Shadow-complexity and trisection genus

TL;DR

This paper introduces a new shadow-complexity invariant for closed 4-manifolds, explores its relationship with the trisection genus, and classifies manifolds based on this invariant, providing new insights into 4-manifold complexity.

Contribution

It defines a parameterized shadow-complexity and establishes bounds relating it to the trisection genus, including exact calculations and classifications for certain manifolds.

Findings

Proves inequality g(W) ≤ 2 + 2 sc_r(W) for r ≥ 1/2.

Determines exact sc_{1/2} values for infinitely many 4-manifolds.

Classifies all 4-manifolds with sc_{1/2} ≤ 1/2.

Abstract

The shadow-complexity is an invariant of closed $4$-manifolds defined by using $2$-dimensional polyhedra called Turaev's shadows, which, roughly speaking, measures how complicated a $2$-skeleton of the $4$-manifold is. In this paper, we define a new version $\mathrm{sc}_{r}$ of shadow-complexity depending on an extra parameter $r\geq0$, and we investigate the relationship between this complexity and the trisection genus $g$. More explicitly, we prove an inequality $g(W) \leq 2+2\mathrm{sc}_{r}(W)$ for any closed $4$-manifold $W$ and any $r\geq1/2$. Moreover, we determine the exact values of $\mathrm{sc}_{1/2}$ for infinitely many $4$-manifolds, and also we classify all the closed $4$-manifolds with $\mathrm{sc}_{1/2}\leq1/2$.