Wedge operations and torus symmetries II

TL;DR

This paper advances the classification of toric spaces by developing methods to identify all characteristic maps on simplicial complexes obtained through wedging, solving a longstanding conjecture and providing a practical classification approach.

Contribution

It introduces a finite classification of seeds with fixed Picard number and a combinatorial puzzle method to find all characteristic maps on wedged complexes.

Findings

Finiteness of seeds with fixed Picard number supporting characteristic maps.

Affirmative proof of Batyrev's 1991 conjecture.

A systematic combinatorial method to classify toric spaces.

Abstract

A fundamental idea in toric topology is that classes of manifolds with well-behaved torus actions (simply, toric spaces) are classified by pairs of simplicial complexes and (non-singular) characteristic maps. The authors in their previous paper provided a new way to find all characteristic maps on a simplicial complex $K(J)$ obtainable by a sequence of wedgings from $K$. The main idea was that characteristic maps on $K$ theoretically determine all possible characteristic maps on a wedge of $K$. In this work, we further develop our previous work for classification of toric spaces. For a star-shaped simplicial sphere $K$ of dimension $n-1$ with $m$ vertices, the Picard number $\operatorname{Pic}(K)$ of $K$ is $m-n$. We refer to $K$ a seed if $K$ cannot be obtained by wedgings. First, we show that, for a fixed positive integer $\ell$, there are at most finitely many seeds of Picard number $\ell$ supporting characteristic maps. As a corollary, the conjecture proposed by V. V. Batyrev in 1991 is solved affirmatively. Second, we investigate a systematic way to find all characteristic maps on $K(J)$ using combinatorial objects called (realizable) puzzles that only depend on a seed $K$. These two facts lead to a practical way to classify the toric spaces of fixed Picard number.