# Journal of Nonlinear Mathematical Physics

Volume 27, Issue 1, October 2019, Pages 95 - 105

# Interrelations of discrete Painlevé equations through limiting procedures

Authors
A. Ramani, B. Grammaticos*
IMNC, CNRS, Université Paris-Diderot, Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France
T. Tamizhmani
SAS, Vellore Institute of Technology, Vellore - 632014, Tamil Nadu, India
*Corresponding author
Corresponding Author
B. Grammaticos
Received 4 June 2019, Accepted 8 July 2019, Available Online 25 October 2019.
DOI
10.1080/14029251.2020.1683981How to use a DOI?
Keywords
discrete Painlevé equations; affine Weyl groups; limiting procedures; canonical forms
Abstract

We study the discrete Painlevé equations associated to the E7(1) affine Weyl group which can be obtained by the implementation of a special limits of E8(1)-associated equations. This study is motivated by the existence of two E7(1)-associated discrete both having a double ternary dependence in their coefficients and which have not been related before. We show here that two equations correspond to two different limits of a E8(1)-associated discrete Painlevé equation. Applying the same limiting procedures to other E8(1)-associated equations we obtained several E7(1)-related equations most of which have not been previously derived.

Open Access

## 1. Introduction

Integrable systems are reputed for the numerous interrelations which establish links among them. Miura and/or Bäcklund transformations are omnipresent in the integrability domain. Once a relation of this type is established between two equations, they cease to be two different ones but are, in fact, the same system under two different guises. Thus establishing relations between known integrable systems is of paramount importance. Painlevé equations, be they continuous or discrete, possess the same rich structure of relations.

This paper focuses on a particular class of discrete Painlevé equations [1], multiplicative ones which are associated to the affine Weyl group [2] E7(1). The main bulk of discrete Painlevé equations with E7(1) symmetry are obtained by deautonomisation of a QRT mapping [3] belonging to one of three families characterised by distinct canonical forms [4] of the A1 QRT matrix. The first two and best known families, referred to as VI and VI′ respectively, correspond to the matrices

A1=(10001q2000q2),(1.1)
and
A1=(0010z21/z20100).(1.2)

(The third family, recently discovered [5], will not be the object of the present study.) The canonical form of the mappings of the VI and VI′ families are

(xn+1xnq2)(xnxn1q2)(xn+1xn1)(xnxn11)=R(xn)(1.3)
and
(xnz2xn+1z2xnxn+1)(xnz2xn1z2xnxn1)=R(xn)(1.4)
respectively. The transformation from a form VI to a from VI′ is straightforward. Separating even and odd indices and thus splitting (1.3) into two equations, we replace the variable xm for m even by q2/xm. We obtain thus an equation of the form (1.4), but written as a QRT-asymmetric one, where moreover we have q = z2. The same procedure, starting from (1.4) and replacing xm for m even by z2/xm leads to a QRT-asymmetric equation of the form (1.3). Given this observation one would expect all the discrete Painlevé equations in form VI, studied in [6], and in form VI′, studied in [7], to be intimately related. An obvious criterion as to which equations may be related is the periodic dependence of their various parameters: if two equations in VI and VI′ forms have the same periodic dependence one would expect them to be somehow related. If we take, for instance, equations (2.16) of [6] and (27) of [7], which both have a quaternary periodic dependence in their parameters, we find that the transformation we outlined above allows to transform one into the other. However a difficulty does exist. It stems from two equations, one in form VI and the other in form VI′ which have the same double ternary dependence and which cannot be related through the transformation introduced above. This paper originated from our aspiration to resolve this minor enigma.

## 2. A short digression

Since in this paper we will be heavily using the QRT formalism [3] for the writing of mappings in symmetric and asymmetric form it is useful to summarise here the conventions that govern it and their consequences for discrete Painlevé equations.

The starting point is a QRT-symmetric mapping relating the variable w over three adjacent points, i.e. wn+1 = F(wn, wn−1). Typically a discrete Painlevé equation involves a variable introducing the secular dependence for which we can simply assume the form sn = αn + β. Moreover a discrete Painlevé equation equation can have parameters with a periodic dependence of the independent variable. To this end we have introduced two useful periodic functions ϕk and χ2k obeying the periodicity relations ϕk(n) = ϕk(n + k) (but where the constant solution of this relation is excluded) and χ2k(n) = −χ2k(n + k). Practically this gives the following forms for ϕk and χ2k:

φk(n)==1k1δ(k)exp(2iπnk),andχ2k(n)==1kη(k)exp(iπ(21)nk).

When dealing with multiplicative equations the expressions above must be understood as defining the logarithms of the periodic functions and thus the ϕ and χ we shall encounter in section 2, 3 and 4 are the exponentials of the right-hand sides of the above expressions.

Starting from a QRT-symmetric mapping one constructs an asymmetric one by separating the evolution for even and for odd values of the indices. One has thus two mappings one giving w2n+1 as a function of w2n and w2n−1 and one giving w2n+2 as a function of w2n+1 and w2n. The QRT convention is to introduce two distinct dependent variables corresponding to indices of different parity. One has thus xm = w2n and ym = w2n+1. Similarly for the secular dependence one introduces zm = s2n and ζm = s2n+1. Given this notation one remarks that ζm = zm+1/2.

Let us illustrate this by producing the asymmetric form of the symmetric mapping

(wn+1+wnsnsn+1)(wn+wn1snsn1)(wn+1+wn)(wn+wn1)=R(wn).(2.1)

Using the conventions explained in the previous paragraph we find for n = 2m, n = 2m + 1 respectively

(ym+xmzmζm)(xm+ym1zmζm1)(ym+xm)(xm+ym1)=R(xm),(2.2a)
(xm+1+ymζmzm+1)(ym+xmzmζm)(xm+1+ym)(ym+xm)=R(ym).(2.2b)

Up to this point the transcription is straightforward. However in the case of discrete Painlevé equations one must also be able to take care of the periodic functions. The periodic function ϕ2(n) is simply equal to γ(−1)n. Thus whenever the equation involves this function the latter will enter as γ in the even-index equation (2.2a) in the example above) and as −γ in the odd-index one (2.2b) in the example). The periodic function χ2(n) is identical to ϕ2(n). For the function ϕ3 we consider the four points ϕ3(2n − 1), ϕ3(2n), ϕ3(2n + 1), and ϕ3(2n + 2). Using the property ϕ3(n) = ϕ3(n + 3) we can rewrite these four quantities as ϕ3(2n − 4), ϕ3(2n), ϕ3(2n − 2), and ϕ3(2n + 2) and the transcription becomes now easy: φ3(m − 2), φ3(m), φ3(m − 1), and φ3(m + 1). In fact, the periodicity property can be used for all ϕs of odd periods. While ϕk(2n) and ϕk(2n + 2) are simply ϕk(m) and ϕk(m + 1), when k = 2p + 1 we have ϕk(2n + 1) = ϕk(2n + 1 − k) = ϕk(2n − 2p) which can be written as φk(mp). The case of even k is more complicated. We start with the remark that the ϕk for even k can be decomposed in terms of a ϕ of lower index and a χ based on the identity ϕ2p(n) = ϕp(n) + χ2p(n). Since we already discussed the case of ϕ2 it suffices to study the transcription of the first few χs. The function χ4(n) is represented in the QRT-asymmetric form by two different functions χ2(m) and χ^2(m) for the even and odd-index equation respectively. A short analysis shows that χ6(n) is transcribed as ϕ3(m) + c in the even-index equation and ϕ3(m − 1) − c in the odd-index one. The function χ8(n) becomes two different functions χ4(m) and χ^4(m) for the even- and odd-index equation respectively. The pattern becomes now clear. For χ2(2p+1) we must introduce ϕ2p+1(m) + c in the even-index equation and ϕ2p+1(mp) − c in the odd-index one. The χ4p on the other hand become two different functions χ2p and χ^2p.

Using the procedure described above one can transcribe any QRT-symmetric discrete Painlevé equation into a QRT-asymmetric one in a straightforward way.

## 3. The case of the two E7(1)-associated equations

The case that motivated the present study is that of two discrete Painlevé equations which have a double ternary periodicity, i.e. one involving two different periodic functions ϕ3(n) and φ^3. This is a feature appearing in equations associated to E7(1) and which does not exist for any of the “lower” groups. The first equation was obtained in [6] under the form

(wnwn+1snsn+1)(wnwn1snsn1)(wnwn+11)(wnwn11)=(wnansn2)(wnbnsn2)(wnc)(wn1/c),(3.1)
where sn has a secular dependence of the form logsn = αn+β plus a periodic dependence involving the two different period-3 functions ϕ3(n) and φ^3(n). The two quantities an and bn obey the relation ab = 1 at the autonomous limit and are such that each of the products as2 and bs2 involve one of the ϕ3(n), φ^3(n). Finally c is a pure constant. The second equation was obtained in [7] and has the form
(wn+1snsn+1wn)(wnwn1snsn1)(wn+1snsn+1wn)(wnsnsn1wn1)=(wndnsn1snsn+1)(wnfnsn)(sn1snsn+1wngn)(snwnhn).(3.2)

Here sn has a secular dependence of the form logsn = αn + β and a periodic dependence involving a period-3 function ϕ3(n). The functions dn and gn have the same ternary periodicity involving a function φ^3(n) as well as an even-odd dependence, which is also present in fn, hn. Neglecting the period-3 dependence we have simply dnfn = 1 and hngn = 1 and moreover gn+1 = dn.

Before addressing the question of whether (3.1) and (3.2) are related in full generality by the simple transformation presented in the previous paragraph, we can simply ask here whether it is possible to transform (3.1) into (3.2), and vice versa, in the case where the ternary dependence is neglected and we keep just the secular one. We start from (3.2) and write it in asymmetric, in the QRT sense, form as

(ymzmζmxm)(xmym1zmζm1)(ymzmζmxm)(xmzmζm1ym1)=(xmdmζm1ζmzm)(xmzm/dm)(ζm1ζmzmxmgm)(zmxm1/gm),(3.3a)
(ymzmζmxm)(xm+1ζmzm+1ym)(ymzmζmxm)(xm+1ymζmzm+1)=(ymgmzm+1ζmzm)(ymζm/gm)(zm+1ζmzmymdm)(ζmym1/dm),(3.3b)
where ζ(m) = z(m + 1/2). Similarly starting from (3.1) we obtain the QRT-asymmetric form
(xmympmρm)(xmym1pmρm1)(xmym1)(xmym11)=(xmapn2)(xmpn2/a)(xmc)(xm1/c),(3.4a)
(xmympmρm)(xm+1ympm+1ρm)(xmym1)(xm+1ym1)=(ymaρm2)(ymρm2/a)(ymc)(ym1/c),(3.4b)
where ρ(m) = p(m + 1/2). We start with equation (3.3), perform the transformation xn = Xn/zn, yn = ζn/Yn and use the relation pn=zn2. We transform thus the form-VI′ equation (3.3) into
(XmYmpmρm)(XmYm1pmρm1)(XmYm1)(XmYm11)=(Xmdmpm2)(Xmpm/dm)(Xmgm/pm)(Xm1/gm),(3.5a)
(XmYmpmρm)(Xm+1Ympm+1ρm)(XmYm1)(Xm+1Ym1)=(Ympm/dm)(Ymdmρm2)(Ymgm)(Ym1/(gmρm)).(3.5b)

A simple glance at the right-hand sides of (3.5) suffices to convince oneself that the terms do not match those of (3.4). Thus even when one considers just the secular dependence one cannot relate the two form-VI and form-VI′ equations.

Does the result above mean that there is no relation whatsoever between equations (3.1) and (3.2)? As we shall show in the next section this is not the case: the two equations have a common ancestor, namely an equation with E8(1) symmetry from which they can be obtained through two different limiting procedures.

## 4. From E8(1) to E7(1) through limiting procedures

Our starting point will be the multiplicative E8(1)-associated equation which has the form

(xn+1zn+1znxn)(xn1zn1znxn)(zn+12zn21)(zn12zn21)(xn+1zn+1znxn)(xn1zn1znxn)(zn+12zn21)(zn12zn21)/(zn+1zn2zn1)=R(xn).(4.1)

Taking the limit x → ∞ while keeping z finite leads to the equation

(xn+1znzn+1xn)(xnxn1znzn1)(xn+1znzn+1xn)(xnznzn1xn1)=S(xn),(4.2)
where the right-hand side S must now be understood as the limit of the right-hand side of (4.1) where we have taken x as well as certain of the parameters which appear in R(xn) to infinity. We remark readily that the equation obtained is of the form VI′.

In practical calculations, using the explicit form of R(x) may turn out to be cumbersome and it is preferable in this case to resort to the ancillary representation we have introduced in [8]. As we showed there, we can introduce the ancillary dependent variable ξ by

xn=ξn+1ξn(4.3)
and use it to rewrite the right-hand side of (4.1) as
R(xn)=zn+1zn2zn1ξnP(ξn)ξn1Q(ξn)ξnQ(ξn)ξn1P(ξn).(4.4)

The P(ξ) and Q(ξ) are given by

P(ξn)=i=18(ξnAni)andQ(ξn)=i=18(Aniξn1),(4.5)
where the Ani are eight quantities which, in principle, depend on the independent variable n. As we had shown in [8] they obey the relation
i=18Ani=zn+12zn42zn12.(4.6)

Before proceeding to the calculations aiming at relating (3.1) and (3.2) we introduce another auxiliary variable qn = q0λn which will be used in order to represent the secular dependence of the various parameters we are going to work with.

In our exhaustive investigation of symmetric E8(1)-associated equations we have, among others, derived a single equation, 4.3.1 in [9], which has a double ternary dependence, just like equations (3.1) and (3.2). The equation was obtained in additive form but its transcription to multiplicative form is straightforward. In the latter case its parameters are:

zn=qnφ3(n)An1=qn3φ2(n)φ^3(n)cAn2=qn3φ2(n)/(φ^3(n)c)An3=qnφ3(n)b/φ2(n)An4=qnφ3(n)/(φ2(n)b)(4.7)
where ϕ2(n) = γ(−1)n. The right-hand side of this equation is a ratio of two quadratic polynomials which means that it was obtained after two simplifications obtained by assuming An7An8=1 and An5An6=1. It has the form
R(xn)=zn+1zn2zn1xn2xnσ1σ4+σ21xn2σ4xnσ3σ4+σ21,(4.8)
where σ1 is the sum of the four Ai, σ2, σ3 the sum of all the products of two and three Ai respectively and σ4 the product of all four Ai. As we have seen, in order to obtain an equation of form VI′ we must take xn → ∞. Clearly, if we wish that the right-hand side remain quadratic, some of the Ai’s must also go to infinity with some going to zero so as to preserve (4.6). Given the form of the Ai in (4.7) this can be done in two different ways. The first possibility is to take both c and b to infinity (while keeping the ratio c/b finite and free). We find in this case the right-hand side
R(xn)=(xnφ2(n)φ^3(n)qn3c/b)(xnφ3(n)qn/φ2(n)(xnqn3φ^3(n)c/(bφ2(n)))(xnqnφ3(n)φ2(n)).(4.9)

Thus is exactly the right-hand side of (3.2) (calling x what is called there w) where sn = qnϕ3(n) (as in equation 4.3.1), dn=φ2(n)φ^3(n)c/b, gn=φ^3(n)c/(bφ2(n)), fn = ϕ3(n)/ϕ2(n) and hn = ϕ2(n). Thus by taking the limit xn → ∞ together with b and c we obtained precisely equation (3.2). However another possibility of limit does exist. We can balance the limit xn → ∞ by taking the γ of the ϕ2(n) to infinity. In that case it is more convenient to split the equation according to the parity of the indices and write it in QRT-asymmetric form.

A2n1=q2n3γφ^3(2n)cA2n2=q2n3γ/(φ^3(2n)c)A2n3=q2nφ3(2n)b/γA2n4=q2nφ3(2n)/(γb)(4.10)
and
A2n+11=q2n+13φ^3(2n+1)c/γA2n+12=q2n+13/(γφ^3(2n+1)c)A2n+13=q2n+1γφ3(2n+1)bA2n+14=q2n+1γφ3(2n+1)/b(4.11)

Taking the limit γ → ∞ we obtain for the even-index equation. In order to use the QRT notations we introduce sm = z2m and ζm = z2m+1. Since zn = qnϕ3(n) we can define φ3(m) = ϕ3(2n) and rewrite sm = q2mφ3(m) which allows us to write ζm = q2m+1ϕ3(2m + 1) = q2m+1ϕ3(2m − 2) = q2m+1φ3 = (m − 1). The right-hand side is

R2n=1ζm1sm2ζm(xmcϕ^3(m)ζm1smζm)(xmζm1smζm/(cϕ^3(m))(xmb/sm)(xm1/(bsm)),(4.12a)
where ϕ^3(m)=φ^3(2m). Similarly for the odd-index right-hand side we find
R2n+1=1smζm2sm+1(ymbζm)(ymζm/b)(ymcϕ^3(m1)/(smζmsm+1))(ym1/(smζmsm+1cϕ^3(m1))),(4.12b)
where φ^3(2m+1)=φ^3(2m2) and thus equal to ϕ^3(m1). Since the resulting equation is in VI′ form we must transform it to a form VI by introducing the transformation xm = Xm/sm, ym = ζm/Ym. We obtain thus the system
(XmYmsm2ζm2)(XmYm1sm2ζm12)(XmYm1)(XmYm11)=(Xmcϕ^3(m)ζm1sm2ζm)(Xmζm1sm2ζm/(cϕ^3(m)))(Xmb)(Xm1/b),(4.13a)
(XmYmsm2ζm2)(Xm+1Ymsm+12ζm2)(XmYm1)(Xm+1Ym1)=(Ymcϕ^3(m1)smζm2sm+1)(Ymsmζm2sm+1/(cϕ^3(m1)))(Ymb)(Ym1/b).(4.13b)

This is just equation (3.1), written in form (3.4).

Thus, by taking two different limits of the same E8(1)-associated equation we obtained the two E7(1)-related equations with double ternary dependence.

## 5. More limits

Having introduced the two possible ways of taking limits in E8(1)-associated equations in order to reduce them to E7(1)-associated ones, one can ask the question whether the case of the two equations analysed in the previous section is unique or more similar cases do exist. It turns out that the latter is indeed the case.

We start from the equations derived in [9] and identified the equations where at least one constant and one ϕ2(n) were present in their parameters. Four such equations are obtained, the ones labeled 4.2.1, 4.2.5, 5.2.1 and 5.2.5 in [9]. In what follows we shall present their different limits. (As pointed out in the previous section, the results of [9] concerned additive equations but their transcription to multiplicative ones is straightforward).

1. a)

Equation 4.2.1 The parameters of this equation are

zn=qnφ5(n+1)2φ5(n1)2/φ5(n)An1=qn3φ2(n)/φ5(n)An2=qn2φ^2(n)φ5(n)cAn3=qn2φ5(n)/(φ^2(n)c)An4=qnφ5(n+1)2φ5(n1)2/(φ5(n)φ2(n)).(5.1)

Note that two different period-2 functions appear here ϕ2(n) = γ(−1)n and φ^2(n)=δ(1)n. In order to balance the limit xn → ∞ we must take the γ of the ϕ2(n) to infinity as well as either of the c or the δ of φ^2(n). It turns out that the two latter choices are perfectly equivalent leading to the same final equation.

Once the equation is VI′ form is obtained it is interesting to convert it into a form VI, using the transformations we outlined in the Introduction. The end result is a QRT-symmetric equation of the form

(XnXn+1qn2qn+12φ52(n+1)φ52(n1)/φ52(n2))(XnXn1qn2qn12φ52(n+1)φ52(n2)/φ52(n+2))(XnYn1)(XnYn11)=(Xnφ52(n+1)φ52(n1)qn4/φ52(n))(Xncφ52(n+1)φ52(n1)ϕ2(n)qn3)(Xncφ52(n+1)φ52(n1)ϕ2(n)/(qnφ52(n)))(Xn1),(5.2)
where φ2(n) must be understood as the limit of φ^2(n)/φ2(n) when both γ and δ go to infinity. This is an E7(1)-associated equation which, at least to the authors knowledge, has not been previously derived.

2. b)

Equation 4.2.5 Before giving the parameters of this equation it is convenient to introduce the variable Zn, a variable simply related to zn by Zn = znzn+1. We can now write the parameters as

Zn=qnqn+1φ6(n)An1=qn2qn1φ2(n)φ6(n+2)c/φ6(n+3)An2=qn2qn+1φ2(n)φ6(n2)φ6(n3)/cAn3=qn1φ6(n)φ6(n+1)φ6(n1)/(φ2(n)φ6(n2)c)An4=qn+1φ6(n1)c/(φ2(n)φ6(n+2)φ6(n+1)).(5.3)

We remark that, here, it is possible to balance the limit xn → ∞ either by taking c to infinity or by taking the γ of the ϕ2(n) to infinity. Taking c to infinity results to an equation in a QRT-symmetric equation of form VI′. Its precise expression is

qn+1qn2qn1φ6(n)φ6(n1)(xn+1qnqn+1φ6(n)xn)(xnxn1qnqn1φ6(n1))(xn+1qnqn+1φ6(n)nxn)(xnqnqn1φ6(n1)xn1)(xnqnqn+1φ6(n2)φ6(n3)φ2(n))(xnqn1φ6(n)φ6(n+1)φ6(n1)/(φ2(n)φ6(n2)))(xnφ6(n3)/(φ6(n+2)φ2(n)qn2qn1))(xnφ2(n)φ6(n+1)φ6(n+2)/(φ6(n1)qn+1)).(5.4)

When we take γ to infinity we obtain a QRT-asymmetric equation in VI′ form. However it is possible to cast it is form VI in which case the equation becomes QRT-symmetric. We are not going to give the intermediate calculations (which are straightforward but rather tedious) and present directly the form of the resulting equation:

(Xn+1XnZn)(XnXn1Zn1)(Xn+1Xn1)(XnXn11)=(XnAn)(XnBn)(XnCn)(XnDn).(5.5)

The various quantities appearing in (5.5) are given by the expressions: logZnalphan + β − 2γ(−1)n + ϕ3(n + 1) + χ6(n) + χ6(n − 1), logAn = αn + β + (−1)n(γn + δ) + ϕ3(n) + χ6(n) and Bn = ZnZn+4/An+5, Cn = Zn+2/An+3, Dn = An+2/Zn+1. (Note that in these expressions we have used parameters and functions without making any attempt to link them to those of the initial equation, lest the result become inextricably complicated.)

Both equations (5.4) and (5.5) are new, derived here for the first time (with, the usual caveat, that the novelty statement concerns the present authors and their knowledge of discrete Painlevé equations).

3. c)

Equation 5.2.1 Here we have six Ani with expressions

zn=qnφ3(n)An1=qn2φ2(n)a/φ3(n)An2=qn2/(φ2(n)φ3(n)a)An3=qnφ3(n)φ^2(n)bAn4=qnφ3(n)φ^2(n)/bAn5=qnφ3(n)c/φ^2(n)An6=qnφ3(n)/(φ^2(n)c)(5.6)

Clearly there are four different ways one can balance the limit xn → ∞: taking all three a, b, c to infinity or the γ of ϕ2(n) and b, c or the δ of φ^2(n) and a or both γ and δ to infinity. All four limits lead to the same equation which is given in a QRT-symmetric VI′ form in the case (a, b, c) → ∞ or can be converted to such a form through a simple gauge choice. Without entering into further details we present the form obtained in the straightforward case when all three a, b, c go to infinity. We find

(xn+1znzn+1xn)(xnxn1znzn1)(xn+1znzn+1xn)(xnznzn1xn1)=(xnzn+1zn1/φ2(n)(xnzn/(cφ^2(n)))(xnczn/φ^2(n))(xn1/(zn+1zn1φ2(n))(xnb/(znφ^2(n)))(xn1/(bznφ^2(n)),(5.7)
where b, c must be understood as the ratios b/a, c/a when all three parameters go to infinity. Equation (5.7) is a discrete Painlevé equation already derived in [7].

4. d)

Equation 5.2.5 Here again we have six Ani. Their expressions are

Zn=qn3qn+13φ5(n)φ5(n+1)An1=qn5φ5(n)φ5(n+1)2φ5(n+2)2a/φ2(n)An2=qn5qn12φ2(n)/(φ5(n)a)An3=qnqn+14φ5(n)φ5(n1)2φ5(n2)2a/φ2(n)An4=qn2φ5(n)/(φ5(n+2)2φ5(n2)2φ2(n)a)An5=qn3φ2(n)φ5(n)bAn6=qn3φ2(n)φ5(n)/b(5.8)

We remark that the limit xn → ∞ can be balanced either by taking both a, b to infinity or by taking to infinity the γ of ϕ2(n). In the first case we obtain a QRT-symmetric discrete Painlevé equation in form VI′:

ZnZn1(xn+1Znxn)(xnxn1Zn1)(xn+1Znxn)(xnZn1xn1)=(xnφ2(n)qn5qn12/φ5(n)(xnbφ2(n)φ5(n)qn3)(xnφ5(n)qn2/(φ2(n)φ5(n+2)2φ5(n2)2))(xnb/(φ2(n)phi5(n)qn3))(xnφ2(n)/(Φ(n)qn5))(xnφ2(n)Φ(n)/(qnqn+14)),(5.9)
where Φ(n) = ϕ5(n)ϕ5(n + 1) 2ϕ5(n + 2) 2. Again, this is an equation which does not appear among those previously derived. Next we turn to the limit obtained when we take the γ of ϕ2(n) to infinity. In this case the equation obtained has a QRT-asymmetric form. However by transforming it into a VI form we find that the resulting discrete Painlevé equation can be cast into a QRT-symmetric form. Without entering into the tedious (but straightforward) details we give the final form of the equation:
(Xn+1XnZn2)(XnXn1Zn12)(Xn+1Xn1)(XnXn11)=(Xnaqn4qn+14φ5(n)/Φ(n))(Xnaqn8φ5(n)Φ(n))(Xnqn2qn12φ5(n1)2φ5(n)4φ5(n+1)2/a)(Xnb)(Xn1/b)(Xnaφ5(n)2/(qn2qn12)).(5.10)

Again, this is a new, to the authors’ knowledge, discrete Painlevé equation.

## 6. Conclusions

This paper was motivated by the existence of two E7(1)-associated discrete Painlevé equations, known under different canonical forms, both having a double ternary dependence in their coefficients. The usual conversion of form-VI to form-VI′ did not suffice in order to link the two equations and thus their relation was somewhat puzzling. In this paper we have shown that these two equations correspond to two different limits of a E8(1)-associated discrete Painlevé equation the coefficients of which do have a double ternary dependence.

Going from discrete Painlevé equations with E8(1) symmetry to ones with E7(1) can be obtained by taking the dependent variable to infinity. In order to balance this and obtain some meaningful equation necessitates taking also some of the parameters to infinity. In every case the independent variable must remain finite. In the case at hand the first of the two limits was a straightforward one, corresponding to taking the limit of a constant appearing explicitly in the equation to infinity. The second limit was somewhat more tricky since it consisted in taking the value of the parameter appearing in the period-2 function to infinity. Since we have ϕ2(n) = γ(−1)n, this means that we have to split the equation into even- and odd-index parts, i.e. write it in QRT-asymmetric form, and then apply the limit.

Having produced the complete list of QRT-symmetric E8(1)-associated discrete Painlevé equations, we asked ourselves whether there existed other cases where two different limits could be implemented and whether these two limits yielded different E7(1)-associated discrete Painlevé equations or the same one written under two different, but equivalent, forms. It turned out that both situations materialised: two of the equations led each to a single one at the limit (out of two and four possibilities respectively) the remaining ones yielding each two distinct equations. In all cases we were able to cast the equations into a QRT-symmetric form, either a VI form or a VI′ one. With one exception, (5.7), all the equations thus obtained had not been derived before (at least as far as the present authors can confirm). In this sense the method introduced here turned out to be a particularly fruitful one.

In our approach we took particular care of balancing the equations so as to lose just one parameter and moreover to be able to do this in at least two different ways. Thus we limited ourselves to the cases where that was possible. Of course it is also possible to implement the limit for E8(1) to E7(1)-associated equations in just a single way, and, in the light of the present results, derive new discrete Painlevé equations. What is potentially even more interesting is to consider the limits where one can go from a discrete Painlevé equation with E8(1) symmetry to one associated to E6(1) or even lower groups. Given the limiting procedure all these equations would have a canonical VI′ form. From our study in [10] we know that such equations do exist and deriving all of them in a systematic approach could be an interesting extension of the present work.

Journal
Journal of Nonlinear Mathematical Physics
Volume-Issue
27 - 1
Pages
95 - 105
Publication Date
2019/10/25
ISSN (Online)
1776-0852
ISSN (Print)
1402-9251
DOI
10.1080/14029251.2020.1683981How to use a DOI?
Open Access

TY  - JOUR
AU  - A. Ramani
AU  - B. Grammaticos
AU  - T. Tamizhmani
PY  - 2019
DA  - 2019/10/25
TI  - Interrelations of discrete Painlevé equations through limiting procedures
JO  - Journal of Nonlinear Mathematical Physics
SP  - 95
EP  - 105
VL  - 27
IS  - 1
SN  - 1776-0852
UR  - https://doi.org/10.1080/14029251.2020.1683981
DO  - 10.1080/14029251.2020.1683981
ID  - Ramani2019
ER  -