Thermodynamics of Markov Processes with Non-extensive Entropy and Free Energy
Liangrong Peng1,2, Hong Qian3, Liu Hong2,3 1College of Mathematics and Data Science, Minjiang University, Fuzhou, 350108, P.R.C. 2Zhou Pei-Yuan Center for Applied Mathematics, Tsinghua University, Beijing, 100084, P.R.C. 3Department of Applied Mathematics, University of Washington, Seattle, WA 98195-3925, U.S.A.a)
(Dated: 1 April 2020)
Statistical thermodynamics of small systems shows dramatic differences from nor- mal systems. Parallel to the recently presented steady-state thermodynamic for- malism for master equation and Fokker-Planck equation, we show that a “ther- modynamic” theory can also be developed based on Tsallis’ generalized entropy (q) PN q (q) S = i=1(pi − pi )/[q(q − 1)] and Shiino’s generalized free energy F = PN q−1 (q) [ i=1 pi(pi/πi) − 1]/[q(q − 1)], where πi is the stationary distribution. dF /dt = (q) (q) (q) (q) −fd ≤ 0 and it is zero iff the system is in its stationary state. dS /dt − Qex = fd (q) where Qex characterizes the heat exchange. For systems approaching equilibrium (q) with detailed balance, fd is the product of Onsager’s thermodynamic flux and force. However, it is discovered that the Onsager’s force is non-local. This is a consequence of the particular transformation invariance for zero energy of Tsallis’ statistics.
Keywords: Tsallis Thermodynamics, Non-equilibrium Steady-State, Master Equa- tion, Fokker-Planck Equation arXiv:1005.1251v3 [math-ph] 31 Mar 2020
a)Author to whom correspondence should be addressed. Electronic mail: [email protected],
1 1. INTRODUCTION
The Boltzmann-Gibbs (B-G) statistical thermodynamics is one of the fundamental theo- ries of modern physics with wide applications in chemistry, biology, and even social sciences. Recently, a complete statistical thermodynamic formalism based on both the B-G entropy and B-G free energy (relative entropy) has been presented for stochastic dynamics based on master equations1,2 and Fokker-Planck equations3,4. The theory introduces several key quantities: entropy production rate, free energy dissipation rate, excess and house-keeping heat, etc. Statistical thermodynamics of small systems requires the traditional thermodynamic quantities such as energy and entropy to be non-extensive5. This extension of the clas- sical B-G entropy provides a framework to study the systems with long-range interactions, multi-fractal structures, long-term memories, or systems whose interaction with macroscopic parts cannot be ignored6. It is therefore natural to ask whether the thermodynamic formal- ism can be developed in parallel for non-extensive entropy and free energy, such as the Tsallis entropy7, Renyi entropy8,9 and many others6,10. Here we focus on the Tsallis entropy, which enjoys the properties of concavity, experimen- tal robustness (Lesche stability), non-extensivity (as a generalization of the B-G entropy), and the finiteness of the rate of entropy production11,12. This work shows that a similar structure does exist for Tsallis thermodynamics, which therefore would be of interest to a wide range applications, like defect turbulence13, optical lattices14, mathematical finance15, and so on. PN Generalized Entropy. The notions of Shannon’s entropy in the form − i=1 fi ln fi PN and relative entropy in the form i=1 fi ln(fi/pi) arise from the asymptotic expressions for
the probability of frequency {fi} within a large set of independent and identically distributed 1 (i.i.d.) random samples, each with an equal probability of N and {pi} respectively. The
−NI(f1,··· ,fN ) exponent I(f1, ··· , fN ) in the asymptotic expression e , known as large deviations rate function, provides the divergence function for an information geometry16. When a set of samples are not independent, the probability for the frequency is no longer a simple multi-nomial distribution; Tsallis entropy is believed to arise in this context in connection with q-binomial distribution17. The Shannon entropy is intimately related to the logarithmic function, y = ln(x), which is the inverse of exponential function x = exp(y). The exponential
2 function is defined through the limit:
1 exp(y) = lim (1 + ry) r . (1) r→0
The inverse function of the right-hand side of Eq. (1) is xr − 1 y = . (2) r If one takes this as the analogue of the y = ln(x), then corresponding to Gibbs’ entropy P − i pi ln pi, one can have a generalization of the entropy:
N X pr − 1 − p i . (3) i r i=1 If we denote r = q − 1, then Eq. (3) becomes
N q−1 X 1 − p S(q) = p i , (4) i q(q − 1) i=1
as q → 1. This is the non-extensive entropy first proposed by Tsallis7, and q ∈ R (q 6= 0, 1) is known as the non-extensive parameter. Here we added an additional factor q to the denominator to maintain the definite sign of free energy and its dissipation rate, which will be shown later. Because of relations in Eqs. (1) and (2), the Tsallis entropy enjoys many of the properties of the Gibbs’ entropy. Relative entropy. Recognizing the correspondence between ln(x) and (xq−1 −1)/[q(q − 18,19 1)] as q → 1, a generalization of the relative entropy with respect to a distribution gi can be similarly introduced20:
" N q−1 # 1 X pi H(q)({p }||{g }) = p − 1 . (5) i i q(q − 1) i g i=1 i
q Statistical internal energy and H as the Gibbs free energy. Assuming {πi} is a stationary distribution to the master equation in Eq. (12) below, one can define a statistical internal energy by applying the Boltzmann’s law in reverse1:
π1−q − 1 (q) = i , (6) i q(q − 1)
in analogous to B-G energy i = ln(1/πi). Then the mean statistical internal energy is
N 1−q X (q) X π − 1 U (q) = pq = pq i . (7) i i i q(q − 1) i=1 i=1
3 q (q) The mean internal energy being the sum of pi i rather than the conventional pii is a peculiar feature of the Tsallis statistics. Then we have generalized free energy:
" N q−1 # 1 X pi F (q) = U (q) − S(q) = p − 1 = H(q)({p }||{π }). (8) q(q − 1) i π i i i=1 i Conditional entropy. Tsallis entropy does not enjoy the standard equation concerning conditional entropy of a pair of correlated random variables A and B19,21:
S(AB) = S(A) + SA(B), (9a)
where X S(AB) = − piqij log (piqij) , (9b) i,j X S(A) = − pi log pi, (9c) i ! X X SA(B) = − pi qij log qij . (9d) i j For Tsallis’s entropy, one has ! " # 1 X 1 X X X S(q)(AB) = 1 − (p q )q = 1 − pq + pq − (p q )q q(q − 1) i ij q(q − 1) i i i ij i,j i i i,j P q X 1 − j qij X = S(q)(A) + pq = S(q)(A) + pq S(q)(B|A = i) i q(q − 1) i i i (q) X (q) (q) (q) ≤ S (A) + pi S (B|A = i) = S (A) + SA (B), i when q > 1. And the inequality sign changes when q < 1. Eq. (9), together with entropy reaches its maximum with uniform distribution 1 1 1 1 S ({p , p , p , ··· , p }) ≤ S , , , ··· , , (10) 1 2 3 N N N N N and entropy is invariant with adding an impossible event
S ({p1, p2, p3, ··· , pN , 0}) = S ({p1, p2, p3, ··· , pN }) , (11)
providing Khinchin’s uniqueness theorem for Gibbs entropy21. It is noted that Tsallis entropy does satisfy both Eqs. (10) and (11). By replacing Eq. (9) with a more general non-extensive one, Abe has shown that the Tsallis entropy follows uniquely from the generalized version of Khinchin’s axioms22.
4 This paper is organised as follows. In Sec. 2 and 3, the Tsallis thermodynamics of master equations and F-P equations are constructed separately, with emphasis on the correct non- negative form of house-keeping heat. In Sec. 4, the underlying mathematical connection and correspondence between the two formalisms is explored and justified for both the discretized F-P equations and tridiagonal master equations, in the limit of infinitesimal jump step in space. Sec. 5 provides two simple but insightful examples to illustrate the application of Tsallis thermodynamics and its difference from B-G thermodynamics. Finally, we summarize our results in Sec. 6. Proofs on theorems in Sec. 4 can be found in appendix for completeness.
2. TSALLIS THERMODYNAMICS OF MASTER EQUATIONS
We now consider a Markov process with master equation N dpi(t) X = (p w − p w ) , i = 1, 2, ··· ,N, (12) dt j ji i ij j=1 which has been widely applied for modeling various irreversible processes, including quantum physics, chemical reactions and open biological systems23–25.
Relative entropy and positive free energy dissipation. We assume both {pi(t)}
and {gi(t)} satisfy the same master equation, with different initial distributions. Then the time derivative of the relative entropy N " q−1 q # d 1 X pi(t) dpi pi(t) dgi H(q)(t) = q − (q − 1) dt q(q − 1) g (t) dt g (t) dt i=1 i i N ( " q−1 q−1# q q 1 X pi pj pi pj = w qp − −(q − 1)g − q(q − 1) ji j g g j g g i,j=1 i j i j N q ( q−1 q ) 1 X wjipj pigj pigj = q − (q − 1) − 1 ≤ 0. (13) q(q − 1) q−1 g p g p i,j=1 gj i j i j
The last step is because wij ≥ 0 when i 6= j, the summand is zero when i = j, and the term in the {· · · }: qzq−1 − (q − 1)zq − 1 (1 − z)2 d 1 − zq = − ≤ 0, (z > 0). (14) q(q − 1) q(q − 1) dz 1 − z
Now if we take a stationary distribution {πi} as the {gi}, then we have the time derivative of generalized free energy F (q): (q) N " q−1 q−1# N dF 1 X pi pj 1 X (q) = w p − = − φ Φ , (15) dt q − 1 ji j π π 2 ij ij i,j=1 i j i,j=1
5 where (p /π )q−1 − (p /π )q−1 φ (t) = p w − p w , Φ(q)(t) = i i j j . (16) ij i ij j ji ij q − 1 Markovian dynamics with detailed balance. Master equation with detailed bal- ance is a special class of stochastic systems which represents closed systems approaching to
equilibrium. For master equations with detailed balance, πiwij = πjwji is equivalent to the Kolmogorov cycle condition: w w ··· w w i0i1 i1i2 in−1in ini0 = 1, (17) wi1i0 wi2i1 ··· winin−1 wi0in
26 for any cycle path i0, i1, ··· , in−1, in, i0 . Then we have q−1 q−1 (q) 2 (piwij) − (pjwji) Φij (t) = q−1 q−1 . (18) (πjwji) + (πiwij) q − 1
(q) In this case, the φij and Φij can be identified as Onsager’s thermodynamic flux and force, respectively. Therefore, we shall call their product the free energy dissipation rate
(q) N dF (q) 1 X (q) − = f = φ Φ ≥ 0. (19) dt d 2 ij ij i,j=1
For standard extensive thermodynamics, the Onsager’s thermodynamic force Φij is “lo-
cally determined”. It depends only on pi, pj, wij, wji and the local energy difference (q) i − j = ln(wij/wji). However, Eq. (18) shows that the Φij here depends also on sta- tionary πi and πj, not just their ratio which can be determined locally (Or equivalently, it (q) (q) depends on the absolute value of i and j , not just their difference). This is a consequence of the particular transformation invariance for zero energy of Tsallis’ statistics. Entropy production rate. With or without detailed balance, we have
(q) N N q−1 q−1 ! dS (t) 1 X 1 X pi − pj = − pq−1 (p w − p w ) = φ dt q − 1 i j ji i ij 2 ij q − 1 i,j=1 i,j=1 N (q) q X (q) (q) = f − φ pq−1 − pq−1 , (20) d 2 ij i i j j i,j=1
(q) where statistical internal energy i is defined in Eq. (6) from the stationary distribution 27 πi. The last term in Eq. (20) can be identified as excess heat exchange rate :
N q X (q) (q) Q(q) = − φ pq−1 − pq−1 . (21) ex 2 ij i i j j i,j=1
6 (q) The Qex = 0 in the steady state. However, for equilibrium steady state with detailed balance, each term in the summand is zero. In contrast, for nonequilibrium steady state, the summand is not zero. But the sum is zero due to Tellegen theorem: The “heat, or kinetic energy” continuously circulates in the system in nonequilibrium steady state. Similar as in the B-G thermodynamics1,2,27, the time derivative of Tsallis entropy can also be separated into two distinct parts
(q) N q−1 q−1 ! dS (t) 1 X pi − pj (q) = φ ≡ e(q) − h , dt 2 ij q − 1 p d i,j=1 where the heat dissipation rate
N (q) 1 X h = φ pq−1 wq−1 − 1 − pq−1 wq−1 − 1 , (22) d 2(q − 1) ij i ij j ji i,j=1 and the entropy production rate
1 X e(q) = p w (p w )q−1 − (p w )q−1 (23) p q − 1 i ij i ij j ji i,j 1 X = φ (p w )q−1 − (p w )q−1 ≥ 0. 2(q − 1) ij i ij j ji i,j
(q) q−1 q−1 The ep is greater than zero since if piwij ≥ pjwji, then ((piwij) − (pjwji) ) /(q−1) ≥ 0. Correct form of house-keeping heat. In B-G thermodynamics, the difference of (q) (q) entropy production rate and free energy dissipation rate ep − fd is also known as the house-keeping heat, which is always greater than zero. However, in Tsallis thermodynamics, the sign of
(q) (q) 1 X pi pj Qg = e(q) − f = p w (p w )q−1 − (p w )q−1 − ( )q−1 + ( )q−1, (24) hk p d q − 1 i ij i ij j ji π π i,j i j
is undefined. Consider a three-state Markov jump process and choose the transition rate matrix as −1 1/2 1/2 W = 1/2 −1 1/2 , 1/2 1/2 −1
whose equilibrium distribution is π = (1/3, 1/3, 1/3). Given the initial distribution p(0) = g(q) (1, 0, 0) and q = 2, it is direct to see Qhk (t = 0) = −2.5 < 0.
7 g(q) Therefore, it is natural to ask whether a part of Qhk can play the same role of house- keeping heat while still being non-negative? Before introducing the correct formula, we (q) 1 P q−1 show that, a direct generalization of house-keeping heat Qhk = q−1 i,j piwij (πiwij) − q−1 (πjwji) from B-G to Tsallis thermodynamics fails to do this. In fact, even though (q) P limq→1 Qhk = i,j piwij[ln(πiwij) − ln(πjwji)] recovers the house-keeping heat in B-G ther- modynamics, it is straightforward to see that by choosing q = 2, the transition rate matrix −3 1 2 W = 2 −3 1 , 2 1 −3
(q) and the initial distribution p(0) = (1, 0, 0), we have Qhk (t = 0) < 0. (q) To be correct, the difference between entropy production rate ep and free energy dissipa- (q) (q) tion rate fd is decomposed into two parts: the house-keeping heat Qhk and non-extensive (q) heat Qne , i.e.
(q) 1 X 1 X pi Q = p w (π w )q−1 − (π w )q−1 − (π w )q − (π w )q, (25) hk q − 1 i ij i ij j ji q π i ij j ji i,j i,j i 1 X pi pj Q(q) = p w (p w )q−1 − (p w )q−1 − ( )q−1 + ( )q−1 ne q − 1 i ij i ij j ji π π i,j i j q−1 q−1 q − 1 1 q q − (πiwij) − (πjwji) + (πiwij) − (πjwji) , (26) q πiwij
(q) (q) where Qhk represents the distance from detailed balance condition, and Qne arises due to the non-extensibility of Tsallis thermodynamics, which vanishes in the limit of q → 1. Based (q) on the polynomial inequality, the sign of Qhk is readily determined as follows.
N Proposition 2.1. Let {πi > 0}i=1 be the steady state of master equations in (12), then the house-keeping heat
(q) 1 X 1 X pi Q = p w (π w )q−1 − (π w )q−1 − (π w )q − (π w )q ≥ 0, hk q − 1 i ij i ij j ji q π i ij j ji i,j i,j i is non-negative definite, which becomes zero if and only if at the equilibrium state.
Proof. The house-keeping heat for master equations can be reformulated as (q) 1 X πjwji πjwji Q = p w (π w )q−1 1 − q( )q−1 + (q − 1)( )q ≥ 0, hk q(q − 1) i ij i ij π w π w i,j i ij i ij
8 q−1 q where in the last step we have used the inequality 1−qz +(q−1)z ≥ 0 for z = πj wji > 0 as q(q−1) πiwij proved in (14). The Q(q) = 0 if and only if z = πj wji = 1 for each pair of (i, j), that is, the hk πiwij system is at equilibrium.
Laws of Tsallis thermodynamics. With above thermodynamic quantities in hand, laws of Tsallis thermodynamics for master equations are reformulated as dS(q) + h(q) = e(q) ≥ 0, (27a) dt d p dS(q) − Q(q) = f (q) ≥ 0, (27b) dt ex d (q) (q) (q) (q) Qex + hd − Qne = Qhk ≥ 0. (27c)
(q) The first relation shows that the entropy production rate ep is always non-negative, as a manifestation of the second law of thermodynamics, while the other two relations go beyond the classical ones. Connections with B-G thermodynamics. By recalling the correspondence between q-logarithm (xq−1 − 1)/(q − 1) and natural logarithm ln(x), as q → 1, we have the entropy, internal energy and free energy in Tsallis thermodynamics become the corresponding items in B-G thermodynamics respectively1,2, i.e.
N (q) X lim S = − pi ln pi, (28a) q→1 i=1 N (q) X lim U = − pi ln πi, (28b) q→1 i=1 N (q) X lim F = pi ln(pi/πi). (28c) q→1 i=1 Similar conclusions also hold for the entropy production rate, free energy dissipation rate and house-keeping heat:
N (q) 1 X piwij lim ep = (piwij − pjwji) ln ≥ 0, (29) q→1 2 p w i,j=1 j ji N (q) 1 X piπj lim f = (piwij − pjwji) ln ≥ 0, (30) q→1 d 2 p π i,j=1 j i N (q) 1 X πiwij lim Q = (piwij − pjwji) ln ≥ 0, (31) q→1 hk 2 π w i,j=1 j ji
9 where the right-hand side of Eqs. (29)-(31) are identified as the corresponding entropy production rate, free energy dissipation rate and house-keeping heat in B-G thermodynamics of master equations1,2. Furthermore, limits of heat dissipation rate and change rate of excess heat
N (q) 1 X wij lim h = (piwij − pjwji) ln , (32) q→1 d 2 w i,j=1 ji N (q) 1 X πi lim Qex = (piwij − pjwji) ln , (33) q→1 2 π i,j=1 j agree exactly with the definitions in B-G thermodynamics. It is noticeable that, the non-extensive heat reduces to zero as q approaches to 1, that (q) is, limq→1 Qne = 0. Thus the house-keeping heat equals to the difference between entropy (q) production and free energy dissipation rates in B-G thermodynamics. This implies that Qne can serve as a characteristic quantity to describe the non-extensivity of the system.
3. TSALLIS THERMODYNAMICS OF FOKKER-PLANCK EQUATIONS
In this Section, we turn to the Tsallis thermodynamics of Fokker-Planck (F-P) equations, which read ∂ ∂ ∂p p(x, t) = − K(x, t),K(x, t) = u(x)p − D(x) , (34) ∂t ∂x ∂x
where u(x) and D(x) > 0 denote the drift and diffusion coefficients respectively. In par- ticular, the steady-state probability distribution is given by a time-independent solution of ∂ ∂t p(x, t) = 0, that is,
∂ ∂π Ks(x) = 0,Ks(x) = u(x)π − D(x) , (35) ∂x ∂x
where π(x) ≥ 0 for x ∈ R. Compared to master equations, the information of transition rate matrix is largely compressed to the draft and diffusion coefficients, which as a consequence will simplify the modeling procedure and numerical computations a lot. Tsallis thermodynamics of F-P equations. By extending the state variables into continuous cases, we can directly borrow the definitions of the generalized entropy, internal
10 energy and free energy from master equations and apply to F-P equations20,
Z pq−1 − 1 S(q)(t) = − dxp , (36a) q(q − 1) Z π1−q − 1 U (q)(t) = dxpq , (36b) q(q − 1) 1 Z p q−1 F (q)(t) = dx p − p , (36c) q(q − 1) π
(q) (q) where (q 6= 0, 1). The free energy dissipation rate fd = −dF /dt associated with F-P equations is given by
1 Z ∂ p 1 Z p ∂K p ∂Ks f (q) = − dx K ( )q−1 = dx q( )q−1 − (q − 1)( )q d q − 1 ∂x π q(q − 1) π ∂x π ∂x Z 1 ∂ p 1 ∂ p Z π2 p ∂ p = − dx K ( )q−1 − Ks ( )q = dx D ( )q−1( ( ))2 (37) q − 1 ∂x π q ∂x π p π ∂x π Z p p K Ks = dx ( )q−1( − )2 ≥ 0, D π p π
where in the second line we have used the integration by parts, and in the last line we have K Ks ∂ p used the relation ( p − π ) = −D ∂x ln( π ). As a result, the free energy dissipation rate 20 (q) is always non-negative , and vanishes if and only if in the steady state. Notice that fd p q−1 possesses a similar form as in classical B-G thermodynamics except for the factor ( π ) . Now we proceed to derive the balance equation of Tsallis entropy.
dS(q)(t) Z Z p K u = − dxKpq−2∂ p = dx pq−1( )2 − pq−1K dt x D p D Z p K Z p K u = dx (Dp)q−1( )2 − dx pq−1(Dq−1 − 1)( )2 + pq−1K D p D p D
(q) (q) ≡ ep − hd ,
where the first contribution
Z p K e(q) = dx (Dp)q−1( )2 ≥ 0, (38) p D p
denotes the entropy production rate, which is always non-negative as a manifestation of the second law of thermodynamics; and the second contribution denotes the heat dissipation rate, or the entropy flux from environment:
Z p K u h(q) = dx pq−1(Dq−1 − 1)( )2 + pq−1K . (39) d D p D
11 House-keeping and non-extensive heats. Similarly, the house-keeping heat together with non-extensive heat is identified as the difference between entropy production and free energy dissipation rate, that is,
Z p K p K Ks e(q) − f (q) = dx (Dp)q−1( )2 − ( )q−1( − )2 ≡ Q(q) + Q(q), (40) p d D p π p π hk ne Z p Ks Q(q) = dx (Dπ)q−1( )2 ≥ 0, (41) hk D π Z p K Ks p K Ks Q(q) = dx (Dp)q−1( )2 − (Dπ)q−1( )2 − ( )q−1( − )2 . (42) ne D p π π p π
(q) (q) Here Qhk represents the distance from detailed balance condition, and Qne arises due to the non-extensibility of Tsallis thermodynamics, which vanishes in the limit of q → 1,
Z s s (q) p K 2 K 2 K K 2 lim Qne = dx ( ) − ( ) − ( − ) = 0. q→1 D p π p π
During above calculations, we use an important relation for F-P equations which is stated through the following lemma.
Lemma 3.1. For q, n ∈ R, such that, q 6= 0, 1 and n ≥ 1, the following integration
Z p p q−1 K Ks (Ks)n dx − = 0, (43) D π p π π as long as the probability distribution vanished at the boundary.
K Ks ∂ p Proof. By using p − π = −D ∂x ln( π ), we have
Z p p K Ks (Ks)n Z ∂ p p dx ( )q−1( − ) = − dx ( )( )q−1(Ks)n D π p π π ∂x π π 1 Z ∂ p 1 Z p ∂ = − dx ( )q(Ks)n = dx( )q(Ks)n−1 Ks = 0, q ∂x π q π ∂x
∂ s where in the last step we have utilized the divergence-free relation as ∂x K = 0 and assumed the probability distribution p(x, t) vanished at the boundary.
Laws of Tsallis thermodynamics. If we further introduce the change rate of excess heat as dU (q) Z ∂ p ∂p Q(q) = = dxpq−2K π2−q ( ) − . (44) ex dt ∂x π ∂x
12 Then, similar as master equations, laws of Tsallis thermodynamics for F-P equations are readily derived as dS(q) + h(q) = e(q) ≥ 0, (45a) dt d p dS(q) − Q(q) = f (q) ≥ 0, (45b) dt ex d (q) (q) (q) (q) Qex + hd − Qne = Qhk ≥ 0. (45c) which are also known as three different faces of the second law of thermodynamics according 28 (q) (q) to Ref. . Again, it should be noted that the non-negativity of ep − fd is not guaranteed for F-P equations in the framework of Tsallis thermodynamics. Connections with B-G thermodynamics. As q → 1, the entropy production rate, free energy dissipation rate and house-keeping heat of F-P equations become Z (q) p K 2 lim ep = dx ( ) ≥ 0, (46) q→1 D p Z p K Ks lim f (q) = dx ( − )2 ≥ 0, (47) q→1 d D p π Z p K K Ks Z p Ks lim Q(q) = dx ( )2 − ( − )2 = dx ( )2 ≥ 0, (48) q→1 hk D p p π D π
by using Lemma 3.1. The right-hand side of Eqs. (46)-(48) are identified as the corre- sponding entropy production rate, free energy dissipation rate and house-keeping heat of F-P equations in B-G thermodynamics1,4. Furthermore, the limit of heat dissipation rate and change rate of excess heat become, respectively, Z u lim h(q) = dx K , (49) q→1 d D Z (q) lim Qex = − dx(K∂xln π), (50) q→1
which agree with the classical definitions in B-G thermodynamics as expected.
4. EMERGENCY OF A UNIFIED FORMULATION IN THE MATHEMATICAL LIMIT
In Ref.29, a unified B-G thermodynamic formalism has been established for both master equations and F-P equations. Particularly, when a Markov process is restrained to one-step
13 jump, the B-G thermodynamics for the master equations and F-P equations has a one-to- one correspondence. However, in Tsallis thermodynamics, due to the non-extensivity and nonlinearity of Tsallis entropy and free energy, above conclusions may not hold any more. In this part, we are going to show that, within the framework of Tsallis thermodynamics (q) discussed in the last two sections, the free energy dissipation rate fd , entropy production (q) (q) rate ep , and house keeping heat Qhk of master equations with a tridiagonal transition rate matrix can be derived exactly from that of F-P equations; and vice versa. Thus a one-to-one correspondence between the Tsallis thermodynamics of master equations and F-P equations can be rigorously established in mathematics.
A. From F-P equations to master equations by direct discretization
First, we start with F-P equations with continuous time and state variables, and derive discrete master equations as its approximation. For notational simplicity, F-P equations in (34) is rewritten into a convenient form
∂ ∂ ∂ p(x, t) = − K(x, t),K(x, t) =u ¯(x)p(x, t) − [D(x)p(x, t)] , (51) ∂t ∂x ∂x
whereu ¯(x) ≡ u(x) + ∂D(x)/∂x. Introduce uniform grids along the x-axis,
x = xi = i∆x = i, i = 0, ±1, ±2, ··· . (52)
∂ By using the forward difference to the drift term − ∂x [¯u(x)p(x, t)], and central difference to ∂2 the diffusion term ∂x2 [D(x)p(x, t)], we discretize the right-hand side of (51) at coordinate
(xi = i, t):
dp (t) 1 1 i = − [¯u p (t) − u¯ p (t)] + [D p (t) − 2D p (t) + D p (t)] , dt i+1 i+1 i i 2 i+1 i+1 i i i−1 i−1 whereu ¯i ≡ u¯(xi),Di ≡ D(xi), and pi(t) denotes the probability distribution at (xi, t). The above equation can be reformulated into the standard form as
dp (t) i = [p (t)w − p (t)w ] + [p (t)w − p (t)w ], i = 0, ±1, ±2, ··· , (53) dt i−1 i−1,i i i,i−1 i+1 i+1,i i i,i+1 by defining the forward and backward transition rates as
1 1 1 w = D , w = D − u¯ , i−1,i 2 i−1 i,i−1 2 i i
14 respectively. Thus we deduce master equations with a tridiagonal transition rate matrix from F-P equations. Now we are at a position to derive the free energy dissipation rate of master equations from F-P equations. Based on results for Tsallis thermodynamics of F-P equations in Sec. 3, the following theorems can be established. In what follows, for notational simplicity, we (q) M F will drop the superscript (q) from fd , and use fd and fd to distinguish the free energy dissipation rates of master equations and F-P equations respectively. Similar notations also apply to other quantities without further mention.
Theorem 4.1. In limit of infinitesimal discretization step → 0, the density of free energy dissipation rate (37) for F-P equations