Connected Operators for the Totally Asymmetric Exclusion Process
Connected Operators for the Totally
Asymmetric Exclusion Process
O. Golinelli, K. Mallick
Service de Physique Théorique, Cea Saclay, 91191 Gif, France
6 April 2007
Abstract
We fully elucidate the structure of the hierarchy of the con-
nected operators that commute with the Markov matrix of the
Totally Asymmetric Exclusion Process (TASEP). We prove for
the connected operators a combinatorial formula that was con-
jectured in a previous work. Our derivation is purely algebraic
and relies on the algebra generated by the local jump operators
involved in the TASEP.
Keywords: Non-Equilibrium Statistical Mechanics, ASEP, Exact
Results, Algebraic Bethe Ansatz.
Pacs numbers: 02.30.Ik, 02.50.-r, 75.10.Pq.
1 Introduction
The Asymmetric Simple Exclusion Process (ASEP) is a lattice model of parti-
cles with hard core interactions. Due to its simplicity, the ASEP appears as a
minimal model in many different contexts such as one-dimensional transport
phenomena, molecular motors and traffic models. From a theoretical point
of view, this model has become a paradigm in the field of non-equilibrium
statistical mechanics; many exact results have been derived using various
methods, such as continuous limits, Bethe Ansatz and matrix Ansatz (for re-
views, see e.g., Spohn 1991, Derrida 1998, Schütz 2001, Golinelli and Mallick
2006).
In a recent work (Golinelli and Mallick 2007), we applied the algebraic
Bethe Ansatz technique to the Totally Asymmetric Exclusion Process (TASEP).
http://arxiv.org/abs/0704.0869v1
Golinelli, Mallick — Connected Operators for TASEP 2
This method allowed us to construct a hierarchy of ‘generalized Hamiltonians’
that contain the Markov matrix and commute with each other. Using the
algebraic relations satisfied by the local jump operators, we derived explicit
formulae for the transfer matrix and the generalized Hamiltonians, generated
from the transfer matrix. We showed that the transfer matrix can be inter-
preted as the generator of a discrete time Markov process and we described
the actions of the generalized Hamiltonians. These actions are non-local be-
cause they involve non-connected bonds of the lattice. However, connected
operators are generated by taking the logarithm of the transfer matrix. We
conjectured for the connected operators a combinatorial formula that was
verified for the first ten connected operators by using a symbolic calculation
program.
The aim of the present work is to present an analytical calculation of the
connected operators and to prove the formula that was proposed in (Golinelli
and Mallick 2007). This paper is a sequel of our previous work, however, in
section 2, we briefly review the main definitions and results already obtained
so that this work can be read in a fairly self-contained manner. In section 3,
we derive the general expression of the connected operators.
2 Review of known results
We first recall the dynamical rules that define the TASEP with n particles
on a periodic 1-d ring with L sites labelled i = 1, . . . , L. The particles move
according to the following dynamics: during the time interval [t, t + dt], a
particle on a site i jumps with probability dt to the neighboring site i+ 1, if
this site is empty. This exclusion rule which forbids to have more than one
particle per site, mimics a hard-core interaction between particles. Because
the particles can jump only in one direction this process is called totally
asymmetric. The total number n of particles is conserved. The TASEP
being a continuous-time Markov process, its dynamics is entirely encoded in
a 2L × 2L Markov matrix M , that describes the evolution of the probability
distribution of the system at time t. The Markov matrix can be written as
Mi , (1)
where the local jump operator Mi affects only the sites i and i + 1 and
represents the contribution to the dynamics of jumps from the site i to i+1.
Golinelli, Mallick — Connected Operators for TASEP 3
2.1 The TASEP algebra
The local jump operators satisfy a set of algebraic equations :
M2i = −Mi, (2)
Mi Mi+1 Mi = Mi+1 Mi Mi+1 = 0, (3)
[Mi,Mj ] = 0 if |i− j| > 1. (4)
These relations can be obtained as a limiting form of the Temperley-Lieb
algebra. On the ring we have periodic boundary conditions : Mi+L = Mi.
The local jumps matrices define an algebra. Any product of the Mi’s will be
called a word. The length of a given word is the minimal number of operators
Mi required to write it. A word, that can not be simplified further by using
the algebraic rules above, will be called a reduced word.
Consider any word W and call I(W ) the set of indices i of the operators
Mi that compose it (indices are enumerated without repetitions). We remark
that, if W is not annihilated by application of rule (3), the simplification
rules (2, 4) do not alter the set I(W ), i.e., these rules do not introduce any
new index or suppress any existing index in I(W ). This crucial property is
not valid for the algebra associated with the partially asymmetric exclusion
process (see Golinelli and Mallick 2006).
Using the relation (2) we observe that for any i and any real number
λ 6= 1 we have
(1 + λMi)
−1 = (1 + αMi) with α =
. (5)
2.2 Simple words
A simple word of length k is defined as a word Mσ(1)Mσ(2) . . .Mσ(k), where σ
is a permutation on the set {1, 2, . . . , k}. The commutation rule (4) implies
that only the relative position of Mi with respect to Mi±1 matters. A simple
word of length k can therefore be written as Wk(s2, s3, . . . , sk) where the
boolean variable sj for 2 ≤ j ≤ k is defined as follows : sj = 0 if Mj is
on the left of Mj−1 and sj = 1 if Mj is on the right of Mj−1. Equivalently,
Wk(s2, s3, . . . , sk) is uniquely defined by the recursion relation
Wk(s2, s3, . . . , sk−1, 1) = Wk−1(s2, s3, . . . , sk−1) Mk , (6)
Wk(s2, s3, . . . , sk−1, 0) = Mk Wk−1(s2, s3, . . . , sk−1) . (7)
The set of the 2k−1simple words of length k will be called Wk. For a simple
word Wk, we define u(Wk) to be the number of inversions in Wk, i.e., the
Golinelli, Mallick — Connected Operators for TASEP 4
number of times that Mj is on the left of Mj−1 :
u(Wk(s2, s3, . . . , sk)) =
(1− sj) . (8)
We remark that simple words are connected, they cannot be factorized
in two (or more) commuting words.
2.3 Ring-ordered product
Because of the periodic boundary conditions, products of local jump opera-
tors must be ordered adequately. In the following we shall need to use a ring
ordered product O () which acts on words of the type
W = Mi1Mi2 . . .Mik with 1 ≤ i1 < i2 < . . . < ik ≤ L , (9)
by changing the positions of matrices that appear in W according to the
following rules :
(i) If i1 > 1 or ik < L, we define O (W ) = W . The word W is well-
ordered.
(ii) If i1 = 1 and ik = L, we first write W as a product of two blocks,
W = AB, such that B = MbMb+1 . . .ML is the maximal block of matrices
with consecutive indices that contains ML, and A = M1Mi2 . . .Mia , with
ia < b− 1, contains the remaining terms. We then define
O (W ) = O (AB) = BA = MbMb+1 . . .MLM1Mi2 . . .Mia . (10)
(iii) The previous definition makes sense only for k < L. Indeed, when
k = L, we have W = M1M2 . . .ML and it is not possible to split W in two
different blocks A and B. For this special case, we define
O (M1M2 . . .ML) = |1, 1, . . . , 1〉〈1, 1, . . . , 1| , (11)
which is the projector on the ‘full’ configuration with all sites occupied.
The ring-orderingO () is extended by linearity to the vector space spanned
by words of the type described above.
2.4 Transfer matrix and generalized Hamiltonians Hk
The algebraic Bethe Ansatz allows to construct a one parameter commuting
family of transfer matrices, t(λ), that contains the translation operator T =
t(1) and the Markov matrix M = t′(0). For 0 ≤ λ ≤ 1, the operator
Golinelli, Mallick — Connected Operators for TASEP 5
t(λ) can be interpreted as a discrete time process with non-local jumps :
a hole located on the right of a cluster of p particles can jump a distance
k in the backward direction, with probability λk(1 − λ) for 1 ≤ k < p,
and with probability λp for k = p. The probability that this hole does
not jump at all is 1 − λ. This model is equivalent to the 3-D anisotropic
percolation model of Rajesh and Dhar (1998) and to a 2-D five-vertex model.
It is also an adaptation on a periodic lattice of the ASEP with a backward-
ordered sequential update (Rajewsky et al. 1996, Brankov et al. 2004),
and equivalently of an asymmetric fragmentation process (Rákos and Schütz
2005).
The operator t(λ) is a polynomial in λ of degree L given by
t(λ) = 1 +
λkHk , (12)
where the generalized HamiltoniansHk are non-local operators that act on the
configuration space. [We emphasize that the notation used here is different
from that of our previous work : t(λ) was denoted by tg(λ) in (Golinelli and
Mallick 2007).]
We have H1 = M and more generally, as shown in (Golinelli and Mallick
2007), Hk is a homogeneous sum of words of length k
1≤i1<i2<...<ik≤L
O (Mi1Mi2 . . .Mik) , (13)
where O () represents the ring ordered product that embodies the periodicity
and the translation-invariance constraints.
For a system of size L with N particles only H1, H2, . . . , HN have a non-
trivial action. Because we are interested only in the case N ≤ L− 1 (the full
system as no dynamics) there are at most L − 1 operators Hk that have a
non-trivial action.
3 The connected operators Fk
3.1 Definition
The generalized Hamiltonians Hk and the transfer matrix t(λ) have non-local
actions and couple particles with arbitrary distances between them. Besides
Hk is a highly non-extensive quantity as it involves generically a number of
terms of order Lk. As usual, the local connected and extensive operators
are obtained by taking the logarithm of the transfer matrix. For k ≥ 1, the
Golinelli, Mallick — Connected Operators for TASEP 6
connected Hamiltonians Fk are defined as
ln t(λ) =
Fk . (14)
Taking the derivative of this equation with respect to λ and recalling that
t(λ) commutes with t′(λ), we obtain
λkFk = λ t(λ)
−1 t′(λ) . (15)
Expanding t(λ)−1 with respect to λ, this formula allows to calculate Fk as a
polynomial function of H1, . . . , Hk. For example F1 = H1, F2 = 2H2 − H
etc... (see Golinelli and Mallick 2007). By using (13), we observe that Fk is
a priori a linear combination of products of k local operators Mi. However
this expression can be simplified by using the algebraic rules (2, 3, 4) and in
fine, Fk will be a linear combination of reduced words of length j ≤ k.
Because of the ring-ordered product that appears in the expression (13)
of the Hk’s, it is difficult to derive an expression of Fk in terms of the local
jump operators. An exact formula for the Fk with k ≤ 10 was obtained
in (Golinelli and Mallick 2007) by using a computer program and a general
expression was conjectured for all k. In the following, the conjectured formula
is derived and proved rigorously.
3.2 Elimination of the ring-ordered product
The expression
λkFk can be written as a linear combination of reduced
words W . We know from formula (13) that at most L− 1 operators Hk are
independent in a system of size L, we shall therefore calculate Fk only for
k ≤ L− 1. Thus, we need to consider reduced words of length j ≤ L − 1.
Let W be such a word, and I(W ) be the set of indices of the operators Mi
that compose W ; our aim is to find the expression of W and to calculate
its prefactor from equation (15). Because the rules (2, 4) do not suppress or
add any new index, the following property is true : if a word W ′ appearing
in λ t(λ)−1 t′(λ) is such that I(W ′) 6= I(W ) then even after simplification,
W ′ will remain different from W . Therefore, the prefactor of W in
is the same as the prefactor of W in
λ tI(λ)
−1 t′I(λ) where tI(λ) = O
(1 + λMi)
with I(W ) ⊂ I . (16)
Because Fk commutes with the translation operator T , then for any r =
1, . . . , L−1, the prefactor of W = Mi1Mi2 . . .Mij is the same as the prefactor
Golinelli, Mallick — Connected Operators for TASEP 7
of T rMT−r = Mr+i1Mr+i2 . . .Mr+ij . Furthermore, any word W of size k ≤
L − 1 is equivalent, by a translation, to a word that contains M1 and not
ML : indeed, there exists at least one index i0 such that i0 /∈ I(W ) and
(i0 + 1) ∈ I(W ) and it is thus sufficient to translate W by r = L− i0.
In conclusion, it is enough to study in expression (15), the reduced words
W with set of indices included in
I∗ = {1, 2, . . . , L− 1} . (17)
Because the index L does not appear in I∗, the ring-ordered product has a
trivial action in equation (16) and we have
tI∗(λ) = (1 + λM1)(1 + λM2) . . . (1 + λML−1) . (18)
We have thus been able to eliminate the ring-ordered product.
3.3 Explicit formula for the connected operators
In equation (18), differentiating tI∗(λ) with respect to λ, we have
t′I∗(λ) =
(1 + λM1) . . . (1 + λMi−1)Mi(1 + λMi+1) . . . (1 + λML−1) . (19)
Using equation (5) we obtain
tI∗(λ)
−1 = (1+αML−1)(1 +αML−2) . . . (1 +αM1) , with α =
. (20)
Noticing that λ(1 + αMi)Mi = −αMi, we deduce
λ tI∗(λ)
−1 t′I∗(λ) = (21)
(1 + αML−1) . . . (1 + αMi+1)Mi(1 + λMi+1) . . . (1 + λML−1) .
The ith term in this sum contains words with indices between i and L − 1.
Because we are looking for the words that contain the operator M1, we must
consider only the first term in this sum, which we note by Q
Q = −α(1 + αML−1) . . . (1 + αM2)M1(1 + λM2) . . . (1 + λML−1) . (22)
In the appendix, we show that
Q = R1 +R2 + . . .+RL−1 , (23)
Golinelli, Mallick — Connected Operators for TASEP 8
where Ri is defined by the recursion :
R1 = −αM1 , (24)
Ri = λRi−1Mi + αMiRi−1 for i ≥ 2 . (25)
To summarize, all the words in
k=1 λ
kFk that contain M1 and not ML are
given by Q = R1+R2+. . .+RL−1. From the recursion relation (25) we deduce
that Ri is a linear combination of the 2
i−1 simple words Wi(s2, s3, . . . , si)
defined in section 2.1. Furthermore, we observe from (25) that a factor λ
appears if si = 1 and a factor α = λ/(λ − 1) appears if si = 0. Therefore,
the coefficient f(W ) of W = Wi(s2, s3, . . . , si) in Q is given by
f(W ) = (−1)u
(1− λ)u+1
= (−1)u
λi+j (26)
where i is the length of W and u = u(W ) is its inversion number, defined in
equation (8). We have thus shown that
f(W ) W =
(−1)u(W )
u(W )+j
λi+j , (27)
where Wi is the set of simple words of length i.
Finally, we recall that the coefficient in
k=1 λ
kFk of a reduced word W
that contains M1 and not ML is the same as its coefficient in Q. Extracting
the term of order λk in equation (27) we deduce that any word W in Fk that
contains M1 and not ML is a simple word of length i ≤ k and its prefactor
is given by (−1)u(W )
u(W )+k−i
The full expression of Fk is obtained by applying the translation operator
to the expression (27); indeed any word in Fk can be uniquely obtained by
translating a simple word in Fk that contains M1 and not ML. We conclude
that for k < L,
Fk = T
(−1)u(W )
k−i+u(W )
W , (28)
where T is the translation-symmetrizator that acts on any operator A as
follows : T A =
i=0 T
i A T−i . The presence of T in equation (28) insures
that Fk is invariant by translation on the periodic system of size L. All simple
words being connected, we finally remark that formula (28) implies that Fk
is a connected operator.
Golinelli, Mallick — Connected Operators for TASEP 9
4 Conclusion
By using the algebraic properties of the TASEP algebra (2-4), we have derived
an exact combinatorial expression for the family of connected operators that
commute with the Markov matrix. This calculation allows to fully elucidate
the hierarchical structure obtained from the Algebraic Bethe Ansatz. It
would be of a great interest to extend our result to the partially asymmetric
exclusion process (PASEP), in which a particle can make forward jumps
with probability p and backward jumps with probability q. In particular, we
recall that the symmetric exclusion process is equivalent to the Heisenberg
spin chain : in this case the connected operators have been calculated only
for the lowest orders (Fabricius et al., 1990). This is a challenging and
difficult problem. In our derivation we used a fundamental property of the
TASEP algebra : the rules (2-4) when applied to a word W either cancel
W or conserve the set of indices I(W ). The algebra associated with PASEP
violates this crucial property because there we have Mi Mi+1 Mi = pq Mi.
Therefore the method followed here does not have a straightforward extension
to the PASEP case.
Appendix: Proof of equation (23)
Let us define the following series
Q1 = −αM1 , (29)
Qi = (1 + αMi)Qi−1(1 + λMi) for i ≥ 2 . (30)
We remark that Q defined in equation (22) is given by Q = QL−1. Let us
consider Ri defined by the recursion (25). The indices that appear in the
words of Qi and Ri belong to {1, 2, . . . , i}. Therefore, we have
[Rj ,Mi] = 0 for j ≤ i− 2 , (31)
because the operators M1,M2, . . . ,Mj that compose Rj commute with Mi.
From equations (31) and (5), we obtain
(1 + αMi)Rj(1 + λMi) = Rj for j ≤ i− 2 . (32)
Furthermore, from (25), we obtain
MiRi−1Mi = λMiRi−2Mi−1Mi + αMiMi−1Ri−2Mi . (33)
Because Mi commutes with Ri−2, we can use the relation MiMi−1Mi = 0 to
deduce that
MiRi−1Mi = 0 . (34)
Golinelli, Mallick — Connected Operators for TASEP 10
Using equation (34), we find
(1 + αMi)Ri−1(1 + λMi) = Ri−1 + λRi−1Mi + αMiRi−1 = Ri−1 +Ri . (35)
From equations (32) and (35), we prove that the (unique) solution of the
recursion relation (30) is given by equation (23), Qi = R1 +R2 + . . .+Ri.
References
• Brankov J. G., Priezzhev V. B. and Shelest R. V., 2004, Generalized
determinant solution of the discrete-time totally asymmetric exclusion
process and zero-range process, Phys. Rev. E 69 066136.
• Derrida B., 1998, An exactly soluble non-equilibrium system: the asym-
metric simple exclusion process, Phys. Rep. 301 65.
• Fabricius K., Mütter K.-H. and Grosse H., 1990, Hidden symmetries in
the one-dimensional antiferromagnetic Heisenberg model, Phys. Rev.
B 42 4656.
• Golinelli O. and Mallick K., 2006, The asymmetric simple exclusion
process: an integrable model for non-equilibrium statistical mechanics,
J. Phys. A: Math. Gen. 39 12679.
• Golinelli O. and Mallick K., 2007, Family of Commuting Operators for
the Totally Asymmetric Exclusion Process, Submitted to J. Phys. A:
Math. Theor., cond-mat/0612351.
• Rajesh R. and Dhar D., 1998, An exactly solvable anisotropic directed
percolation model in three dimensions, Phys. Rev. Lett. 81 1646.
• Rajewsky N., Schadschneider A. and Schreckenberg M., 1996, The
asymmetric exclusion model with sequential update, J. Phys. A: Math.
Gen. 29 L305.
• Rákos A. and Schütz G. M., 2005, Current distribution and random
matrix ensembles for an integrable asymmetric fragmentation process,
J. Stat. Phys. 118 511.
• Schütz G. M., 2001, Exactly solvable models for many-body systems far
from equilibrium in Phase Transitions and Critical Phenomena, vol.
19, C. Domb and J. L. Lebowitz Ed., Academic Press, San Diego.
• Spohn H., 1991, Large scale dynamics of interacting particles, Springer,
New-York.
http://arxiv.org/abs/cond-mat/0612351
	Introduction
	Review of known results
	The TASEP algebra
	Simple words
	Ring-ordered product
	Transfer matrix and generalized Hamiltonians Hk
	The connected operators Fk
	Definition
	Elimination of the ring-ordered product
	Explicit formula for the connected operators
	Conclusion
