On a class of Darboux-integrable semidiscrete equations

Tam metin

(1)Zheltukhin et al. Advances in Difference Equations (2017) 2017:182 DOI 10.1186/s13662-017-1241-z. RESEARCH. Open Access. On a class of Darboux-integrable semidiscrete equations Kostyantyn Zheltukhin1* , Natalya Zheltukhina2 and Ergun Bilen1 *. Correspondence: [email protected] Department of Mathematics, Middle East Technical University, Universiteler Mahallesi, Dumlupinar Bulvari 1, Ankara, 06800, Turkey Full list of author information is available at the end of the article 1. Abstract We consider a classification problem for Darboux-integrable hyperbolic semidiscrete equations. In particular, we obtain a complete description for a special class of equations admitting four-dimensional characteristic x-rings and two-dimensional characteristic n-rings. For all described equations, the corresponding x- and n-integrals are constructed. Keywords: semidiscrete equations; Darboux integrability; characteristic rings. 1 Introduction Classification problems play an important role in the study of integrable equations. For classification of hyperbolic equations, it is convenient to define integrability in terms of characteristic rings. The notion of a characteristic ring was introduced by Shabat for integrable hyperbolic equations of exponential type (see [, ]) and then used by Zhiber to study general integrable hyperbolic equations (see [–]). Later, Habibullin extended this notion to the case of semidiscrete and discrete equations (see [–]). For more details on characteristic rings, see survey paper []. We consider semidiscrete hyperbolic equations that admit nontrivial x- and n-integrals, so-called Darboux-integrable equations []. It was proved in [] that a semidiscrete hyperbolic equation is Darboux integrable if and only if its characteristic x- and n-rings are finite-dimensional. Description of all equations with characteristic x- and n-rings of finite dimensions is a very difficult classification problem. The majority of known Darbouxintegrable semidiscrete equations possess x- and n-rings of dimensions not exceeding five (see [, , ]). Necessary and sufficient conditions for a characteristic x-ring to be fourdimensional were obtained in [] (also see [] for a characterization of five-dimensional characteristic x-rings). In [] the conditions for a two-dimensional characteristic n-ring were obtained. We use these conditions to explicitly derive integrable equations with fourdimensional characteristic x-rings and two-dimensional characteristic n-rings. Consider the equation tx = f (x, t, t , tx ),. (). where the function t(n, x) depends on the discrete variable n and continuous variable x. ∂ ∂k t and t = t(n + , x). It is also convenient to denote t[k] = ∂x We use the notations tx = ∂x k t, © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made..

(2) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 2 of 14. k ∈ N, and tm = t(n + m, x), m ∈ Z. It was proved in [] that if equation () has a fourdimensional characteristic x-ring, then the function f has the form f = A(x, t, t )M(x, t, tx ) + B(x, t, t )tx + C(x, t, t ).. (). In this work, we assume that the function M depends only on tx and f does not depend on x, that is, we study equations of the form tx = A(t, t )M(tx ) + B(t, t )tx + C(t, t ).. (). It turns out that we have to consider two cases of f linear and nonlinear in tx . The results of our investigation are given in the following theorems. Theorem  Let f be a linear function of tx . Equation () has a four-dimensional characteristic x-ring and a two-dimensional characteristic n-ring if and only if. f=. γ (t) γ (t) tx – σ (t) + σ (t ), γ (t ) γ (t ). where the functions γ and σ satisfy either of the relations . γ (t)σ (t) = γ (t) B + B γ (t)σ (t). .  γ (t)σ (t) = γ (t) B + B γ (t)σ (t). or. with arbitrary constants B and B . Theorem  Let f be a nonlinear function of tx . Equation () has a four-dimensional characteristic x-ring and a two-dimensional characteristic n-ring if and only if. f=. c η(t)η(t ) tx. or f =. c ec (t+t ) – P, tx + P. where c , c , and P are arbitrary constants, and η is an arbitrary function of one variable, or f=. √.  B –  tx + Ptx + Q + Btx + P(B – ), . where B, P, and Q are arbitrary constants. The paper is organized as follows. First, we give proofs of Theorems  and , and in the last section, we provide x- and n- integrals for equations found in Theorems  and ..

(3) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 3 of 14. 2 Proofs of Theorem 1 and Theorem 2 2.1 Preliminary results In what follows, all calculations are done on the set of solutions of equation (), that is, we consider . . . , t– , t , t , . . . and tx , txx , txxx , . . . as independent dynamical variables. The derivatives of . . . , t– , t , t , . . . and shifts of tx , txx , txxx , . . . are expressed in terms of the dynamical variables using (). Let us formulate necessary and sufficient conditions so that equation () has a characteristic x-ring of dimension four and a characteristic n-ring of dimension two. First, we consider the n-ring. The following theorem was proved in []. Theorem  Equation () has a characteristic n-ring of dimension two if and only if D. ft ftx. = –ft ,. (). where D is the shift operator: Dg(n, x) = g(n + , x). We remark that equality () implies that ∂ ft = ∂t ftx. (). since ft does not depend on t . We use this observation later. For the characteristic x-ring, we have to consider two cases: ftx tx = , that is, f is a linear function of tx , and ftx tx = , that is, f is a nonlinear function of tx . The following theorems were proved in []. Theorem  Equation () with ftx tx =  has a characteristic ring Lx of dimension four if and only if . ft K(m) –m+ D m ftx. =. K(m) + m – ft , m. (). where K is the vector field K= and m =. ∂ ∂ ∂ + tx + f + ··· , ∂x ∂t ∂t –(fxtx +tx ftx t +fftx t )+ft +ftx ft ftx. .. Theorem  Equation () with ftx tx =  has a characteristic ring Lx of dimension four if and only if D. ftx tx tx ftx tx. =. ftx tx tx ftx – ftx tx ftx tx ftx. (). and ˜ = mf ˜ tx – (fx + tx ft + ft f ), Dm ˜ = where m. fxtx +tx ftx t +fftx t –ft –ftx ft ftx tx. .. ().

(4) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 4 of 14. In the same way as in equation (), we have ∂ m =  and ∂t. ∂ ˜ = . m ∂t. (). For convenience of the reader, let us give definitions of x- and n-integrals and of Darboux-integrable semidiscrete equations. Definition  A function F(x, t, t , . . . , tk ) is called an x-integral of equation () if Dx F(x, t, t , . . . , tk ) =  for all solutions of (). Here Dx is the operator of total differentiation with respect to x: Dx g(n, x) = (d/dx)g(n, x). A function G(x, t, tx , . . . , t[m] ) is called an n-integral of equation () if DG(x, t, tx , . . . , t[m] ) = G(x, t, tx , . . . , t[m] ) for all solutions of (). Equation () is called Darboux integrable if it admits a nontrivial x-integral and a nontrivial n-integral.. 2.2 Proof of Theorem 1 We assume that f is a linear function of tx . Thus f (t, t , tx ) = A(t, t )tx + B(t, t ),. (). and equation () becomes tx = A(t, t )tx + B(t, t ).. (). The proof of the Theorem  is based on the following lemmas. Lemma  Let ftx tx = . Then the characteristic n-ring of equation () has dimension two if and only if f (t, t , tx ) =. c γ (t)σ (t) c γ (t) c tx + – + σ (t ), γ (t ) γ (t ) γ (t ). (). where γ and σ are functions of one variable, and c , c are constants. Proof It follows from condition () that ft At Bt = tx + ftx A A. (). does not depend on t . Hence AAt and BAt do not depend on t . So we can write A(t, t ) = γ (t)ϕ(t ) and B(t, t ) = l(t)ϕ(t ) + σ (t ) for some functions γ , ϕ, and σ . The function f takes.

(5) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 5 of 14. form f (t, t , tx ) = γ (t)ϕ(t )tx + l(t)ϕ(t ) + σ (t ).. (). Applying condition () to f given by (), we get l (t ) γ (t ) γ (t)ϕ(t )tx + l(t)ϕ(t ) + σ (t ) + γ (t ) γ (t ) = –γ (t)ϕ (t )tx – l(t)ϕ (t ) – σ (t ).. (). By comparing the coefficients of tx in () we get γ (t ) ϕ (t ) + = , γ (t ) ϕ(t ) so that ϕ(t ) = γ c(t ) , where c is some constant. Substituting this ϕ into equation () and collecting the terms independent of tx , we get γ (t )σ (t ) + γ (t )σ (t ) + l (t ) = .. (). Solving (), we find l(t) = –γ (t)σ (t) + c˜  ,. (). where c is some constant. Substituting ϕ and l found into equation (), we get equation (). We can check that condition () is satisfied for function (). Now we can rewrite equation () as tx =. c γ (t)σ (t) c γ (t) c tx + – + σ (t ), γ (t ) γ (t ) γ (t ). (). where γ and σ are functions of one variable, and c , c are constants. The equation can be simplified by introducing the new variable τ = L(t),. (). where L satisfies L (t) = γ (t). Equation () becomes τx = c τx + c – c Q(τ ) + Q(τ ). (). for some function Q of one variable. We can check that condition () is satisfied for the new equation. Hence our change of variable does not affect the dimension of the characteristic n-ring. In the next lemma, we give conditions for equation () to have a four-dimensional characteristic x-ring..

(6) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 6 of 14. Lemma  Equation () has a four-dimensional characteristic x-ring if and only if Q(τ ) = A τ  + A τ. or. Q(τ ) = A eατ + A e–ατ. (). for some constants A , A , and α. Proof Applying condition () to function f = c τx + c – c Q(τ ) + Q(τ ), we get c τx + c – c Q(τ ) + Q(τ ) c Q (τ ) – Q (τ ) Q (τ ) – Q (τ ) + =. Q (τ )(c – c Q(τ ) + Q(τ )) – Q (τ ) + Q (τ ) Q (τ ) – Q (τ ). c Q (τ ) – Q (τ ) Q (τ )(c – c Q(τ ) + Q(τ )) τ + Q (τ ) – Q (τ ). + x Q (τ ) – Q (τ ) Q (τ ) – Q (τ ). By comparing the coefficients of τx in this equality, we get c D. c Q (τ ) – Q (τ ) Q (τ ) – Q (τ ). =. c Q (τ ) – Q (τ ) , Q (τ ) – Q (τ ) . . )–Q (τ ) which implies that either c =  and cQQ (τ(τ )–Q (τ ) is constant or c Q (τ ) – Q (τ ) = . In the second case, we also get c = . Thus, equation () has the form. τx = τx + d(τ , τ ).. (). Equations of this form were completely classified in [] (together with their x– and ncharacteristic rings). It follows from [] that Q must have the form given in the statement of the lemma. Returning to the original variable t in equation () with Q given by equation (), we get Theorem .. 2.3 Proof of Theorem 2 We assume that f is a nonlinear function of tx . Thus f (t, t , tx ) = A(t, t )M(tx ) + B(t, t )tx + C(t, t ),. (). and equation () becomes tx = A(t, t )M(tx ) + B(t, t )tx + C(t, t ).. (). The proof of the Theorem  is based on the following lemmas. Lemma  Let equation () have a characteristic n-ring of dimension two, and let M be a nonlinear function. Then the function M satisfies M = –. α  M + α  tx + α  , α  M + α  tx + α . where α M + α tx + α = .. ().

(7) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 7 of 14. Proof If the dimension of the characteristic n-ring is two, then ( fftt )t = . Hence, for f x given by equation (), we have . At M + tx Bt + Ct AM + B =. t. (Att M + tx Btt + Ctt )(AM + B) – (At M + Bt )(At M + tx Bt + Ct ) = . (AM + B). This can be rewritten as M (α M + α tx + α ) = –(α M + α tx + α ). (). for some constants αi , i = , , . . . , . Note that if α M + α tx + α = , then either M =  or α M + α tx + α = . In both cases, we get that f is a linear function of tx . Hence we can assume that α M + α tx + α = , and we can write equality (). The above lemma allows us to express the derivative M in terms of M. We can also express the shift DM in terms of M. Indeed, as it was proved in [] (see Lemma ), if equation () has a four-dimensional characteristic x-ring and ftx tx = , then Df = –H (t, t , t )tx + H (t, t , t )f + H (t, t , t ). (). for some functions H , H , and H . Therefore, D(AM + Btx + C) = –H tx + H (AM + Btx + C ),. (). DM = Q (t, t , t )M + Q (t, t , t )tx + Q (t, t , t ). (). and. for some functions Q , Q and Q . We use expressions () and () for the derivative and shift of M in the next lemma. Lemma  Let equation () have a characteristic n-ring of dimension two. Then M has either of the forms M = tx+P , or M = tx + Ptx + Q, or M = tx . Proof Consider the vector field X = ∂t∂x . We can easily check that DX = ft XD. Thus x DX(M) = ft X(DM). Using equation () for X(M) and equation () for DM, we get x. . α  M + α  tx + α  –D α  M + α  tx + α . =.  X(Q M + Q tx + Q ). AM + B. Using equation () and equation () once more, we get –. α˜ (Q M + Q tx + Q ) + α˜ (AM + Btx + C) + α˜ α˜ (Q M + Q tx + Q ) + α˜ (AM + Btx + C) + α˜ =. M+α tx +α Q – Q αα M+α  tx +α . M+α tx +α B – A αα M+α  tx +α. ,.

(8) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 8 of 14. where Dαk = α˜k . Hence we can write R M – (R tx + R )M + R tx + R tx + R = . (). for some functions Rk , k = , , . . . , . Then, we find that. M=. (R tx + R ) ±. (R tx + R ) – R (R tx + R tx + R ) R. if R = . or M=. R tx + R tx + R R  tx + R . if R = .. Since the function f = AM + Btx + C has a linear term Btx and a free term C, we can assume that M has the form given in the statement of the lemma. Now we consider each value of M obtained in the lemma, separately. We start with the simple case M = tx . Lemma  Equation () cannot have a four-dimensional characteristic x-ring if M = tx . Proof We can easily check that, for any f = A(t, t )tx + B(t, t )tx + C(t, t ), condition () is not satisfied. Hence equation () cannot have a four-dimensional char acteristic x-ring if M = tx . Let us consider the case M =.  . tx +P. Lemma  Let M = tx +P , and let equation () have a four-dimensional characteristic xring and a two-dimensional characteristic n-ring. Then equation () takes either of the forms. tx =. c∗ η(t)η(t ) tx. ∗∗. or. tx =. c∗ ec (t+t ) – P. tx + P. (). Proof We have. f (t, t , tx ) =. A(t, t ) + B(t, t )tx + C(t, t ). tx + P. Applying condition () to f , we get. B(tx. + P). (tx + P)(B(tx + P) + A) (tx + P) = . + (C + P – BP)(tx + P) + A (B(tx + P) – A). ().

(9) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 9 of 14. From this equality we get B(C + P – BP)(tx + P) + AB(tx + P) + A(C + P – BP)(tx + P) = . In this equality the coefficients of (tx + P)k , k = , , , must be zero. So we find B(C + P – BP) = ,. AB = ,. and A(C + P – BP) = .. Since A(t, t ) =  (otherwise ftx tx = ), we find B =  and C = –P. Thus we have f (t, t , tx ) =. A(t, t ) – P. tx + P. (). Using condition (), we get At (t , t )A(t, t ) At (t, t ) = A(t , t )(tx + P) tx + P or At (t , t ) At (t, t ) = . A(t , t ) A(t, t ). (). At (t ,t ) does not depend A(t ,t ) ∂ so ∂t∂t ln A(t, t ) = .. It follows that. on t , so. ∂ ∂t ∂t. ln A(t , t ) = , and. At (t,t ) A(t,t ). does not. depend on t, Hence we get A(t, t ) = ϕ(t)η(t ) for some functions ϕ and η. Using equation (), we obtain ϕϕ(t(t)) = ηη(t(t)) , which implies that ϕ(t ) = c∗ η(t ), where c∗ is some constant. Hence we have f=. c∗ η(t)η(t ) – P. tx + P. (). From condition () it follows that (tx + P) η (t) Pη (t) Pη (t ) ˜ = – + (tx + P) – m η(t) η(t ) η(t). () ∗∗. does not depend on t . So, either P =  or η (t ) = c∗∗ η(t ), which implies η(t ) = ec t with some constant c∗∗ . Thus we obtain equations (). We can easily check that these equations have a two-dimensional characteristic n-ring and a four-dimensional characteristic x-ring. Let us consider the case M =. . tx + Ptx + Q.. Lemma  Let M = tx + Ptx + Q, and let equation () have a four-dimensional characteristic x-ring and a two-dimensional characteristic n-ring. Then equation () takes the form tx =. √.  B –  tx + Ptx + Q + Btx + P(B – ), . where B, Q, P are constants, and B = .. ().

(10) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 10 of 14. Proof We have f = A(t, t ) tx + Ptx + Q + B(t, t )tx + C(t, t ).. (). Applying condition (), we find BP + tx + A tx + tx P + Q – (AP + Atx + B tx + tx P + Q). –P – C – Btx – A tx + Ptx + Q , = Q + (C + Btx + A tx + Ptx + Q)(C + P + Btx + A tx + Ptx + Q). or .  Q + C + Btx + A tx + Ptx + Q C + P + Btx + A tx + Ptx + Q . · BP + tx + A tx + tx P + Q .  . = AP + Atx + B tx + tx P + Q P + C + Btx + A tx + Ptx + Q . Comparing the coefficients of ( tx + Ptx + Q)i (tx )j for i, j = , , , we get AB(C + P – BP) = , A –C  – CP + A P – Q +  B –  Q = ,  A + B –C + (B – )P = , –A P(C + P) + B Q – B C  + CP + Q + A –P + Q = , A (C + P) P – Q + B (C + P)Q – BP C  + CP + Q – A Q = . We can check that these equalities are satisfied if and only if and C  + CP = A P – Q – Q + B Q.. C = PB – P Simplifying, we get C = PB – P,. and either. B = A + . or P = Q.. In the case P = Q, we have that M = tx + Ptx + Q is a linear function of tx . Therefore we have to study only the case B = A + . Thus we have f=. . P B(t, t ) –  tx + Ptx + Q + B(t, t )tx + B(t, t ) –  , . (). where B = . In the same way, we check that condition () in the form (D fftt ) – (ft ) =  is x satisfied for this function f if and only if Bt (t, t )  B (t, t ) – . =. Bt (t , t ) .  B (t , t ) – . ().

(11) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 11 of 14. Hence we can write Bt (t, t ) = A(t, t )ϕ(t ), Bt (t, t ) = ±A(t, t )ϕ(t). (). for some function ϕ. Using condition (), let us show that B can only be a constant function. We have  ˜ = μ tx tx + ptx + q + μ tx + ptx + q  + μ tx + ptx + q , m. (). where PB(Bt + Bt ) , (P – Q)(B – ) √ P B – (Bt + Bt ) μ = , (P – Q)(B – ). (). μ =. μ =. (). (Q – P + P B)Bt + QBBt . (P – Q)(B – ). (). ˜ does not depend on t , we have that μ , μ , and μ also do not depend on t . Using Since m (), we have μ =. P(ϕ(t ) ± ϕ(t)) . P – Q. (). Since μ does not depend on t , either ϕ is a constant function or P = . Note that in both cases, we get μ =  and μ = . We start with the case where φ is some constant C. Using equation (), we have μ =. C(Q – P ) + C(P ± Q)B . √ (P – Q) B – . (). Differentiating this equality with respect to t , we get =. CBt ((P ± Q) + (Q – P )B) . (P – Q)(B – ) . ,. (). which gives Bt =  or ((P ± Q) + (Q – P )B) = . Both equalities imply that B is a constant. Now we consider the case P = . Then, using equation (), we have μ =. ϕ(t ) ± Bϕ(t) Q(Bt + BBt ) =– √ . –Q(B – ) B – . (). Differentiating this equality with respect to t , we get =. Bt (Bϕ(t ) ± ϕ(t)) . (B – ) . ,. (). ϕ(t) which implies that either Bt =  or B = ± ϕ(t . In both cases, we get that B is a constant ) function. Indeed, if Bt = , then Bt =  by equation (), so B is a constant function, and if.

(12) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 12 of 14. φ(t) B = ± φ(t , then μ = ± ϕ  (t) – ϕ  (t ), and since μ does not depend on t , we get that φ ) is a constant function, and hence B is a constant function. Using the equality B = A + , we get the statement of the lemma. The proof of Theorem  easily follows from the above lemmas.. 3 Examples The functions f given in the Theorem  lead to the following examples. Example  The equation tx =. γ (t) γ (t) tx – σ (t) + σ (t ), γ (t ) γ (t ). where functions γ and σ satisfy the relation . γ (t)σ (t) = γ (t) B + B γ (t)σ (t) ,. has an x-integral F = γ (t)tx – σ (t).. (L(t )–L(t ))(L(t )–L(t)) , (L(t )–L(t ))(L(t )–L(t )). B , B ∈ R, where L(t) =. t . γ (τ ) dτ , and an n-integral I =. Example  The equation tx =. γ (t) γ (t) tx – σ (t) + σ (t ), γ (t ) γ (t ). where functions γ and σ satisfy the relation .  γ (t)σ (t) = γ (t) B + B γ (t)σ (t) ,. has an x-integral F = γ (t)tx – σ (t).. (eL(t) –eL(t ) )(eL(t ) –eL(t ) ) , (eL(t) –eL(t ) )(eL(t ) –eL(t ) ). B , B ∈ R, where L(t) =. t . γ (τ ) dτ , and an n-integral I =. The functions f given in the Theorem  lead to the following examples. Example  The equation tx =. c η(t)η(t ) tx. has an x-integral F =. t . η– (τ ) dτ –. t . η– (τ ) dτ and an n-integral I =. tx c η(t). +. Example  The equation tx =. c ec (t+t ) –P tx + P. has an x-integral F = e–c t +c Px – e–c t +c Px and an n-integral I =. tx +P c ec t. +. ec t . tx +P. η(t) . tx.

(13) Zheltukhin et al. Advances in Difference Equations (2017) 2017:182. Page 13 of 14. Example  The equation tx =. √.  B –  tx + Ptx + Q + Btx + P(B – ) . has an x-integral F = –B – B + B –  t + B – B +  t + –B + B –  t + t and an n-integral. √ n tx + Ptx + Q + tx + .P . I = B – B –  In all examples, we can check that F is an x-integral and I is an n-integral by direct calculations.. Competing interests The authors declare that they have no competing interests. Authors’ contributions All authors contributed equally to this work. All authors read and approved the final manuscript. Author details 1 Department of Mathematics, Middle East Technical University, Universiteler Mahallesi, Dumlupinar Bulvari 1, Ankara, 06800, Turkey. 2 Department of Mathematics, Bilkent University, Bilkent, Ankara, 06800, Turkey.. Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Received: 27 January 2017 Accepted: 13 June 2017 References 1. Shabat, AB, Yamilov, RI: Exponential systems of type I and Cartan matrices. Preprint BBAS USSR Ufa (1981) (in Russian) 2. Leznov, AN, Smirnov, VG, Shabat, AB: Internal symmetry group and integrability conditions for two-dimensional dynamical systems. Teor. Mat. Fiz. 51, 10-21 (1982) (in Russian) 3. Sokolov, VV, Zhiber, AV: On the Darboux integrable hyperbolic equations. Phys. Lett. A 208, 303-308 (1995) 4. Zhiber, AV, Sokolov, VV, Ya, SS: On nonlinear Darboux-integrable hyperbolic equations. Dokl. Akad. Nauk, Ross. Akad. Nauk 343, 746-748 (1995) (in Russian) 5. Zhiber, AV, Sokolov, VV: Exactly integrable hyperbolic equations of Liouville type. Russ. Math. Surv. 56, 61-101 (2001) 6. Zhiber, AV, Murtazina, RD: On the characteristic Lie algebras for the equations uxy = f (u, ux ). J. Math. Sci. (N.Y.) 151, 3112-3122 (2008) 7. Kostrigina, OS, Zhiber, AV: Darboux-integrable two-component nonlinear hyperbolic systems of equations. J. Math. Phys. 52, 033503 (2011) 8. Habibullin, IT: Characteristic algebras of fully discrete hyperbolic type equations. SIGMA 1, 023 (2005) 9. Habibullin, IT, Pekcan, A: Characteristic Lie algebra and the classification of semi-discrete models. Theor. Math. Phys. 151, 781-790 (2007) 10. Habibullin, IT: Characteristic algebras of discrete equations. In: Difference Equations, Special Functions and Orthogonal Polynomials, pp. 249-257. World Scientific, Hackensack (2007) 11. Habibullin, IT, Gudkova, EV: Classification of integrable discrete Klein-Gordon models. Phys. Scr. 81, 045003 (2011) 12. Habibullin, IT, Zheltukhina, N, Pekcan, A: On some algebraic properties of semi-discrete hyperbolic type equations. Turk. J. Math. 32, 277-292 (2008) 13. Habibullin, IT, Zheltukhina, N, Pekcan, A: On the classification of Darboux integrable chains. J. Math. Phys. 49, 102702 (2008) 14. Habibullin, IT, Zheltukhina, N, Pekcan, A: Complete list of Darboux integrable chains of the form t1x = tx + d(t, t1 ). J. Math. Phys. 50, 102710 (2009) 15. Habibullin, IT, Zheltukhina, N, Sakieva, A: On Darboux-integrable semi-discrete chains. J. Phys. A 43, 434017 (2010) 16. Habibullin, IT, Zheltukhina, N: Discretization of Liouville type nonautonomous equations. J. Nonlinear Math. Phys. 23, 620-642 (2016) 17. Zhiber, AB, Murtazina, RD, Habibullin, IT, Shabat, AB: Characteristic Lie rings and integrable models in mathematical physics. Ufa Math. J. 4, 17-85 (2012) 18. Darboux, G: Leçons sur la théorie générale des surfaces et les applications géométriques du calculus infinitésimal, vol. 2. Gautier Villas, Paris (1915).

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