# The C-complex clasp number of links

## TL;DR

This paper investigates the minimal number of clasps in C-complexes bounded by a link, relating it to linking numbers and crossing changes, thereby advancing understanding of link complexity.

## Contribution

It establishes that for 2-component links, the linking number determines the minimal number of clasps, and for 3-component links, the triple linking number provides an additional bound.

## Key findings

- Linking number determines minimal clasps for 2-component links.
- Triple linking number bounds clasps in 3-component links.
- C-complex clasp number relates to crossing changes to boundary links.

## Abstract

In the 1980's Daryl Cooper introduced the notion of a C-complex (or clasp-complex) bounded by a link and explained how to compute signatures and polynomial invariants using a C-complex. Since then this was extended by works of Cimasoni, Florens, Mellor, Melvin, Conway, Toffoli, Friedl, and others to compute other link invariants. Informally a C-complex is a union of surfaces which are allowed to intersect each other in clasps. The purpose of the current paper is to study the minimal number of clasps amongst all C-complexes bounded by a fixed link $L$. This measure of complexity is related to the number of crossing changes needed to reduce $L$ to a boundary link. We prove that if $L$ is a 2-component link with nonzero linking number, then the linking number determines the minimal number of clasps amongst all C-complexes. In the case of 3-component links, the triple linking number provides an additional lower bound on the number of clasps in a C-complex.