Recursion operators and the hierarchies of MKdV equations related to $D_4^{(1)}$, $D_4^{(2)}$ and $D_4^{(3)}$ Kac-Moody algebras

V. S. Gerdjikov, A. A. Stefanov, I. D. Iliev +4

We constructed the three nonequivalent gradings in the algebra $D_4\simeq so(8)$. The first one is the standard one obtained with the Coxeter automorphism $C_1=S_{α_2} S_{α_1}S_{α…