On maximal commutative subalgebras of Poisson algebras associated with involutions of semisimple Lie algebras

Abstract: For any involution $\sigma$ of a semisimple Lie algebra $\mathfrak g$, one constructs a non-reductive Lie algebra $\mathfrak k$, which is called a $\mathbb Z_2$-contraction of $\mathfrak g$. In this paper, we attack the problem of describing maximal commutative subalgebras of the Poisson algebra $S(\mathfrak k)$. This is closely related to the study of the coadjoint representation of $\mathfrak k$ and the set, $\mathfrak k^*_{reg}$, of the regular elements of $\mathfrak k^*$. By our previous results, in the context of $\mathbb Z_2$-contractions, the argument shift method provides maximal commutative subalgebras of $S(\mathfrak k)$ whenever $codim(\mathfrak k^* \setminus \mathfrak k^*_{reg})\ge 3$. Our main result here is that $codim(\mathfrak k^* \setminus \mathfrak k^*_{reg})\ge 3$ if and only if the Satake diagram of $\sigma$ has no trivial nodes. (A node is trivial, if it is white, has no arrows attached, and all adjacent nodes are also white.) The list of suitable involutions is provided. We also describe certain maximal commutative subalgebras of $S(\mathfrak k)$ if the (-1)-eigenspace of $\sigma$ in $\mathfrak g$ contains regular elements.