Bäcklund Transformation for the BC-Type Toda Lattice
We study an integrable case of n-particle Toda lattice: open chain with boundary terms containing 4 parameters. For this model we construct a Bäcklund transformation and prove its basic properties: canonicity, commutativity and spectrality. The Bäcklund transformation can be also viewed as a discret...
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2007
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irk-123456789-1473742019-02-15T01:24:12Z Bäcklund Transformation for the BC-Type Toda Lattice Kuznetsov, V. Sklyanin, E. We study an integrable case of n-particle Toda lattice: open chain with boundary terms containing 4 parameters. For this model we construct a Bäcklund transformation and prove its basic properties: canonicity, commutativity and spectrality. The Bäcklund transformation can be also viewed as a discretized time dynamics. Two Lax matrices are used: of order 2 and of order 2n+2, which are mutually dual, sharing the same spectral curve. 2007 Article Bäcklund Transformation for the BC-Type Toda Lattice / V. Kuznetsov, E. Sklyanin // Symmetry, Integrability and Geometry: Methods and Applications. — 2007. — Т. 3. — Бібліогр.: 22 назв. — англ. 1815-0659 2000 Mathematics Subject Classification: 70H06 http://dspace.nbuv.gov.ua/handle/123456789/147374 en Symmetry, Integrability and Geometry: Methods and Applications Інститут математики НАН України |
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We study an integrable case of n-particle Toda lattice: open chain with boundary terms containing 4 parameters. For this model we construct a Bäcklund transformation and prove its basic properties: canonicity, commutativity and spectrality. The Bäcklund transformation can be also viewed as a discretized time dynamics. Two Lax matrices are used: of order 2 and of order 2n+2, which are mutually dual, sharing the same spectral curve. |
| format |
Article |
| author |
Kuznetsov, V. Sklyanin, E. |
| spellingShingle |
Kuznetsov, V. Sklyanin, E. Bäcklund Transformation for the BC-Type Toda Lattice Symmetry, Integrability and Geometry: Methods and Applications |
| author_facet |
Kuznetsov, V. Sklyanin, E. |
| author_sort |
Kuznetsov, V. |
| title |
Bäcklund Transformation for the BC-Type Toda Lattice |
| title_short |
Bäcklund Transformation for the BC-Type Toda Lattice |
| title_full |
Bäcklund Transformation for the BC-Type Toda Lattice |
| title_fullStr |
Bäcklund Transformation for the BC-Type Toda Lattice |
| title_full_unstemmed |
Bäcklund Transformation for the BC-Type Toda Lattice |
| title_sort |
bäcklund transformation for the bc-type toda lattice |
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Інститут математики НАН України |
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2007 |
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http://dspace.nbuv.gov.ua/handle/123456789/147374 |
| citation_txt |
Bäcklund Transformation for the BC-Type Toda Lattice / V. Kuznetsov, E. Sklyanin // Symmetry, Integrability and Geometry: Methods and Applications. — 2007. — Т. 3. — Бібліогр.: 22 назв. — англ. |
| series |
Symmetry, Integrability and Geometry: Methods and Applications |
| work_keys_str_mv |
AT kuznetsovv backlundtransformationforthebctypetodalattice AT sklyanine backlundtransformationforthebctypetodalattice |
| first_indexed |
2025-07-11T01:57:02Z |
| last_indexed |
2025-07-11T01:57:02Z |
| _version_ |
1837313885872324608 |
| fulltext |
Symmetry, Integrability and Geometry: Methods and Applications SIGMA 3 (2007), 080, 17 pages
Bäcklund Transformation
for the BC-Type Toda Lattice?
Vadim KUZNETSOV † and Evgeny SKLYANIN ‡
† Deceased
‡ Department of Mathematics, University of York, York YO10 5DD, UK
E-mail: eks2@york.ac.uk
Received July 13, 2007; Published online July 25, 2007
Original article is available at http://www.emis.de/journals/SIGMA/2007/080/
Abstract. We study an integrable case of n-particle Toda lattice: open chain with boundary
terms containing 4 parameters. For this model we construct a Bäcklund transformation
and prove its basic properties: canonicity, commutativity and spectrality. The Bäcklund
transformation can be also viewed as a discretized time dynamics. Two Lax matrices are
used: of order 2 and of order 2n + 2, which are mutually dual, sharing the same spectral
curve.
Key words: Bäcklund transformation; Toda lattice; integrability; boundary conditions; clas-
sical Lie algebras
2000 Mathematics Subject Classification: 70H06
1 Introduction
In the present paper we study the Hamiltonian system of n one-dimensional particles with
coordinates xj and canonical momenta Xj , j = 1, . . . , n:
{Xj , Xk} = {xj , xk} = 0, {Xj , xk} = δjk, (1.1)
characterized by the Hamiltonian
H =
n∑
j=1
1
2
X2
j +
n−1∑
j=1
exj+1−xj + α1ex1 +
1
2
β1e2x1 + αne−xn +
1
2
βne−2xn (1.2)
containing 4 arbitrary parameters: α1, β1, αn, βn.
The model was missing from the early lists of integrable cases of the Toda lattice [1, 2] based
on Dynkin diagrams for simple affine Lie algebras. Its integrability was proved first in [3, 4, 5].
As for the more recent classifications, in [6] the model is enlisted as the case (i). In [7, 8]
particular cases of the Hamiltonian (1.2) are assigned to the C(1)
n case with ‘Morse terms’. For
brevity, we refer to the model as ‘BC-Toda lattice’ emphasising the fact that each boundary
term is a linear combination of the term ∼ α corresponding to the root system B and of the
term ∼ β corresponding to the root system C, see [1, 2, 7, 8].
In section 2 we review briefly the known facts about the integrability of the model using
the approach developed in [3, 4] and based on the Lax matrix L(u) of order 2 and the corre-
sponding quadratic r-matrix algebra. In particular, we construct explicitly a generating function
of the complete set of commuting Hamiltonians Hj (j = 1, . . . , n) which includes the physical
Hamiltonian H (1.2).
?This paper is a contribution to the Vadim Kuznetsov Memorial Issue ‘Integrable Systems and Related Topics’.
The full collection is available at http://www.emis.de/journals/SIGMA/kuznetsov.html
mailto:eks2@york.ac.uk
http://www.emis.de/journals/SIGMA/2007/080/
http://www.emis.de/journals/SIGMA/kuznetsov.html
2 V. Kuznetsov and E. Sklyanin
In Section 3 we describe the main result of our paper: construction of a Bäcklund transfor-
mation (BT) for our model as a one-parametric family of maps Bλ : (Xx) 7→ (Y y) from the
variables (Xx) to the variables (Y y). We construct the BT choosing an appropriate gauge (or
Darboux) transformation of the local Lax matrices. In Section 4, adopting the Hamiltonian
point of view developed in [9, 10], we prove the basic properties of the BT:
1. Preservation of the commuting Hamiltonians Bλ : Hj(X,x) 7→ Hj(Y, y).
2. Canonicity: preservation of the Poisson bracket (1.1).
3. Commutativity: Bλ1 ◦ Bλ2 = Bλ2 ◦ Bλ1 .
4. Spectrality: the fact that the graph of the BT is a Lagrangian manifold on which the
2-form
Ω ≡
n∑
j=1
(
dXj ∧ dxj − dYj ∧ dyj
)
− d lnΛ ∧ dλ (1.3)
vanishes. Here Λ is an eigenvalue of the matrix L(λ). In other words, the parameter λ of
the BT and its exponentiated canonical conjugate Λ lie on the spectral curve of L(u):
det
(
Λ− L(λ)
)
= 0. (1.4)
We also prove the following expansion of Bλ in λ−1
Bλ : f 7→ f − 2λ−1{H, f}+O(λ−2), λ→∞. (1.5)
which allows to interpret the BT as a discrete time dynamics approximating the continuous-time
dynamics generated by the Hamiltonian (1.2).
In Section 5 we construct for our system an alternative Lax matrix L(v). The new Lax
matrix of order 2n + 2 is dual to the matrix L(u) of order 2 in the sense that they share the
same spectral curve with the parameters u and v having been swapped:
det
(
v − L(u)
)
= (−1)n+1v det
(
u− L(v)
)
. (1.6)
In the same section we provide an interpretation of the BT in terms of the ‘big’ Lax mat-
rix L(v) and establish a remarkable factorization formula for λ2 − L2(v).
The concluding Section 6 contains a summary and a discussion. All the technical proofs and
tedious calculations are removed to the Appendices.
2 Integrability of the model
In demonstrating the integrability of the model we follow the approach to the integrable chains
with boundary conditions developed in [3, 4] and use the notation of [9, 10].
The Lax matrix L(u) for the BC-Toda lattice is constructed as the product
L(u) = K−(u)T t(−u)K+(u)T (u) (2.1)
of the following matrices (T t stands for the matrix transposition).
The monodromy matrix T (u) is itself the product
T (u) = `n(u) · · · `1(u) (2.2)
of the local Lax matrices
`j(u) ≡ `(u;Xj , xj) =
(
u+Xj −exj
e−xj 0
)
, (2.3)
Bäcklund Transformation for the BC-Type Toda Lattice 3
each containing only the variables Xj , xj describing a single particle. Note that trT (u) is the
generating function for the Hamiltonians of the periodic Toda lattice.
The matrices K±(u) containing the information about the boundary interactions are defined
as [3, 4]
K−(u) =
(
u −α1
α1 β1u
)
, K+(u) =
(
u −αn
αn βnu
)
. (2.4)
The significance of the Lax matrix L(u) is that its spectrum is invariant under the dynamics
generated by the Hamiltonian (1.2), the corresponding equations of motion dG/dt ≡ Ġ = {H,G}
for an observable G being
ẋj = Xj , j = 1, . . . , n (2.5)
and
Ẋj = exj+1−xj − exj−xj−1 , j = 2, . . . , n− 1, (2.6a)
Ẋ1 = ex2−x1 − α1ex1 − β1e2x1 , (2.6b)
Ẋn = −exn−xn−1 + αne−xn + βne−2xn . (2.6c)
To prove the invariance of the spectrum of L(u) we introduce the matrices Aj(u)
Aj(u) =
(
−u exj
−e−xj−1 0
)
, j = 2, . . . , n− 1, (2.7)
A1(u) =
(
−u ex1
−α1 − β1ex1 0
)
, An+1(u) =
(
−u αn + βne−xn
−e−xn 0
)
, (2.8)
which satisfy the easily verified identities
˙̀
j = Aj+1`j − `jAj , j = 1, . . . , n, (2.9)
−K̇+ = 0 = K+An+1(u) +At
n+1(−u)K+, (2.10a)
K̇− = 0 = A1(u)K− +K−A
t
1(−u). (2.10b)
From (2.2) and (2.9) it follows immediately that
Ṫ (u) = An+1(u)T (u)− T (u)A1(u). (2.11)
Then, using (2.1) and (2.10), we obtain the equality
L̇(u) =
[
A1(u), L(u)
]
(2.12)
implying that the spectrum of L(u) is preserved by the dynamics.
There are only two spectral invariants of a 2 × 2 matrix: the trace and the determinant.
From (2.3) it follows that det `(u) = 1 and, respectively, detT (u) = 1, so, by (2.1), the determi-
nant of L(u)
d(u) ≡ detL(u) = detK−(u) detK+(u) = (α2
1 + β1u
2)(α2
n + βnu
2) (2.13)
contains no dynamical variables Xx. The trace
t(u) ≡ trL(u) = trK−(u)T t(−u)K+(u)T (u), (2.14)
4 V. Kuznetsov and E. Sklyanin
however, does contain dynamical variables and therefore can be used as a generating function of
the integrals of motion, which can be chosen as the coefficients of the polynomial t(u) of degree
2n+ 2 in u. Note that t(−u) = t(u) due to the symmetry
Kt
±(−u) = −K±(u). (2.15)
The leading coefficient of t(u) at u2n+2 is a constant (−1)n. Same is true for its free term
t(0) = trK+(0)K−(0) = −2αnα1 (2.16)
due to the identity
MK±(0)M t = detM ·K±(0), (2.17)
which holds for any matrix M .
We are left then with n nontrivial coefficients Hj
t(u) = (−1)nu2n+2 − 2αnα1 +
n∑
j=1
Hju
2j (2.18)
which are integrals of motion Ḣj = 0 since ṫ(u) = 0 due to (2.12).
The conserved quantities Hj are obviously polynomial in X, e±x. Their independence can
easily be established by setting e±x = 0 in (2.3) and analysing the resulting polynomials in X.
It is also easy to verify that the physical Hamiltonian (1.2) is expressed as
H =
(−1)n+1
2
Hn. (2.19)
The quantities Hj are also in involution
{Hj ,Hk} = 0 (2.20)
with respect to the Poisson bracket (1.1). Together with the independence of Hj , it constitutes
the Liouville integrability of our system.
The commutativity (2.20) of Hj or, equivalently, of t(u)
{t(u1), t(u2)} = 0 (2.21)
is proved in the standard way using the r-matrix technique [3, 4].
Let 1 be the unit matrix of order 2 and for any matrix L define
1
L ≡ L⊗ 1,
2
L ≡ 1⊗ L. (2.22)
We have then the quadratic Poisson brackets [10, 11]
{
1
`(u1),
2
`(u2)} = [r(u1 − u2),
1
`(u1)
2
`(u2)], (2.23)
and, as a consequence,
{
1
T (u1),
2
T (u2)} = [r(u1 − u2),
1
T (u1)
2
T (u2)], (2.24)
with the r-matrix
r(u) =
P
u
, (2.25)
where P is the permutation matrix Pa⊗ b = b⊗ a.
Bäcklund Transformation for the BC-Type Toda Lattice 5
Let
r̃(u) = rt1(u) = rt2(u), (2.26)
t1 and t2 being, respectively, transposition with respect to the first and second component of
the tensor product C2 ⊗ C2.
Then for both T (u) = T (u)K−(u)T t(−u) and T (u) = T t(−u)K+(u)T (u) we obtain the same
Poisson algebra [3, 4]
{
1
T (u1),
2
T (u2)} = r(u1 − u2)
1
T (u1)
2
T (u2)−
1
T (u1)
2
T (u2)r(u1 − u2)
−
1
T (u1)r̃(u1 + u2)
2
T (u2) +
2
T (u2)r̃(u1 + u2)
1
T (u1), (2.27)
which ensures the commutativity (2.21) of t(u).
3 Describing Bäcklund transformation
In this section we shall construct a Bäcklund transformation (BT) for our model. We shall stay
in the framework of the Hamiltonian approach proposed in [9] and follow closely our previous
treatment of the periodic Toda lattice [9, 10], with the necessary modifications taking into
account the boundary conditions.
We are looking thus for a one-parametric family of maps Bλ : (Xx) 7→ (Y y) from the
variables (Xx) to the variables (Y y) characterised by the properties enlisted in the Introduction:
Invariance of Hamiltonians, Canonicity, Commutativity and Spectrality.
The invariance of the commuting Hamiltonians Hj , or of their generating polynomial t(u) =
trL(u) will be ensured if we find an invertible matrix M1(u, λ) intertwining the matrices L(u)
depending on the variables Xx and Y y:
M1(u, λ)L(u;Y, y) = L(u;X,x)M1(u, λ). (3.1)
To find M1(u, λ) let us look for a gauge transformation
Mj+1(u, λ)`(u;Yj , yj) = `(u;Xj , xj)Mj(u, λ), j = 1, . . . , n, (3.2)
implying that detMj does not depend on j. From (3.2) and (2.2) we obtain
Mn+1(u, λ)T (u;Y, y) = T (u;X,x)M1(u, λ). (3.3)
Let J be the the standard skew-symmetric matrix of order 2
J =
(
0 1
−1 0
)
, J t = −J, J2 = −1, (3.4)
and define the antipode Ma as
Ma ≡ −JMJ (3.5)
for any matrix M of order 2. It is easy to see that
M tMa = MaM t = detM. (3.6)
Transposing (3.3) and using (3.6) together with the the fact that detMj is independent of j
we obtain the relation
T t(−u;X,x)Ma
n+1(−u, λ) = Ma
1 (−u, λ)T t(−u;Y, y). (3.7)
6 V. Kuznetsov and E. Sklyanin
We shall be able to obtain (3.1) if we impose two additional relations
K−(u)Ma
1 (−u, λ) = M1(u, λ)K−(u), (3.8a)
K+(u)Mn+1(u, λ) = Ma
n+1(−u, λ)K+(u). (3.8b)
Then, starting with the right-hand side L(u;X,x)M1(u, λ) of (3.1) and using (2.1) and (3.3)
we obtain
L(u;X,x)M1(u, λ) = K−(u)T t(−u;X,x)K+(u)T (u;X,x)M1(u, λ)
= K−(u)T t(−u;X,x)K+(u)Mn+1(u, λ)T (u;Y, y) (3.9)
Using then (3.8b) to move Mn+1(u, λ) through K+(u), then using (3.7) and finally (3.8a) we
get, step by step,
L(u;X,x)M1(u, λ) = K−(u)T t(−u;X,x)Ma
n+1(−u, λ)K+(u)T (u;Y, y)
= K−(u)Ma
1 (−u, λ)T t(−u;Y, y)K+(u)T (u;Y, y)
= M1(u, λ)K−(u)T t(−u;Y, y)K+(u)T (u;Y, y)
= M1(u, λ)L(u;Y, y) (3.10)
arriving finally at (3.1).
We have thus to find a set of matrices Mj(u, λ), j = 1, . . . , n + 1 compatible with the
conditions (3.2) and (3.8). A quick calculation shows that the so called DST-ansatz for Mj used
in [9, 10] for the periodic Toda lattice contradicts the conditions (3.8).
The philosophy advocated in [10] requires that the ansatz for the gauge matrix Mj(u) be
chosen in the form of a Lax matrix satisfying the r-matrix Poisson bracket (2.23) with the same
r-matrix (2.25) as the Lax operator `(u). It was shown in [10] that the so-called DST-ansatz
MDST
j (u, λ) =
(
u− λ+ sjSj −sj
Sj −1
)
(3.11)
serves well for the periodic Toda case. The above ansatz is however not compatible with the
boundary conditions (3.8) and we have to use a more complicated ansatz for Mj in the form of
the Lax matrix for the isotropic Heisenberg magnet (XXX-model):
Mj(u, λ) =
(
u− λ+ sjSj s2jSj − 2λsj
Sj −u− λ+ sjSj
)
, detMj(u, λ) = λ2 − u2. (3.12)
The same gauge transformation was used in [12] for constructing a Q-operator for the quan-
tum XXX-magnet.
Substituting (3.12) into (3.2) we obtain the relations
Xj = −λ+ s−1
j exj + sj+1e−xj , (3.13a)
Yj = λ− s−1
j eyj − sj+1e−yj , (3.13b)
Sj = 2λs−1
j − s−2
j exj − s−2
j eyj , (3.13c)
Sj+1 = e−xj + e−yj , (3.13d)
for j = 1, . . . , n, and from (3.8), respectively,
S1 =
2(α1 + β1λs1)
1 + β1s21
, Sn+1 =
2(λsn+1 − αn)
βn + s2n+1
. (3.14)
Bäcklund Transformation for the BC-Type Toda Lattice 7
Eliminating the variables Sj , we arrive to the equations defining the BT (j = 1, . . . , n):
Xj = −λ+ s−1
j exj + sj+1e−xj , (3.15a)
Yj = λ− s−1
j eyj − sj+1e−yj . (3.15b)
The variables sj , j = 1, . . . , n + 1 in (3.15) are implicitly defined as functions of x, y and λ
from the quadratic equations
(e−xj−1 + e−yj−1)s2j − 2λsj + (exj + eyj ) = 0, j = 2, . . . , n (3.16a)
(2α1 + β1ex1 + β1ey1)s21 − 2λs1 + (ex1 + ey1) = 0, (3.16b)
(e−xn + e−yn)s2n+1 − 2λsn+1 + (2αn + βne−xn + βne−yn) = 0. (3.16c)
Like in the periodic case [9, 10], the BT map Bλ : (Xx) 7→ (Y y) is described implicitly by
the equations (3.15). Unlike the periodic case, we have extra variables sj . It is more convenient
not to express sj from equations (3.16) and to substitute them into (3.15) but rather define the
BT by the whole set of equations (3.15) and (3.16).
Equations (3.15) and (3.16) are algebraic equations and therefore define (Y y) as multivalued
functions of (Xx), which is a common situation with integrable maps [13].
In this paper, to avoid the complications of the real algebraic geometry we allow all our
variables to be complex.
4 Properties of the Bäcklund transformation
Having defined the map Bλ : (Xx) 7→ (Y y) in the previous section, we proceed to establish its
properties from the list given in the Introduction.
4.1 Preservation of Hamiltonians
The equality Hj(X,x) = Hj(Y, y) ∀ λ, or, equivalently, t(u;X,x) = t(u;Y, y) holds by construc-
tion, being a direct consequence of (3.1).
4.2 Canonicity
The canonicity of the BT means that the variables Y (X,x;λ) and y(X,x;λ) have the same
canonical Poisson brackets (1.1) as (Xx). An equivalent formulation can be given in terms of
symplectic spaces and Lagrangian manifolds. Consider the 4n-dimensional symplectic space V4n
with coordinates XxY y and symplectic 2-form
Ω4n ≡
n∑
j=1
(
dXj ∧ dxj − dYj ∧ dyj
)
. (4.1)
Equations (3.15) and (3.16) define a 2n-dimensional submanifold Γ2n ⊂ V4n which can be
considered as the graph Y = Y (X,x;λ), y = y(X,x;λ) of the BT (the parameter λ is assumed
here to be a constant). The canonicity of the BT is then equivalent to the fact that the manifold
Γ2n is Lagrangian, meaning that: (a) it is isotropic, that is nullifies the form Ω4n
Ω4n|Γ2n
= 0, (4.2)
and (b) it has maximal possible dimension for an isotropic manifold: dim Γ2n = 1
2 dimV4n.
8 V. Kuznetsov and E. Sklyanin
One way of proving the canonicity is to present explicitly the generating function Φλ(y;x) of
the canonical transformation, such that
Xj =
∂Φλ
∂xj
, Yj = −∂Φλ
∂yj
. (4.3)
The required function is given by the expression
Φλ(y;x) =
n∑
j=1
fλ(yj , sj+1;xj , sj) + ϕ
(0)
λ (s1) + ϕ
(n+1)
λ (sn+1), (4.4)
where
fλ(yj , sj+1;xj , sj) = λ(2 ln sj − xj − yj) + s−1
j (exj + eyj )− sj+1(e−xj + e−yj ), (4.5a)
ϕ
(0)
λ (s1) = −λ ln
(
1 + β1s
2
1)−
2α1√
β1
arctan
(√
β1s1
)
, (4.5b)
ϕ
(n+1)
λ (sn+1) = λ ln
(
βn + s2n+1
)
− 2αn√
βn
arctan
(
sn+1√
βn
)
, (4.5c)
and sj(x, y;λ) are defined implicitly through (3.16).
Equalities (4.3) can be verified by a direct, though tedious, computation. Another, more
elegant, way is to use the argument from [10] based on imposing a set of constraints in an
extended phase space, see Appendix A.
4.3 Commutativity
The commutativity Bλ1 ◦Bλ2 = Bλ2 ◦Bλ1 of the BT follows from the preservation of the complete
set of Hamiltonians and the canonicity by the standard argument [9, 10] based on Veselov’s
theorem [13] about the action-angle representation of integrable maps.
4.4 Spectrality
The spectrality property formulated first in [9] generalises the canonicity by allowing the pa-
rameter λ of the BT to be a dynamical variable like x and y.
Let us extend the symplectic space V4n from section 4.2 to a (4n + 2)-dimensional space
V4n+2 by adding two more coordinates λ, µ and defining the extension Ω4n+2 of symplectic
form Ω4n (4.1) as
Ω4n+2 ≡ Ω4n − dµ ∧ dλ =
n∑
j=1
(
dXj ∧ dxj − dYj ∧ dyj
)
− dµ ∧ dλ. (4.6)
Define the extended graph Γ2n+1 of the BT by equations (3.15) and a new equation
µ = − ∂
∂λ
Φλ(y;x). (4.7)
The 2-form Ω4n+2 obviously vanishes on Γ2n+1, and the manifold Γ2n+1 is lagrangian.
An amazing fact is that eµ is proportional to an eigenvalue of the matrix L(λ), see (1.4). In
fact, the two eigenvalues of L(λ) can be found explicitly to be
Λ = (α2
n + βnλ
2)
1 + β1s
2
1
βn + s2n+1
n∏
j=1
(
−s−2
j exj+yj
)
, (4.8a)
Bäcklund Transformation for the BC-Type Toda Lattice 9
Λ̃ = (α2
1 + β1λ
2)
βn + s2n+1
1 + β1s21
n∏
j=1
(
−s2je−xj−yj
)
, (4.8b)
see Appendix B for the proof.
Having the explicit formulae (4.8a) for Λ and (4.4) for Φλ(y;x) one can easily verify that
Λ = (−1)n(α2
n + βnλ
2) eµ. (4.9)
4.5 Bäcklund transformation as discrete time dynamics
One of applications of a BT is that it might provide a discrete-time approximation of a continuous-
time integrable system [14, 15]. Indeed, iterations of the canonical map Bλ generate a discrete
time dynamics. Furthermore, if we find a point λ = λ0 that (a) the map Bλ0 becomes the iden-
tity map, and (b) in a neighbourhood of λ0 the infinitesimal map Bλ0+ε ∼ ε{H, ·} reproduces
the Hamiltonian flow with the Hamiltonian (1.2), we can claim that Bλ is a discrete time ap-
proximation of the BC-Toda lattice. An attractive feature of this approximation is that, unlike
some others [14], the discrete-time system and the continuous-time one share the same integrals
of motion.
In our case λ0 = ∞. Letting ε = λ−1 and assuming the ansatz
yj = xj +O(ε), j = 1, . . . , n (4.10)
we obtain from (3.16a) and (3.16b) the expansion
sj = εexj +O(ε2), j = 1, . . . , n (4.11a)
and from (3.16c) the expansion
sn+1 = ε(αn + βne−xn) +O(ε2). (4.11b)
Substituting then expansions (4.10) into equation (3.13d) we obtain
Sj = 2e−xj−1 +O(ε), j = 2, . . . , n+ 1 (4.12a)
and substituting expansion (4.11) for s1 into formula (3.14) for S1 we obtain
S1 = (2α1 + β1ex1) +O(ε). (4.12b)
Then from (3.12) we have
−εMj = 1 + ε
(
u1 + 2Aj
)
+O(ε2), j = 1, . . . , n+ 1, (4.13)
where Aj coincides with the matrix (given by (2.7) and (2.8)) which describes the continuous-
time dynamics of the Lax matrix. From (3.2) we obtain then
`(u;Yj , yj) = `(u;Xj , xj)− 2ε
(
Aj+1`(u;Xj , xj)− `(u;Xj , xj)Aj
)
+O(ε2), (4.14)
for j = 1, . . . , n+ 1. Comparing the result to (2.9) we get the expansion (1.5).
10 V. Kuznetsov and E. Sklyanin
5 Dual Lax matrix
Many integrable systems possess a pair of Lax matrices sharing the same spectral curve with
the parameters u and v swapped like in (1.6), see [16] for a list of examples and a discussion.
In particular, the periodic n-particle Toda lattice has two Lax matrices: the ‘small’ one, of
order 2 [11], and the ‘big’ one, of order n [17]. For various degenerate cases of the BC-Toda
lattice ‘big’ Lax matrices are also known [2, 7, 8, 17].
In this section we present a new Lax matrix of order 2n+2 for the most general, 4-parametric
BC-Toda lattice. Here we describe the result, removing the detailed derivation to Appendix C.
Let Ejk be the square matrix of order 2n+ 2 with the only nonzero entry (Ejk)jk = 1. The
Lax matrix L(v) is then described for the generic case n ≥ 3 as
L(v) =
n∑
j,k=1
LjkEjk
=
n∑
j=2
exj−xj−1Ej,j−1 +
n∑
j=1
(
−XjEjj + Ej,j+1
)
−
n−1∑
j=1
exj+1−xjE2n+2−j,2n+1−j +
n∑
j=1
(
XjE2n+2−j,2n+2−j − E2n+2−j,2n+3−j
)
+
(
αne−xn +
βn
2
e−2xn
) (
En+1,n − En+2,n+1
)
+
βn
2
e−xn−xn−1
(
En+3,n − En+2,n−1
)
− En+1,n+2
−
(
α1ex1 +
β1
2
e2x1
) (
E2n+2,2n+1 + v−1E1,2n+2
)
+
β1
2v
ex1+x2
(
E2,2n+1 − E1,2n
)
− vE2n+2,1 (5.1)
and consists of a bulk ‘Jacobian’ strip (the main diagonal and two adjacent diagonals) which
reproduces the Lax matrix for the open Toda lattice together with boundary blocks containing
parameters α1β1αnβn. We do not consider here the special case of small dimensions n = 1, 2
when the two boundary blocks interfere with each other and the structure of the Lax matrices
becomes more complicated
To help visualise the matrix L(v) we present an illustration for the case n = 3, using the
shorthand notation ξj ≡ exj , ηj ≡ eyj :
L(v) =
−X1 1 0 0 0 −β1
2v
ξ1ξ2 0 α1
v
ξ1+
β1
2v
ξ2
1
ξ2
ξ1
−X2 1 0 0 0 β1
2v
ξ1ξ2 0
0 ξ3
ξ2
−X3 1 0 0 0 0
0 0 α3
ξ3
+
β3
2ξ23
0 −1 0 0 0
0 − β3
2ξ2ξ3
0 −α3
ξ3
− β3
2ξ23
X3 −1 0 0
0 0 β3
2ξ2ξ3
0 − ξ3
ξ2
X2 −1 0
0 0 0 0 0 − ξ2
ξ1
X1 −1
−v 0 0 0 0 0 −α1ξ1−β1
2
ξ2
1 0
. (5.2)
The matrix L(v) possesses the symmetry
L(v) = −CvLt(v)C−1
v , (5.3)
Bäcklund Transformation for the BC-Type Toda Lattice 11
where
Cv = −vE2n+2,2n+2 +
2n+1∑
j=1
Ej,2n+2−j =
0 0 . . . 0 1 0
0 0 . . . 1 0 0
. . . . . . . . . . . . . . . . . .
0 1 . . . 0 0 0
1 0 . . . 0 0 0
0 0 . . . 0 0 −v
(5.4)
(note that C−1
v = Cv−1).
The matrix L(v) shares the same spectral curve with the ‘small’ Lax operator L(u) satisfying
the determinantal identity (1.6) and thus generates the same commuting Hamiltonians Hj .
The Lax matrix L(v) of order 2n + 2 seems to be new. When one or more of the constants
α1β1αnβn vanish it degenerates (with a drop of dimension) into known Lax matrices for simple
affine Lie algebras [2, 7, 8, 17]. For the general 4-parametric case a Lie-algebraic interpretation
of L(v) is still unknown. In particular, it is an interesting question whether L(v) satisfies a kind
of r-matrices Poisson algebra.
Inozemtsev [5] presented a different Lax matrix for the BC-Toda lattice, of order 2n instead
of 2n+ 2 and with a more complicated dependence on the spectral parameter. The relation of
these two Lax matrices is yet to be investigated.
For the dynamics (2.5), (2.6) we have an analog of the Lax equation (2.12):
˙L(v) = [A(v),L(v)] (5.5)
with A(v) defined as
A(v) =
n∑
j=1
(
XjEjj − Ej,j+1 −XjE2n+2−j,2n+2−j + E2n+2−j,2n+3−j
)
+ En+1,n+1
+ vE2n+2,1 −
β1
2
e2x1
(
E2n+2,2n+1 + v−1E1,2n+2
)
+
βn
2
e−2xn
(
En+1,n − En+2,n
)
(5.6)
and satisfying
A(v)Cv + CvAt(v) = 0. (5.7)
The analog of the formula (3.1) for the Bäcklund transformation is
M(v, λ)L(v;Y, y) = L(v;X,x)M(v, λ), (5.8a)
M̃(v, λ)L(v;X,x) = L(v;Y, y)M̃(v, λ), (5.8b)
where M(v) is given by
M(v) =
n∑
j,k=1
MjkEjk = −
n∑
j=2
ξj
ηj−1
Ej,j−1 +
n∑
j=1
(
sj+1
ηj
− ξj
sj
)
Ejj + Ej,j+1 (5.9)
+
n−1∑
j=1
ηj+1
ξj
E2n+2−j,2n+1−j +
n∑
j=1
(
sj+1
ξj
− ηj
sj
)
E2n+2−j,2n+2−j − E2n+2−j,2n+3−j
+
(
αn
ξn
+
βn
2ξ2n
) (
En+1,n − En+2,n+1
)
+
βn
2ξnξn−1
(
En+3,n − En+2,n−1
)
− En+1,n+2
−
(
α1ξ1+
β1ξ
2
1
2
)(
E2n+2,2n+1+v−1E1,2n+2
)
+
β1ξ1ξ2
2v
(
E2,2n+1−E1,2n
)
− vE2n+2,1,
12 V. Kuznetsov and E. Sklyanin
(using again the notation ξj ≡ exj , ηj ≡ eyj ) and M̃(v) is defined as
M̃(v) ≡ CvMt(v)C−1
v . (5.10)
One of common ways to obtain a Bäcklund transformation is from factorising a Lax matrix
in two different ways, see [18] for Toda lattices and [13] for other integrable models. For our
model we also have a remarkable factorisation, only instead of L(v) we have to take its square:
λ2 − L2(v;X,x) = M(v, λ)M̃(v, λ), (5.11a)
λ2 − L2(v;Y, y) = M̃(v, λ)M(v, λ). (5.11b)
6 Discussion
The method for constructing a Bäcklund transformation presented in this paper seems to be
quite general and applicable as well to other integrable sl(2)-type chains with the boundary
conditions treatable within the framework developed in [3, 4].
There is little doubt that a similar BT can be constructed for the D-type Toda lattice and
a more general Inozemtsev’s Toda lattice [5] with the boundary terms like
a1
sinh2 x1
2
+
b1
sinh2 x1
+
an
sinh2 xn
2
+
bn
sinh2 xn
since those, as shown in [20], can also be described in the formalism based on the boundary K
matrices (2.1) and the Poisson algebra (2.27).
The ‘big’ Lax matrix L(v) still awaits a proper Lie-algebraic interpretation. Obtaining a BT
from the factorisation of λ2 − L2 like in (5.11) might prove to be useful for other integrable
systems related to classical Lie algebras.
It is well known that the quantum analog of a BT is the so-called Q-operator [21], see also [9].
Examples of Q-operators for quantum integrable chains with a boundary have been constructed
recently for the XXX magnet [12] and for the Toda lattices of B, C and D types [22]. Our
results for the BC-Toda lattice agree with those of [22], the generating function of the BT being
a classical limit of the kernel of the Q-operator. Hopefully, our results will help to construct the
Q-operator for the general 4-parametric quantum BC-Toda lattice.
A Proof of canonicity
Here we adapt to the BC-Toda case the argument from [10] developed originally for the periodic
case. The trick is to obtain the graph Γ2n of the BT as a projection of another manifold in
a bigger symplectic space, the mentioned manifold being Lagrangian for trivial reason.
Consider the 8-dimensional symplectic space W8 with coordinates XxY ySsT t and the sym-
plectic form
ω8 ≡ dX ∧ dx+ dS ∧ ds− dY ∧ dy − dT ∧ dt. (A.1)
The matrix relation
M(u, λ;T, t)`(u;Y, y) = `(u;X,x)M(u, λ;S, s) (A.2)
is equivalent to 4 relations
X = −λ+ s−1ex + te−x, (A.3a)
Bäcklund Transformation for the BC-Type Toda Lattice 13
Y = λ− s−1ey − te−y, (A.3b)
S = 2λs−1 − s−2ex − s−2ey, (A.3c)
T = e−x + e−y, (A.3d)
defining a 4-dimensional submanifold G4 ⊂W8. The fact that G4 is Lagrangian, that is ω8|G4
= 0,
is proved by presenting explicitly the generating function
fλ(y, t;x, s) = λ(2 ln s− x− y) + s−1(ex + ey)− t(e−x + e−y), (A.4)
such that
X =
∂fλ
∂x
, S =
∂fλ
∂s
, Y = −∂fλ
∂y
, T = −∂fλ
∂t
. (A.5)
An alternative proof [10] is based on the fact that `(u) and M(u, λ) are symplectic leaves of
the same Poisson algebra (2.23).
Relation (A.2) defines thus a canonical transformation from XxSs to Y yT t.
Let us take n copies W
(j)
8 of W8 decorating the variables XxY ySsT t with the indices
j = 1, . . . , n and impose on them n matrix relations obtained from (A.2) by adding subscript j to
all variables. We obtain then a Lagrangian manifold G4n = ⊗n
j=1G
(j)
4 in the 8n-dimensional sym-
plectic space W8n = ⊕n
j=1W
(j)
8 with the symplectic form ω8n =
n∑
j=1
ω
(j)
8 and the corresponding
canonical transformation with the generating function
n∑
j=1
fλ(yj , tj ;xj , sj).
Let us also introduce 4 additional variables T0, t0 and Sn+1, sn+1 serving as coordinates in the
4-dimensional symplectic space W4 with the symplectic form ω4 ≡ dSn+1 ∧ dsn+1 − dT0 ∧ dt0.
The relations
T0 =
2(α1 + β1λt0)
1 + β1t20
, Sn+1 =
2(λsn+1 − αn)
βn + s2n+1
(A.6)
define then a 2-dimensional Lagrangian submanifold G2 ⊂ W4 characterised by the generat-
ing function ϕ = ϕ
(0)
λ (t0) + ϕ
(n+1)
λ (sn+1) with ϕ
(0)
λ and ϕ
(n+1)
λ defined by (4.5b) and (4.5c),
respectively:
T0 = −∂ϕλ
∂t0
, Sn+1 =
∂ϕλ
∂sn+1
. (A.7)
We end up with the (8n + 4)-dimensional symplectic space W8n+4 = W8n +W4, symplectic
form ω8n+4 = ω8n+ω4, and the (4n+2)-dimensional Lagrangian submanifold G4n+2 = G4n×G2 ⊂
W8n+4 defined by the generating function
Fλ = ϕ
(0)
λ (t0) + ϕ
(n+1)
λ (sn+1) +
n∑
j=1
fλ(yj , tj ;xj , sj). (A.8)
The final step is to impose 2n+ 2 constraints
Tj = Sj+1, tj = sj+1, j = 0, . . . , n, (A.9)
which define a subspace W6n+2 ⊂W8n+4 of dimension (8n+4)−(2n+2) = 6n+2 and respective
2n-dimensional submanifold G2n = G4n+2 ∩W6n+2.
Constraints (A.9) allow to eliminate the variables Tt. The space W6n+2 splits then into the
direct sum W6n+2 = V4n + W2n+2 of the space W4n with coordinates XjxjYjyj (j = 1, . . . , n)
14 V. Kuznetsov and E. Sklyanin
and W2n+2 with coordinates Sjsj (j = 1, . . . , n + 1). Using (A.9) we obtain that dTj ∧ dtj −
dSj+1 ∧ dsj+1 = 0 and therefore the symplectic form ω8n+4 restricted on W6n+2
ω8n+4|W6n+2
=
n∑
j=1
(
dXj ∧ dxj − dYj ∧ yj
)
, (A.10)
degenerates: it vanishes on W2n+2 and remains nondegenerate on V4n. In fact, on V4n the form
ω8n+4 coincides with the standard symplectic form (4.1).
ω8n+4|V4n
= Ω4n. (A.11)
After the elimination of the variables Tt from equations (A.3) and (A.6), the resulting set
of equations defining the submanifold G2n = G4n+2 ∩ W6n+2 ⊂ W6n+2 coincides with equa-
tions (3.13) and (3.14) defining the BT.
As we have seen in Section 3, the variables Sjsj can also be eliminated leaving a 2n dimen-
sional submanifold Γ2n ⊂ V4n coinciding with the graph of the BT discussed in Section 4.2.
By construction, Γ2n is the projection of G2n from W6n+2 onto V4n parallel to W2n+2. Further-
more, Γ2n is Lagrangian since ω8n+4 vanishes on G4n+2, therefore on G2n = G4n+2 ∩W6n+2, and
therefore on Γ2n. The canonicity of the BT is thus established geometrically, without tedious
calculations.
The same argument as in [10] shows that the generating function Φλ of the Lagrangian
submanifold Γ2n is obtained by setting tj = sj+1 in (A.8), which produces formula (4.4).
B Proof of spectrality
Here we provide the proof of formulae (4.8) for the eigenvalues of L(λ). For the proof we use
an observation from [10] and show that the eigenvectors of L(λ) are given by null-vectors of
M1(±λ, λ).
After setting u = −λ in (3.12) the matrix Mj becomes a projector
Mj(−λ, λ) =
(
−2λ+ sjSj s2jSj − 2λsj
Sj sjSj
)
=
(
−2λ+ sjSj
Sj
)
(1 sj) (B.1)
with the null-vector
σj ≡
(
−sj
1
)
, Mj(−λ, λ)σj = 0. (B.2)
Let us set u = −λ in the matrix equality (3.1) and apply it to the vector σ1. By (B.2), the
right-hand side gives 0. Therefore, L(−λ)σ1 should be proportional to the same null-vector σ1
of Mj(−λ, λ), and σ1 is an eigenvector of L(−λ).
To find the corresponding eigenvalue Λ, use the factorised expression (2.1) of L(−λ) and
apply it to σ1. Using (2.3) we obtain
`(−λ;Yj , yj)σj = −sje−yjσj+1, (B.3)
hence
T (u;Y, y)σ1 = σn+1
n∏
j=1
(
−sje−yj
)
. (B.4)
From (3.5) and (3.12) we obtain
Ma
j (u, λ) =
(
−u− λ+ sjSj −Sj
2λsj − s2jSj u− λ+ sjSj
)
, (B.5)
Bäcklund Transformation for the BC-Type Toda Lattice 15
hence
Ma
j (λ, λ) =
(
−2λ+ sjSj −Sj
(2λsj − s2j )Sj sjSj
)
=
(
−1
sj
)
(2λ− sjSj Sj), (B.6)
the corresponding null-vector being
σ̃j ≡
(
Sj
sjSj − 2λ
)
, Ma
j σ̃j = 0. (B.7)
A direct calculation using (2.3) and (3.13d) yields
`tj(λ;Yj , yj)σ̃j+1 = sje−xj σ̃j (B.8)
and, consequently,
T t(λ;Y, y)σ̃n+1 = σ̃1
n∏
j=1
(
sje−xj
)
. (B.9)
From (2.4) we get, respectively, the identities
K+(−λ)σn+1 =
1
2
(βn + s2n+1)σ̃n+1, K−(−λ)σ̃1 = 2
α2
1 + β1λ
2
1 + β1s21
σ̃1. (B.10)
Using the above formulae we are able to move σ1 through all the factors constituting L(−λ)
and obtain the equality
L(−λ;Y, y)σ1 = Λσ1, (B.11)
where Λ is given by (4.8a). Note that Λ is an eigenvalue of L(λ) as well since Λ(λ) = Λ(−λ).
The second eigenvalue Λ̃ (4.8b) of L(λ) is obtained from
ΛΛ̃ = detL(λ) ≡ d(λ) = (α2
n + βnλ
2)(α2
1 + β1λ
2), (B.12)
see (2.13).
C Derivation of the dual Lax matrix
To construct the ‘big’ Lax operator L(v) from the ‘small’ one L(u) we use the technique developed
for the periodic the periodic Toda lattice [10, 19], with the necessary corrections to accommodate
the boundary conditions.
Let θ1 be an eigenvector of L(u) with the eigenvalue v:
L(u)θ1 = vθ1, θ1 =
(
ϕ1
ψ1
)
. (C.1)
Reading off the factors constituting the product L(u), see (2.1), (2.2), define recursively the
vectors θj
θj =
(
ϕj
ψj
)
, j = 1, . . . 2n+ 2, (C.2)
by the relations
θj+1 = `(u;Xj , xj)θj , j = 1, . . . , n, (C.3a)
16 V. Kuznetsov and E. Sklyanin
θn+2 = K+(u)θn+1, (C.3b)
θn+j+3 = `t(−u;Xn−j , xn−j)θn+j+2, j = 0, . . . , n− 1, (C.3c)
and close the circuit with the equation
vθ1 = K−(u)θ2n+2, (C.3d)
which is equivalent to (C.1).
A recursive elimination of ψj results in the equations
uϕ1 = ϕ2 −X1ϕ1 +
(
α1
v
ex1 +
β1
v
e2x1
)
ϕ2n+2 −
β1
v
e2x1X1ϕ2n+1 +
β1
v
ex1+x2ϕ2n, (C.4a)
uϕj = ϕj+1 −Xjϕj + exj−xj−1ϕn−1, j = 2, . . . , n (C.4b)
uϕn+1 = ϕn+2 + αne−xnϕn, (C.4c)
uϕn+2 = −ϕn+3 +Xnϕn+2 + (αne−xn + βne−2xn)ϕn+1
− βne−2xnXnϕn + βne−xn−xn−1ϕn−1, (C.4d)
uϕj = −ϕj+1 +X2n+2−jϕj − exj−3−xj−4ϕj−1, j = n+ 3, . . . , 2n+ 1, (C.4e)
uϕ2n+2 = vϕ1 − α1ex1ϕ2n+1. (C.4f)
In order to simplify the 6-terms relations (C.4a) and (C.4d) and make the matrix L(v) more
symmetric we perform an additional reversible change of variables
ϕ1 = ϕ̃1 +
β1
2v
e2x1ϕ̃2n+1, (C.5a)
ϕj = ϕ̃j , j = 2, . . . , n+ 1, (C.5b)
ϕn+2 = ϕ̃n+2 +
βn
2
e−2xnϕ̃n, (C.5c)
ϕj = −ϕ̃j , j = n+ 3, . . . , 2n+ 2. (C.5d)
The resulting equations for ϕ̃j read
uϕ̃1 = ϕ̃2 −X1ϕ̃1 −
(
α1
v
ex1 +
β1
2v
e2x1
)
ϕ̃2n+2 −
β1
2v
ex1+x2ϕ̃2n, (C.6a)
uϕ̃2 = ϕ̃3 −X2ϕ̃2 + ex2−x1ϕ̃1 +
β1
2v
e2x1ϕ̃2n+1, (C.6b)
uϕ̃j = ϕ̃j+1 −Xjϕ̃j + exj−xj−1ϕ̃n−1, j = 3, . . . , n (C.6c)
uϕ̃n+1 = −ϕ̃n+2 +
(
αne−xn +
βn
2
e−2xn
)
ϕ̃n, (C.6d)
uϕ̃n+2 = −ϕ̃n+3 +Xnϕ̃n+2 −
(
αne−xn +
βn
2
e−2xn
)
ϕ̃n+1 −
βn
2
e−xn−xn−1ϕ̃n−1, (C.6e)
uϕ̃n+3 = −ϕ̃n+4 +Xn−1ϕ̃n+3 − exn−xn−1ϕ̃n+2 +
βn
2
e−2xn , (C.6f)
uϕ̃j = −ϕ̃j+1 +X2n+2−jϕ̃j − exj−3−xj−4ϕ̃j−1, j = n+ 4, . . . , 2n+ 1, (C.6g)
uϕ̃2n+2 = −vϕ̃1 −
(
α1ex1 +
β1
2
e2x1
)
ϕ̃2n+1. (C.6h)
Introducing the vector Θ with 2n+2 components ϕ̃j , j = 1, . . . , 2n+2 we can rewrite relations
(C.6) in the matrix form
L(v)Θ = uL(v)Θ (C.7)
with the matrix L(v) given by (5.1). It follows from (C.7) that u is an eigenvalue of L(v).
Bäcklund Transformation for the BC-Type Toda Lattice 17
The rest of the formulae of Section 5 are obtained by a straitforward calculation not much
different from the periodic case [10, 19].
Acknowledgements
This work has been partially supported by the European Community (or European Union)
through the FP6 Marie Curie RTN ENIGMA (Contract number MRTN-CT-2004-5652).
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http://arxiv.org/abs/solv-int/9711010
http://arxiv.org/abs/nlin.SI/0009009
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http://arxiv.org/abs/nlin.SI/0403028
http://arxiv.org/abs/solv-int/9712016
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http://arxiv.org/abs/0705.2886
1 Introduction
2 Integrability of the model
3 Describing Bäcklund transformation
4 Properties of the Bäcklund transformation
4.1 Preservation of Hamiltonians
4.2 Canonicity
4.3 Commutativity
4.4 Spectrality
4.5 Bäcklund transformation as discrete time dynamics
5 Dual Lax matrix
6 Discussion
A Proof of canonicity
B Proof of spectrality
C Derivation of the dual Lax matrix
References
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