Stability Conditions for Multigraded Rings

Abstract: Let $D$ be a finitely generated abelian group and $S$ a $D$-graded ring. We introduce a geometric semistability condition for points $x \in \Spec(S)$, characterized by maximal-dimensional orbit cones $σ(x)$. This set of geometrically semistable points $X^{\mathrm{gss}}$ yields a new framework for the $D$-graded Proj construction, which is equivalently given as the geometric quotient of $D(S_+) = \Spec(S) \setminus V(S_+)$ by the torus $\Spec(S_0[D])$, where $S_+ \unlhd S$ is the ideal generated by all relevant elements. We show that orbit cones are unions of relevant cones $\CC_D(f)$. This yields a chamber decomposition of the weight space $σ(S) = \overline{\Cone}(d \in D \mid S_d \neq 0)$, determined entirely by relevant elements. In particular, we obtain $\Proj^D(S) = X^{\mathrm{gss}}\sslash \Spec(S_0[D])$. As an application, for a simplicial toric (pre-)variety $X$ with full-dimensional convex support and $S = \Cox(X)$, this chamber decomposition of its weight space recovers the secondary fan of $X$. Consequently, when $d \in D = \Cl(X)$, the space $\Proj^D(S)$ is exactly the direct limit of all GIT quotients $\BA^n \sslash_{χ^d} \Spec(S_0[D])$ of $X$.