediated charge transfer .  to provide a microscopic model of the main features described above, we developed a numerical approach based on the analogy between a single protein and an impedance network .the charge transport is obtained by introducing a procedure of stochastic changes of the network impedances .the investigations are performed on the br , whose current - voltage ( i - v ) responses were carried out on a wide range of applied voltages , then extended to pr , on which experiments are only at the preliminary stage of low applied bias .former experiments were carried out on br , probably the best known protein in the seven - helix tmp family .samples of br in its natural membrane were used in a metal - insulator - metal structure and their i - v characteristics measured in dark and also in green light .the samples exhibited a small conductivity ( less than 1 ps / cm ) at low applied field .this conductivity was found to increase as the protein sample is irradiated with green light and/or when the electrical field overcomes a critical value . in particular , green light produces a higher current with respect to dark conditions , as a consequence of the modification of the chromophore - opsin complex .the super - linear i - v characteristics point toward a tunneling charge - transfer mechanism . in particular , a transition between a direct tunneling ( dt ) regime and a fowler - nordheim ( fn ) tunneling regime is revealed ( see ). first measurements of i - v characteristics in pr were reported in ref .the experiment was performed on a thin - film device with a contact distance of 50 , just about times longer than the br nanolayers used in the previous experiments .the film was obtained by dehydrating pr in a buffer solution .the protein activity after dehydration was tested by absorbtion measurements that confirmed the preservation of the structure and the function after the treatment .accordingly , the experiments performed on br and pr evidence the possibility to obtain , _ in vitro _ like so _ in vivo _ , a significant photocurrent , a result of great interest for optoelectronic applications .      following the indication of experiments ,we assume that charge transfer through the amino acids of the single protein is the main mechanism of transport .therefore , a microscopic interpretation of experiments is given in terms of the single protein electrical properties , taken as intrinsic characteristics of the protein morphology .to this purpose we make use of the impedance network protein analogous ( inpa ) technique already detailed in . in short , each protein is associated with a topological network , consisting of nodes and links , as pictured in .each node corresponds to a single amino acid , and its position is taken as that of the related atom .each couple of nodes is connected with a link when their distance is less than an assigned interaction radius , ( usually in the range  ) , so that the network captures the protein topology .the graph turns into an impedance network when an elemental impedance is associated with each link . in the present case of a static responseonly resistances are considered .accordingly , the elemental resistance between the -th and -th nodes , say , is taken as : , where , is the cross - sectional area between the spheres centered on the nodes , respectively , is the distance between these nodes , is the resistivity . by positioning the input andoutput electrical contacts on the first and last node , respectively , for a given applied bias the network is solved within a linear kirchhoff scheme . by construction , this network produces a parameter - dependent static i - v characteristic . to account for the super - linear current at increasing applied voltages , a tunneling mechanism of charge transferis included by using a stochastic approach within a monte carlo scheme . in particular ,the resistivity value of each link is chosen between a low value , taken to fit the current at the highest voltages , and a high value , which depends on the voltage drop between network nodes as : where is the maximum resistivity value taken to fit the i - v characteristic at the lowest voltages ( ohmic response ) and is the height of the tunneling barrier between nodes .  ), is the variance of x. ]  ) , is the variance of x. ]  the transmission probability of each tunneling process is given by : \  \hspace{0.06 cm }  ( ev_{i , j }  < \phi ) , \hspace{0.4 cm } \mathcal{p}^{fn}_{ij}=\exp \left[-\alpha\ \frac{\phi}{ev_{i , j}}\sqrt{\frac{\phi}{2 } } \right ] \  \hspace{0.06 cm }  ( ev_{i , j } \ge\phi ) \  \label{eq:1}\ ] ] where is the potential drop between the couple of amino acids , , and is the electron effective mass , here taken the same of the bare value .the investigation is carried out through a comparative analysis of pr and br properties .the inpa model is calibrated on the experiments performed on br monolayers . in particular , starting from the experimental i - v characteristic the model parameters used to get the best agreement are :  , j ,  , and   . reports the i - v characteristics calculated for a single protein of br and pr , on a wide bias range . in the common range of applied voltages, calculations agree with experiment when properly rescaled to take into account the sample size .reports the data on scale axes appropriate to evidence the transition between dt and fn tunneling regimes , the latter being evidenced by a sharp increase ( over ten orders of magnitude ) at high voltages of the dependent variable .the average chord - conductance , , is calculated over a large ensemble of configurations , and is taken as a measurable global - quantity and order parameter in recent investigations . as a further step ,the conductance fluctuations which emerge as a result of the same numerical simulations are estimated .indeed , these fluctuations are essential to determine the average value of the global quantity since they are particularly high close to the transition between the dt and fn regimes , as generally expected near to a phase transition .indeed , the probability distribution function ( pdf ) of these fluctuations , normalized to its first and second moment , is found to be well described by the generalized gumbel distribution , , where is a positive number , the variance , with .furthermore , , with , and indicating the gamma , digamma and trigamma function , respectively . finally , is a normalized pdf with zero mean and unitary variance ; it is a skewed function , recently used to describe finite - size systems in both critical and not critical conditions .the numerical value of the shaping parameter is related to the system internal dynamic . in the present case , the lack of a gaussian behaviour is due to the existence of a low and upper bound on the conductance values .the values of which , in a wide range of applied bias , better describe the pdfs for br and pr , are and , respectively ( see ) .integer values of allow to interpret the as the distribution of the -th minimum / maximum of the independent variables , i.e. should be correlated with the existence of a single dominant conductance value . otherwise , the interpretation of on integer value remains an open question .microscopic investigations of the i - v and conductance characteristics of the two light receptors br and pr reproduce available experiments and predict the pr behaviour on a bias range not yet explored . from experiments , the protein activity after dehydration was tested by absorption measurements that confirmed the preservation of the structure and the function after the treatment . from theory ,the pdf of the fluctuations of the conductance , , taken as order parameter are found to follow the generalized gumbel distribution , for both proteins .these results confirm the inpa approach as a powerful numerical method to investigate protein electrical properties on the basis of their tertiary structure .19 i casuso , l fumagalli , j samitier , _ et al ._ , h melikyan , b -e khishigbadrakh , a babajanyan , _ et al ._ , y jin , n friedman , m sheves , _ et al .e alfinito , j -f millithaler and l reggiani h m berman , j westbrook , z feng , _ et al . _ a noullez , and j -p pinton , ; s c chapman , g rowlands and n w watkins , e alfinito and l reggiani ,

Abstract: we report a theoretical / computational approach for modeling the current - voltage characteristics of sensing proteins . the modeling is applied to a couple of transmembrane proteins , bacteriorhodopsin and proteorhodopsin , sensitive to visible light and promising biomaterials for the development of a new generation of photo - transducers . the agreement between theory and experiments sheds new light on the microscopic interpretation of charge transfer in proteins and biological materials in general .   # 1#2#3#4#1 _ # 2 _ * # 3 * # 4


when planning cbm strategies , see , e.g. , , , , the typical assumption in the literature is that the asset at hand is monitored continuously and one can intervene and repair the asset at any given moment .however , in practice , especially for highly complicated high - tech systems , such as off - shore wind turbines , baggage handling systems in airports and interventional x - ray machines , downtimes are extremely costly and scheduling maintenance tasks is challenging , see , e.g. , , . one solution suggested in the literature to overcome this problem is the planning of opportunistic maintenance .most of the works on opportunistic maintenance consider only unscheduled opportunities , rarely treating the case of scheduled opportunities , see , e.g. , and the references therein .different from existing research on opportunistic maintenance policies , we consider both scheduled and unscheduled opportunities for cbm .furthermore , we consider distinct costs for maintenance based on the type of the opportunity . for a single - component modelwe derive the optimal long - run average cost cbm policy and calculate the corresponding long - run average cost .thereafter , we compare it with the corresponding cost for only one type of opportunity . our work can be viewed as a variation of the work of in the direction of cbm .the paper is organised as follows .in , we present the model and describe the exact setting that motivated our work . in the subsequent two sections we prove the two main results : in , we derive the optimal policy and in , we calculate the long - run average cost . in, we numerically compare the optimal replacement policy to other policies .finally , in the last section , we present some concluding remarks and ideas on future research .we consider a single component that is monitored continuously and whose condition is fully observable .we assume that the condition of the component can only degrade over time and it can be classified as _, _ satisfactory _ and _ failed_. this type of models are known in the literature as delay time degradation models , see ( * ? ? ?* chapter 14 ) .we will refer to the state of perfect condition as state , the state of satisfactory condition as state and the failure state as state .furthermore , we assume that as soon as a component failure occurs , the component is instantaneously replaced by an `` as good as new '' component .so , in the mathematical formulation of the model , we may assume , due to the instantaneous replacement at failure , that the model evolves between only states and .the component spends an exponential amount of time with rate in state , .the above model formulation implies that initially the component starts in state ( perfect state ) , then after an exponential amount of time with rate , the component deteriorates and the condition of the component goes to state ( satisfactory state ) .the component spends an exponential amount of time with rate in state 1 , after which a failure occurs . at a failurethe component is instantaneously replaced by an `` as good as new '' component and the condition is restored to ( perfect state ) .a schematic evolution of the condition of the component and the corresponding times of transitions are depicted in figure [ fig:1 ] .^{\mathrm{exp}(\mu_2)}&1\ar@/^.75pc/[r]^{\mathrm{exp}(\mu_1)}&2  \ar@/^.75pc/[r]^{\mathrm{exp}(\mu_2)}&\cdots\ \ \ \\ } \ ] ]  we assume that we have two types of opportunities in which we can preventively replace the component before failure : the scheduled and the unscheduled opportunities .the scheduled opportunities correspond to pre - arranged opportunities occurring according to a fixed schedule .these opportunities can be attributed to either service / maintenance agreements or to regulation imposition checks .we assume that the scheduled opportunities occur at epochs , with .this is also in accordance with what happens in practice as maintenance actions once planned are typically not rescheduled .the unscheduled opportunities correspond to random opportunities triggered by failures of other unrelated components of the same system or failures of other systems in close proximity .we assume that these unscheduled opportunities occur according to a poisson process at rate .  the unscheduled and scheduled opportunities , abbreviated by uso and so , respectively , serve as opportunities for the monitored component to be replaced preventively .such a preventive replacement is assumed to cost less than a corrective replacement upon failure , which costs say .moreover , incorporating a planning perspective , we may assume that the preventive replacement cost at a scheduled opportunity , say , is less than or equal to the corresponding cost at an unscheduled opportunity , say , that is .our aim is to determine a policy when to replace the component based on its condition and the opportunity type , i.e. scheduled or unscheduled .more explicitly , we will need to formally define the state space , which refers to the condition of the component , the action space and the decision epochs .the state space is governed by the process depicting the condition of the component , i.e. the markov chain evolving between the states .the action space consists of only two actions : replace the component by an `` as good as new '' or do nothing .lastly , the decision epochs are the epochs of the scheduled and unscheduled opportunities . in, we depict so by ( ) and uso by ( o ) .( -6,0 ) ( -6,2.5 ) node[right , text width=2.7cm]condition of the component ; ( -6.1 , 1.5 ) node[left]2 ( -5.9,1.5 ) ; ( -6.1 , .5 ) node[left]1 ( -5.9,.5 ) ; ( -6,0 ) ( 7.1,0 ) node[below , xshift=1ex]time ; ( -6,1.5 ) ( -2,1.5 ) ; ( -4.5,0 ) node[yshift=-.2pt] node[below , yshift=-1ex ] ; ( -3,0 ) node[yshift=-.2pt] node[below , yshift=-1ex ] ; ( -2.4,0 ) circle ( 0.1 ) ; ( -2,.5 ) ( 2.35,.5 ) ; ( -2,1.5 ) ( -2,0 ) node[below]sc ; ( -1.5,0 ) node[yshift=-.2pt] node[below , yshift=-1ex ] ; ( 0,0 ) node[yshift=-.2pt] node[below , yshift=-1ex ] ; ( .6,0 ) circle ( 0.1 ) ; ( 1.5,0 ) node[yshift=-.2pt] node[below , yshift=-1ex ] ; ( 1.8,0 ) circle ( 0.1 ) ; ( 2.35,1.5 ) ( 2.35,0 ) node[below]sc ; ( 2.35,1.5 ) ( 6.5,1.5 ) ; ( 3,0 ) node[yshift=-.2pt] node[below , yshift=-1ex ] ; ( 4.2,0 ) circle ( 0.1 ) ; ( 4.5,0 ) node[yshift=-.2pt] node[below , yshift=-1ex ] ; ( 6,0 ) node[yshift=-.2pt] node[below , yshift=-1ex ] ; ( 6.5,1.5 ) ( 6.5,0 ) node[below]sc ; ( 6.5,.5 ) ( 7,.5 ) ;  ( -6,-.5 ) ( -2,-.5 ) node [ midway , below , yshift=-12pt ] ; ( -2,-.5 ) ( 2.35,-.5 ) node [ midway , below , yshift=-12pt ] ; ( 2.35,-.5 ) ( 6.5,-.5 ) node [ midway , below , yshift=-12pt ] ; at ( 3.75,2.5 ) scheduled opportunities ; ( .7,2 ) circle ( 0.1 ) node[right , xshift=.45ex]unscheduled opportunities poisson process ; at ( 2.2,3 ) state changes ( sc ) ;this section is devoted to the proof of our first main result on the optimal policy on when to replace the component . to this purpose, we set up our problem as a markov decision problem ( mdp ) . due to the stochastic nature of the problem, it does not suffice to know the type of the decision epoch ( scheduled or unscheduled opportunity ) , but it is also required to keep track of the remainder time till the next scheduled opportunity .the remaining time until the next scheduled opportunity may impact our decision , i.e. the optimal policy may depend on the residual time till the next scheduled opportunity .thus , for the full description of the condition ( state ) of the component , we will use a triplet descriptor , [ page - ref ] where indicates the condition of the component . if , then this means that the condition of the system is about to change andthere is no decision associated with this epoch , while if or , this means that this is a decision moment of the type of a scheduled or unscheduled opportunity .finally , the third element indicates the remaining time until the scheduled opportunity . note that if then . the introduction of the time in the full description of the condition of the component renders the model inhomogeneous , and for this reason we use techniques that stem from semi - markov decision problems ( smdp ) .+   in case of equal costs at scheduled and unscheduled opportunities , the long - run optimal average cost policy does not depend on time .in particular , the optimal policy for state is to do nothing and for state the optimal policy depends on the model parameters .[ thm : opt1 ] under the assumption that , the long - run optimal average cost policy is : for state to do nothing . for state to replace ( at both scheduled and unscheduled opportunities ) if , and to do nothing otherwise .  for the proof of theorems [ thm : opt1 ] and [ thm : opt2 ], we derive the policy that minimises the long - run average cost per time unit . to this purpose ,we state the bellman optimality equations for the decision process with action space , see , e.g. , , .[ thm : puterman ] for a decision process with an embedded unichain , there exists a scalar and a value function satisfying the system of equations where is the expected sojourn time of the smdp in state with transition probabilities under action , and is the stationary total expected cost of the smdp in state between two consecutive decision epochs under action .the existence of such a scalar ensures the existence of a policy that minimises the long - run average cost .the bellman optimality equations [ bellman ] for the model at hand become :  in this paragraph we explain in detail how equation is obtained .state , as explained before , see page , is not associated with any decision .therefore , there is no minimum operator appearing on the right hand side of the equation and the corresponding cost , , is equal to zero . for the other terms appearing on the right hand side of equation, it suffices to note that there are three possible evolutions in terms of the state of the system either a so or a sc or an uso , where the time till the next so is equal to , while the time till the sc and uso are exponentially distributed with rates and , respectively . in particular , the expected sojourn time of the smdp in state can be calculated as the minimum of a deterministic time and two exponentially distributed times , which can be easily verified to be equal to .the set of equations [ eq : v1sc][eq : so ] was obtained using very similar arguments .+ note that in [ eq : uso , eq : so ] , inside the minimum , the left terms correspond to the action `` replace '' , while the right terms correspond to the action `` do nothing '' .furthermore , for , [ eq : uso , eq : so ] yield that it is never optimal to replace . in order to decide if it is optimal or not to replace the component in state at scheduled and unscheduled opportunities , cf .[ eq : uso , eq : so ] , we define the following auxiliary functions , for ,\ ] ] and rewrite [ eq : v2sc , eq : v1sc , eq : uso , eq : so ] , for , from this point onward in the proof , we will use that . at a scheduled opportunity, we can either replace a component in state or do nothing .we first assume that we replace the component at a scheduled opportunity , i.e. thus , equation [ visot ] for implies that .furthermore , for , equation [ eq : fis ] yields and .combining the last two results yields since the functions are continuous functions in  ] the function is monotone . moreover ,if or equivalently if is non - increasing , we can extend the above approach throughout the interval  ] , but that would contradict assumption .hence , for  ] , we now need to identify the , more concretely the point such that .if is strictly positive this contradicts assumption , yielding .all in all , if the policy at a scheduled opportunity is to replace the component in state 1 , then the optimal long - run average cost policy at an unscheduled opportunity is to replace in state if , cf . .a schematic representation of the function is depicted in figure [ fig:3 ] .similarly to the case that we replace the component at a scheduled opportunity , we may assume that we do not replace the component at a scheduled opportunity and prove that then , it is also optimal not to replace the component at an unscheduled opportunity if . at the equality , we are cost - wise indifferent between the two actions .   in case of different preventive costs at scheduled and unscheduled opportunities ,the optimal policy , in certain cases , becomes time - dependent , i.e. there exists a threshold such that it is optimal to replace if the residual time until the next scheduled opportunity is greater than or equal to the threshold and do nothing otherwise .[ thm : opt2 ] under the assumption that , the optimal long - run average cost policy is : for state to do nothing . for state to replace at scheduled opportunities and at unscheduled opportunities occurring in , with ,if , and to do nothing otherwise .similarly to the proof of , we need to make certain assumptions here regarding the actions at the given opportunities .in particular , we distinguish three cases , each corresponding to a different set of actions . _ case ( i ) : _ ; _ case ( ii ) : _ ; _ case ( iii ) : _ .the proof of this theorem is identical in structure to the proof of theorem [ thm : opt1 ] and for this reason it is omitted .schematic representations of the function for cases ( i ) and ( ii ) are depicted in figures [ fig:4a ] and [ fig:4b ] , respectively .0.48 .,title="fig : " ]  0.48 .,title="fig : " ]in the previous section , we derived the optimal long - run average cost policy . in this section , for any given time dependent control limit policy , we calculate the long - run average cost per time unit in theorem [ them : costs ] .  [ them : costs ] consider a given policy that states that in state we do nothing , and in state we replace at scheduled opportunities and at unscheduled opportunities for which the remaining time until the next scheduled opportunity is greater than , and we do nothing otherwise . under thisgiven policy , the long - run average cost is .\label{eq : replacet}\end{aligned}\ ] ]  for the calculation of the long - run average cost , we employ techniques from renewal - reward theory . to this purpose, we consider as renewal epochs the instants of the scheduled opportunities , thus , the inter - renewal times are deterministic and equal to . for the determination of the average cost during a renewal cycle ,we define the following probabilities where .then , the average cost in a cycle can be obtained as from which , once we calculate the unknown probability , , we can immediately derive the long - run average cost by simply dividing by .+ the probabilities in [ eq : propbsilam ] satisfy the following system of differential equations solving - , knowing that for all , and , , i.e. they are continuous functions , we obtain where    in case of only _ scheduled opportunities _ , which corresponds to the case or equivalently to the case , the long - run expected cost equals   in case of only _ unscheduled opportunities _ , which corresponds to the case , the condition of the component can be fully described using a double descriptor which is independent of time , and thus the new model formulation falls into the framework of regular mdps . it can be easily shown that : for state the optimal policy is to do nothing .for state 1 the optimal policy is to replace if and to do nothing otherwise .furthermore , under the optimal policy the average long - run cost is equal to   in case of only _ corrective replacements _ , the long - run average cost is equal to a small numerical example , we examine the effect of , and on the long - run average cost , while keeping all other parameters fixed .we choose , , and . in this case , the long - run average cost in case of only corrective replacements is equal to . in table 1 , we show the long - run average cost for various choices of , and for three different policies : the optimal policy ( ) , the policy that we replace at only the scheduled opportunities ( ) , and finally , the policy under which we always replace at all opportunities ( ) .it seems that the use of the proposed policies can considerably improve upon the long - term average cost when compared to performing only corrective replacements .in most of the numerical scenarios we performed , as long as the choice of the parameters is reasonable , we did not notice considerable differences between the three policies , viz . .   & & + & & & & & & & & + 0.1 & 2840.41 & 2885.56 & 3384.70 & 3384.86 & 3422.03 & 3802.49 & 3807.90 & 3823.32 + 0.5 & 2840.41 & 3042.07 & 3384.09 & 3384.86 & 3538.91 & 3784.63 & 3807.90 & 3867.21 + 1 & 2840.41 & 3194.24 & 3383.38 & 3384.86 & 3636.35 & 3768.42 & 3807.90 & 3899.32 + 2 & 2840.41 & 3401.88 & 3382.15 & 3384.86 & 3747.82 & 3747.68 & 3807.90 & 3932.53 +  + & & + & & & & & & & & + 0.1 & 3378.56 & 3403.48 & 3719.49 & 3720.28 & 3738.77 & 3979.00 & 3985.81 & 3989.58 + 0.5 & 3378.56 & 3489.66 & 3716.57 & 3720.28 & 3796.18 & 3956.81 & 3985.81 & 3998.72 + 1 & 3378.56 & 3573.11 & 3713.40 & 3720.28 & 3842.96 & 3937.06 & 3985.81 & 4003.48 + 2 & 3378.56 & 3686.18 & 3708.32 & 3720.28 & 3894.71 & 3912.27 & 3985.81 & 4006.06 +  + & & + & & & & & & & & + 0.1 & 3792.57 & 3797.66 & 3916.43 & 3916.70 & 3921.39 & 4052.18 & 4055.71 & 4055.51 + 0.5 & 3792.57 & 3816.41 & 3915.40 & 3916.70 & 3937.26 & 4039.84 & 4055.71 & 4053.46 + 1 & 3792.57 & 3836.68 & 3914.21 & 3916.70 & 3951.98 & 4027.59 & 4055.71 & 4049.58 + 2 & 3792.57 & 3868.78 & 3912.11 & 3916.70 & 3970.48 & 4010.20 & 4055.71 & 4041.61 +in this paper , we derived the optimal replacement policy for a 3-state component degrading over time with corrective replacements at failures and preventive replacements at both scheduled and unscheduled opportunities .we also explicitly calculated the long - run average cost for certain time - dependent policies . in future work , we want to extend our analysis to models of components with more than three states , to models with additional features such as repairs , and to models in which the condition of the component is only partially observable .the work of sz .klosi is supported by the data science flagship framework , a cooperation between the eindhoven university of technology and philips .the work of s. kapodistria is supported by an nwo gravitation project , networks , and a tki project , daisy4offshore .the authors would like to thank m. barbieri , j. korst , and v. pronk ( all philips research ) and o. j. boxma ( eindhoven university of technology ) for their time and advice in the preparation of this work .

Abstract: motivated by original equipment manufacturer ( oem ) service and maintenance practices we consider a single component subject to replacements at failure instances and two types of preventive maintenance opportunities : scheduled , which occur due to periodic system reviews of the equipment , and unscheduled , which occur due to failures of other components in the system . modelling the state of the component appropriately and incorporating a realistic cost structure for corrective maintenance as well as condition - based maintenance ( cbm ) , we derive the optimal cbm policy . in particular , we show that the optimal long - run average cost policy for the model at hand is a control - limit policy , where the control limit depends on the time until the next scheduled opportunity . furthermore , we explicitly calculate the long - run average cost for any given control - limit time dependent policy and compare various policies numerically .


in the last decades computer science has undergone a spectacular development that has left its marks on most aspects of our everyday life while truly revolutionizing a number of domain specific human activities .however , brute computing force growing at exponential rate was not the answer to the problem of complexity as signaled already in the late sixties when the term software crisis " got coined .thanks to a dynamic it market marked by fierce competition , software development has been constantly maturing ever since .unfortunately , computational science remained immune to all those environmental factors that drive the development of the software industry . even though it has always made much better use of the improving hardware capabilities by squeezing every last drop of performance out of the available computing resources , in terms of efficiency and software quality it has fallen much behind modern trends .recently , almost forty years later , scientists realized too the obstacles standing in the way of large scale scientific software projects .the two papers send a clear message to the computational science community : _ a change of paradigm is necessary ! _ in and this message is reinforced in the context of small and medium size projects that make up the bulk of the activity in the community .it is pointed out that there is an acute need for shifting towards a reuse oriented paradigm which would improve dramatically the efficiency and quality of computational research .component software is a relatively recent buzzword in the software industry many viewing it as the holy grail of software reuse .these ideas found a fertile soil in computational science thanks to grid computing that enjoys high priority on the research agenda of most developed regions such as eu , north - america and japan .grid computing , even though still very much in its infancy , has been preparing the path for component software for some years .driven by the challenges of grid computing the fundamental ideas from component software have quickly been taken up by a few groups and a recommendation for a common component architecture ( cca ) emerged .the dominance of this approach is secured by the common component architecture forum , an organization backed by several us based universities . at the core of cca stands a language interoperability tool , called babel , and the scientific interface definition language ( sidl) . starting from a manually written sidl file babel can generate stub and skeleton source code for a number of programming languages commonly used in scientific computing .the programmer can include the implementation part and after compilation the functions can seamlessly communicate across the boundaries of programming languages .several toy and production frameworks have been developed based on babel .they provide a visual programming environment for assembling components into computational projects .there is no doubt that this approach can be viable on the long run yet presently it is not much more than an intellectual experiment as the amount of training for a developer trying to produce a cca compliant component represents a threshold that are too high for most to consider the solution . the developer is required to learn sidl and the constituting object - oriented programming concepts .existing numeric libraries can not be turned into a component without carrying out changes in the function signatures . beside the involved extra work ,another reason why many would not take this step is because , as long as cca is not an established standard , spoiling " the code s compliance with previous prescriptions would not pay off . especially not for a natural scientists or engineers who want the most bang for the buck " in their own research and do not share the excitement of computer scientists over using a well designed but fresh standard that apparently makes life only more difficult for quite a few years to come .no wonder that the associated public component repository , alexandria , is still empty several years after project launch .another notable solution is the iris explorer of the numerical algorithms group ( nag) .apart from the wide - spread flow - based visual programming environment on the user side , on the component development side the author is assisted by a module builder application that , after collecting sufficient information on the input and output ports via self - explanatory gui forms , it wraps and builds the code into an iris module . however , documenting the code in terms of its interface is completely manual and avoiding mismatches between the source and its description is the responsibility of the author . using custom datatypes constitutes another challenge for the developer .the two main reasons , though , why the nag solution fails to have the impact we envision for the future computational framework is that , on the one hand it is limited to mathematical problems and data visualization , on the other it is a commercial product . as a result , in spite of the iris explorer centre of excellence initiative public repository has not received new modules since 2002 .nevertheless , since the nag solution is endowed with a multitude of deasirable features , we consider that its user base should have an important word to say in conceiving the new high impact solution .determined to avoid the aforementioned pitfalls the comodi initiative is meant to be a pragmatic approach with well - defined objectives formulated in view of the ultimate goal of moving computational science out of its nadir and giving it a hand in starting on the road towards modern programming practices while harvesting along the way all the efficiency and quality benefits of a new reuse oriented paradigm .comodi emerges from a set of premises that are not exclusively its own .even though not always explicitly formulated by the authors of other solutions yet stand at the basis of these approaches . the following premises are the distilled conclusions of a preliminary survey made with a mixed group of computational scientists on the occasions of conferences , workshops and via an online form on the comodi website :  1 .  computational science is not computer science : the roots of computational science are in natural sciences and engineering not in computer science ; 2 .computational science demands a new paradigm . the efficiency and quality of the scientific software development process and the reproducibility of virtual computer experiments " need major improvements .3 .  computational science will not take up any new technologies in the short run unless simpler than those that are in use today ; 4 .the community itself should decide on the new paradigm .a restricted group of promoters can only set the process off and catalyze it ; 5 .a few , relatively homogeneous groups make up the bulk of the community .the new solution should target these groups instead of trying to be fully comprehensive .out of the above five points the last three are comodi specific but the extra emphasis on point 3 is the one that makes comodi essentially different from the rest .this latter postulate translates into a fundamental requirement that we can term as _ zero effort threshold"_. we can make it more specific by formulating a series of commandments that will guide the design process . a clear distinction needs to be made between what comodi means to end - users on the one hand , and to component developers or authors on the other .the former group is primarily involved in assembling components into projects and executing them while component authors design and implement new components .since the two activities require different skills and work methods the requirements set for the employed tools in each case also differ . for user satisfactionthe solution has to be endowed with the following features :  * user friendly graphical interface ; * intuitive representation of data and processes such that the elements of low - level programming , c and fortran programmers are accustomed to , can be clearly identified ; * high - level flow - based visual programming environment ; * possibility for low - level control ; * platform independence of the framework and of the components ; * comprehensive component repository ; * free !  in order to fully support developers it is imperative that no compliance criteria are set for the computational code neither in terms of structure nor used data types .in other words , any valid code written in the supported programming languages should automatically be ready for comodi .therefore the following restrictions apply to adapting regular code to comodi :  * no change in the source code .neither in the interface nor in the implementation ; * no extra coding . connectivity is achieved by supplementing author provided source - code with automatically generated glue - code ; * no need for the author to know other languages / standards then the ones used for implementing the code ; * no platform dependence .the capabilities of the system the development is carried out on is extended by on - line servers providing compilation as web service ; * no language dependence .c / c++ , fortran , java , python and other present and future languages should be able to communicate seamlessly ; * low performance overhead ; * support for both open source and commercial components .  for closely matching the low - level approach of computational scientist s to programming , components need to be the lowest possible granularity units of code , namely functions and procedures . in and they are also referred to as _ logical components _ so that they could be clearly distinguished from higher granularity _ physical components _ or _ packages _ that are units of deployment .a physical component can contain a large number of logical components that are packed together and then uploaded as one file into the repository .clearly , the above commandments are easier to state than to comply with .communicating data across , language and platform boundaries represents a serious technical challenge .the tip of the iceberg includes the following issues :  * language dependence : e.g. pascal uses the call stack differently ; * architecture dependence : file systems , little endian vs big endian ; * compiler dependence : the same data type may be represented differently .e.g. 16 bits vs 32 bits ; * exporting type definitions .  for bridging over languagesthere are several solutions at hand such as translating all types of source code into a single language , e.g. c , or compile them to an intermediate language similar to java bytecodes or the microsoft il for .net .alternatively , following the design of iris explorer one can provide an api for the different languages or create stub and skeleton code based on some interface description as done with babel for the common component architecture ( cca ) .however , none of the above can fully live up to the expectations formulated in the commandments from the previous section as they either require extra care from the developer or interfere with his / her source code or induce a significant performance overhead .we suggest that the responsibility of all wiring related issues should be assumed by generated glue - code .similarly to iris and unlike babel , interface glue - code is generated after the implementation which apparently is not a healthy programming practice as this would prescribe defining all interfaces first and then stuffing the implementation into the code .however , babel glue - code is generated from an sidl file written manually by the developer .this means a new language to be mastered which would make comodi much less appealing . besides , the target segment of comodi are scientists that already have programmed for a few years and ready to try comodi in view of the zero effort threshold .these either already have their implementation ready or will develop it using their own familiar programming environment .iris does a better job in this respect but falls short of effectively minimizing the effort of the developer and when it comes to user defined datatypes the developer bumpes into apis and an involved type definition procedure .thus , the problem of interoperability in itself is not impossible to circumvent .the question remains though at what cost ? by cost " we mean the compromises that need to be made when there are no available alternatives for simultaneously respecting all of the above formulated general requirements to their full extent .our small prototypes arm us with confidence but complete certainty is conditioned by a solution that has been tested against a wide range of different computational tasks .merging the reuse oriented paradigm with that of distributed computing represents a double effort threshold for the community , on the user and developer side alike .both areas are in an experimental stage .therefore we recommend that comodi should focus on providing a viable solution for the problems it is targeting , namely efficiency and quality of scientific software development , without getting involved in the problem of efficient computing and storage resource sharing .nevertheless , all design decisions should be taken in view of the requirements for adapting the framework to the grid in the near future .the complete solution is made up of the following major elements :  * high - level visual programming environment for computational projects ; * standardized scientific component descriptor language ( cdl ) ; * component developer tools for adapting regular code to the framework ; * distributed component repository ; * compilation web service .  as it has already been suggested in the previous section , depending on the programming activities the above software elements support they can be divided into two fundamental groups : the _ user side _ and the _ developer side_. on figure [ f : architecture ] we can see the sketch of the comodi architecture .the responsibilities of each part are summarized in table [ t : roles ] ..responsibilities of the two major parts of comodi [ cols="<,<",options="header " , ]   the developer layer contains a user friendly _ graphical user interface _ ( gui ) , a _ component developer tool _ ( cdt ) with a _parser_. the cdt , after semi - automatically collecting information pertaining to the content , behavior , and representation of the component , generates a _ component descriptor file _( cdf ) in the xml based _ component desciptor language _ ( cdl ) and the source of the _ glue - code _ that will intermediate the communication of the component within the comodi framework . at this stagethe cdf will contain all communication related information such as exported functions and data types .it describes both syntactically and semantically the component , supports the programming style of computational scientists as far as data structures , and it is extensible .its complexity is expected to grow together with the user community and the number of application areas . by semi - automaticalwe mean that the _ parser _ , which stands at the basis of the tool , inspects the source file and generates a primary cdf .using the gui , the developer only has to confirm the exported ports , provide human readable documentation for the component , set default values and add representation related information .the cdt then contacts on - line _ compilation servers _ and returns ready - made binaries for the platforms of the developer s choice .the compiled library together with the descriptor file is uploaded by the developer to a place where it can be accessed publicly while the cdt registers the component in the _ component repository_.    the deployed component is a package containing the component s source code - if the developer chooses to make the source open - the component descriptor file , the binaries for both the computational- and the generated glue - code , and further resources .components are packed into standard zip or tar.gz format and registered in the _ remote component repository_. upon use within the comodi framework , the component is downloaded and stored in the _ local component repository_.  the sources provided by the component developer suffer no changes whatsoever during the component creation process .all glue - code comes as additional functions in a separate file .not touching the source of the developer has the benefit of the compiled component being usable both within and outside the comodi framework making comodi components fully compatible with traditional programming environments .  in order to make comodi itself easily extensible it has to be component - based .this requires the separation of the framework into a _ kernel layer _ and several other modules built on the top of it .it is possible to enforce a very general view on this component architecture and deal uniformly with computational components and components that are intimately related to the framework itself . in this approach ,anything apart from the kernel is a component , be it a simple numerical component or a heavyweight gui .however , this uniformity , while simplifying the integration of components vital to the proper functioning of comodi , will come dear as it compromises the postulated simplicity of wiring computational components by users .therefore it is sensible not to sacrifice the support of user and component developer activities in favor of those related to the development of comodi .in the previous section we pointed out that `` clever '' glue - code is the key to following the comodi commandments .it can come in two flavors depending on whether it mediates an incoming function call through a provides port or an outgoing call via a uses port .we shall prepend the term glue - code with the words `` uses '' or `` provides '' whenever this aspect will be of relevance .connectors represent a similar concept intimately related to component software .there is no fundamental difference between connectors and glue - code .both are meant to bridge over incompatibilities that are not essential from the point of view of the composed client - server system .as such they can be handled automatically or semi - automatically .the most important difference between the two consists in the fact that glue - code is generated while connectors , as components in general , are hand - made .glue - code is tightly associated with a component within the boundaries of the same physical component ( deployed unit ) .glue - code is a kind of integrated connector that can communicate with anybody at one end but it only connects to a well - defined component located in the same physical component .glue - code is included into the component instead being part of the framework for a number of reasons :  * there are functionalities that can only be set statically , depending on the content of the source code ; * it can be better optimized for performance : in order to satisfy the requirement of low overhead it is necessary that the framework does not intermediate the communication between components .instead , it will wire up the connections by setting direct component - to - component references ; * freedom of the component author to further optimize it ; * keeps the framework platform independent ; * in view of the approaching era of grid computing the autonomy of components should be increased .the obvious disadvantage of this solution is the components increased size .however , this increase is not expected to be a relevant problem .since glue - code takes over all the burden of making regular code connectible it has several responsibilities :  * _ call stack management : _ bridge the difference in handling the call stack in various languages * _ parameter passing management : _ when the two involved languages ca nt automatically do it by themselves . for example , when calling fortran from c , parameters can only be passed by reference . * _ linking : _ by linking we mean the process of setting all call references between components .these references are set according to parameter strings extracted from an xml encoded file containing the description of the assembled computational application .they are stored in static variables of glue - code segments and are passed to the computational code during runtime ( see figure [ f : linking ] ) .the runtime entry point is called once also during link - time and the children references are requested from the glue - code function in charge with wiring . as a result , the wiring function can be completely avoided during runtime .the actual computational code , referred to as business logic " in figure [ f : linking ] , will be in the body of a function that receives all necessary information , including the child references , as parameters .this has the additional benefit of self - containment .the function is also fully functional outside the context of the framework without confusing extra arguments .a more detailed discussion on the wiring mechanism can be found in .+  * _ handling data and code aggregates : _ translates between data structures of different granularity .for example , structures records and objects can be decomposed into a set of variables and vice versa .this is especially important when binding c++ code to c. * _ managing default values and references : _ the author of a component is encouraged to provide default values for the parameters of both uses and provide ports .there are at least two benefits to this feature .for once , users can gradually explore the capabilities of a component by using a reduced set of parameters at each port .secondly , default parameters endow a given port with the flexibility of connecting to ports that require a different number of parameters . * _ managing global naming conventions : _ since the components are autonomous , authors must respect only the rules of the programming language in which the component is developed .problems may arise with the identifiers ( names of functions , procedures , types , variables , constants , parameters ) if two or more authors give the same name to two or more entities .the glue - code generator translates the identifiers between local and global naming conventions . * _ managing remote calls : _ it realizes a _ stub - skeleton _ architecture for local and remote component calls .the details implementing connectors in comodi is a topic on its own right and it is beyond the scope of this paper. the reader can refer to chapter 10 . in and .one of the particularities of the comodi solution is that the process of converting the source code into a component is automated to the maximum possible extent .therefore the component developer tool must `` gain insight '' into the the source code with the help of the parser .the aim of the _ parser _ is to extract the necessary information from the programs source code in order to elaborate a full documentation of the component .the output of the parser is an xml document that is piped into the glue code generator and also serves as raw material for the content of the component descriptor file .the parser analyzes lexically and syntactically the source of the developers computational code .the semantic analysis is outside the scope of the parser .the used information is :  * identifiers * types * variables * constants * functions * procedures * parameters * comments for the documentation * special directives  the architecture of the comodi parser consists of two blocks : _ the programming language description and recognition part _ ( ebnf parser ) and _ the program recognition part _ , which carries out the lexical and the syntactical analysis based on a self - constructive automata - system ( fig .[ f : parser ] ) .the description of the programming languages is given in ebnf format ( standard : iso / iec 14977 ) .the _ ebnf parser _ reads the ebnf description of the programming language ( fortran , c , c++ , pascal , java , etc . ) , and builds an automata - system , responsible for the lexical and the syntactical analysis of the source code .the automata - system contains a set of modified push - down automata , interconnected into a network , each automaton each possessing an inner stack .this system is equivalent to a single non - deterministic push - down automaton , capable of recognizing context - free languages .the lexical elements and the syntax of the majority of the used imperative programming languages , and also their recognition rules , can be defined with context - free grammars .so , our automata - system can recognize the majority of imperative programming language , once their ebnf definitions are available .the automata - system receives as input the program source code each token is an element of the alphabet starts from an initial state , and after reading the symbol next in the line , it changes the inner - state according to the transitional function . during this transitionit modifies the stack . after reading all the input symbols ,if the automata - system is in one of the final states , and the stack is empty , the automata - system recognizes the input source code and builds correctly the tables . in all other cases it fails .our prototype show that the above algorithm seems to work well for languages such as c or fortran .we have presented the general requirements and a few design guidelines for a complete reuse oriented solution for computational scientists .the lack of impact of present solutions is blamed on the involved high effort threshold , in many cases made worse by a closed source and restrictive copyrights .therefore , the corner stone of comodi is the zero effort threshold requirement for component developers .we argue that the community needs a solution that allows a smooth , effortless transition to the new paradigm . once there, high - tech solutions will automatically be accepted by the community .we also claim that a change of paradigm requires a solution that is widely used and supported , situation that is only conceivable within an opensource project .comodi should be a joint effort of computer scientists assuring the quality of the code and computational scientists collectively and actively contributing to refining the requirements .if the contribution of either of the two sides gets out of balance comodi ends up as just another interesting case study in computer science or , conversely , becomes an unreliable pile of code impossible to maintain .our prototypes indicate that the suggested architecture is feasible .thus , the main challenge is not of technical nature but rather consists in sparking the interest of the community in developing and using comodi .this work is supported by the national university research council of romania with grant no ..  11 _ comodi homepage , _p. naur and b. randell , _ software engineering : report on 1968 nato conference , _ nato , 1969 d. post , _ the coming crisis in computational science _ , proceedings of the ieee international conference on high performance computer architecture : workshop on productivity and performance in high - 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04 , 2000 ( http://www.extreme.indiana.edu/ccat/papers/hpdc2000.pdf ) _ common component architecture ( cca ) forum homepage , _ http://www.cca-forum.org _ babel homepage , _ http://www.llnl.gov/casc/components/babel.html _ ccafe homepage , _ http://www.cca-forum.org/baallan / ccafe _ xcat homepage , _ http://www.extreme.indiana.edu/xcat _scirun homepage , _ http://www.sci.utah.edu _ numerical algorithms group homepage _ , http://www.nag.co.uk/ _ iris explorer centre of excellence homepage , __ open data explorer homepage , _lzr , b. prv , _ comodi : component wiring in a framework for scientific computing _ , studia babe - bolyai , series informatica , vol .xlix , no . 2 ( 2004 ) 103 a. mayer , s. mcgough , m. gulamali , l. young , j. stanton , s. newhouse , j. darlington , _ meaning and behaviour in grid oriented components _ , proceedings of the third international workshop on grid computing , springer - verlag ( 2002 ) ( www.lesc.ic.ac.uk/iceni/pdf/grid2002.pdf )

Abstract: the computational module integrator ( comodi ) is an initiative aiming at a component based framework , component developer tool and component repository for scientific computing . we identify the main ingredients to a solution that would be sufficiently appealing to scientists and engineers to consider alternatives to their deeply rooted programming traditions . the overall structure of the complete solution is sketched with special emphasis on the component developer tool standing at the basis of comodi .