New approaches to $\mathfrak{gl}_N$ weight system

TL;DR

This paper introduces new methods for computing the $rak{gl}_N$ weight system using permutation invariants and the Harish-Chandra isomorphism, simplifying calculations in the universal enveloping algebra.

Contribution

It proposes two novel approaches—permutation invariants and Harish-Chandra isomorphism—for more efficient computation of the $rak{gl}_N$ weight system.

Findings

Permutation invariant approach simplifies calculations.

Harish-Chandra isomorphism reduces computations to commutative algebra.

Demonstrated methods with multiple examples.

Abstract

The present paper has been motivated by an aspiration for understanding the weight system corresponding to the Lie algebra $\mathfrak{gl}_N$. The straightforward approach to computing the values of a Lie algebra weight system on a general chord diagram amounts to elaborating calculations in the noncommutative universal enveloping algebra, in spite of the fact that the result belongs to the center of the latter. The first approach is based on a suggestion due to M. Kazarian to define an invariant of permutations taking values in the center of the universal enveloping algebra of $\mathfrak{gl}_N$. The restriction of this invariant to involutions without fixed points (such an involution determines a chord diagram) coincides with the value of the $\mathfrak{gl}_N$ -weight system on this chord diagram. We describe the recursion allowing one to compute the $\mathfrak{gl}_N$ -invariant of permutations and demonstrate how it works in a number of examples. The second approach is based on the Harish-Chandra isomorphism for the Lie algebras $\mathfrak{gl}_N$. This isomorphism identifies the center of the universal enveloping algebra $\mathfrak{gl}_N$ with the ring $\Lambda^*(N)$ of shifted symmetric polynomials in $N$ variables. The Harish-Chandra projection can be applied separately for each monomial in the defining polynomial of the weight system; as a result, the main body of computations can be done in a commutative algebra, rather than noncommutative one.