Constraints on Dark Energy state equation with varying pivoting redshift
  We assume the DE state equations w(a) = w_0+w_a(a_p-a), and study the
dependence of the constraints on w_0 and w_a coefficients on the pivoting
redshift 1+z_p=1/a_p. Coefficients are fitted to data including WMAP7, SNIa
(Union 2.1), BAO's (including WiggleZ and SDSS results) and H_0 constraints.
The fitting algorithm is CosmoMC. We find specific differences between the
cases when neutrino mass is allowed or disregarded. More in detail: i) The z_p
value yielding uncorrelated constraints on w_0 and w_a is different in the two
cases, holding ~0.25 and ~0.35, respectively. (ii) If we consider the intervals
allowed to w_0, we find that they shift when z_p increases, in opposite
directions for vanishing or allowed neutrino mass. This leads to no overlap
between 1sigma intervals already at z_p >~0.4. (iii) The known effect that a
more negative state parameter is required to allow for neutrino mass displays
its effects on w_a, rather than on w_0. (iv) The w_0-w_a constraints found by
using any pivot z_p can be translated into constraints holding at a specific
z_p value (0 or the z_p where errors are uncorrelated). When we do so, error
ellipses exhibit a satisfactory overlap.
