A. Zabrodin, A. Zotov / Nuclear Physics B 927 (2018) 550–565 553
Theorem 1. Equations of motion (1.11) for the Ruijsenaars–Schneider model follow from the set
of N + M equations
˙q
i
=
N
k=i
ϑ(q
i
− q
k
+ η)
ϑ(q
i
− q
k
)
M
γ =1
ϑ(q
i
− μ
γ
− η)
ϑ(q
i
− μ
γ
)
,
˙μ
α
=
M
γ =α
ϑ(μ
α
− μ
γ
− η)
ϑ(μ
α
− μ
γ
)
N
k=1
ϑ(μ
α
− q
k
+ η)
ϑ(μ
α
− q
k
)
,
(1.12)
where ϑ(z) should be replaced by sinh(z) and z in hyperbolic and rational cases respectively.
The variables {μ
α
} satisfy gl
M
Ruijsenaars–Schneider equations of motion:
¨μ
α
=
M
γ =α
˙μ
α
˙μ
γ
(2E
1
(μ
αγ
) − E
1
(μ
αγ
+ η) − E
1
(μ
αγ
− η)), α = 1 ...M.
(1.13)
In the elliptic case N = M, while in hyperbolic and rational cases N and M are arbitrary.
Note that equations (1.12) are well kno
wn in the theory of time discretization [22] (and/or
Bäcklund transformations [16]) of the Ruijsenaars–Schneider model.
2
Here we give a direct
proof (without usage of the discrete time dynamics) likewise it was presented in [29] for the
Calogero–Moser models.
Let N ≥ M for definiteness. It wa
s shown in [2] for the Calogero–Sutherland models that M
integrals of motion coincide, and other N − M are equal to some constants. We give a proof of a
similar result for the Ruijsenaars–Schneider models using determinant identities from [11,5].
Next, we sho
w that equations (1.12) follow from some multi-pole ansatz for a pair of complex
functions satisfying (complexified version of) the ILW equation with discrete Laplacian. The
latter was suggested in [24–26]:
∂
t
log(F
+
(z) − F
−
(z) + f
0
) = F
+
(z) + F
−
(z) − F
+
(z + η) − F
−
(z − η) .
(1.14)
It can be reduced to the following equation for a single real function:
f
t
= f Tf,
(1.15)
where f = f(x, t), x ∈ R and
Tf(x)=
ı
2π
−
+1/2
ˆ
−1/2
E
1
(y − x + η) + E
1
(y − x − η) − 2E
1
(y − x)
f(y)dy.
(1.16)
It should be mentioned that a relation between the Ruijsenaars–Schneider (and Calogero–
Moser) models and ILW–Benjamin–Ono equations is known [3,10] from the collective field
theory description of integrable many-body systems, which is adapted to the N →∞limit. Re-
lated algebraic structures and possible applications can be found in [7,18–20].
The paper is or
ganized as follows. In the next section we prove Theorem 1 and coincidence of
(a part of) action variables for the Ruijsenaars–Schneider models (1.11) and (1.13). In Section 3
we review the ILW equation with discrete Laplacian following [24] and describe its relation to
the self-dual form (1.12).
2
We are grateful to Yu. Suris for drawing our attention to this.