Article
Ufa Mathematical Journal
Volume 13, Number 2, pp. 115-134
On mKdV equations related to Kac-Moody algebras $A_5^{(1)}$ and $A_5^{(2)}$.
Gerdjikov V.S.
DOI:10.13108/2021-13-2-115
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We outline the derivation of the mKdV equations related to the Kac--Moody algebras $A_5^{(1)}$ and $A_5^{(2)}$.
First we formulate their Lax representations and provide details how they
can be obtained from generic Lax operators related to the algebra $sl(6)$
by applying proper Mikhailov type reduction groups $\mathbb{Z}_h$. Here $h$ is the Coxeter number of the relevant Kac--Moody algebra. Next we adapt Shabat's method for constructing the fundamental analytic solutions of the Lax operators $L$. Thus we are able to reduce the direct and inverse spectral problems for $L$ to Riemann--Hilbert problems (RHP) on the union of $2h$ rays $l_\nu$. They leave the origin of the complex $\lambda$-plane partitioning it into equal angles $\pi/h$. To each $l_\nu$ we associate a subalgebra $\mathfrak{g}_\nu$ which is a direct sum of $sl(2)$--subalgebras. In this way, to each regular solution of the RHP we can associate scattering data of $L$ consisting of scattering matrices $T_\nu \in \mathcal{G}_\nu$ and their Gauss decompositions. The main result of the paper states how to find the minimal sets of scattering data $\mathcal{T}_k$, $k=1,2$,
from $T_0$ and $T_1$ related to the rays $l_0$ and $l_1$. We prove that each of the minimal sets $\mathcal{T}_1$ and $\mathcal{T}_2$ allows one to reconstruct both the scattering matrices $T_\nu$, $\nu =0, 1, \dots 2h$ and the corresponding potentials of the Lax operators $L$.