Root and weight semigroup rings for signed posets

Abstract

We consider a pair of semigroups associated to a signed poset, called the root semigroup and the weight semigroup, and their semigroup rings, $R_P^\mathrm{rt}$ and $R_P^\mathrm{wt}$, respectively. Theorem 4.1.5 gives generators for the toric ideal of affine semigroup rings associated to signed posets and, more generally, oriented signed graphs. These are the subrings of Laurent polynomials generated by monomials of the form $t_i^{\pm 1},t_i^{\pm 2},t_i^{\pm 1}t_j^{\pm 1}$. This result appears to be new and generalizes work of Boussicault, F\'eray, Lascoux and Reiner, of Gitler, Reyes, and Villarreal, and of Villarreal. Theorem 4.2.12 shows that strongly planar signed posets $P$ have rings $R_P^\mathrm{rt}$, $R_{P^{\scriptscriptstyle\vee}}$ which are complete intersections, with Corollary 4.2.20 showing how to compute $\Psi_P$ in this case. Theorem 5.2.3 gives a Gr\"obner basis for the toric ideal of $R_P^{\mathrm{wt}}$ in type B, generalizing Proposition 6.4 of F\'eray and Reiner. Theorems 5.3.10 and 5.3.1 give two characterizations (via forbidden subposets versus via inductive constructions) of the situation where this Gr\"obner basis gives a complete intersection presentation for its initial ideal, generalizing Theorems 10.5 and 10.6 of F\'eray and Reiner.