Open-Closed TQFT String Operations for Disc Cobordisms, Simultaneous Saddle Interactions, and Constant Homology Classes

TL;DR

This paper studies genus zero open-closed TQFT string operations, focusing on disc cobordisms and simultaneous saddle interactions, revealing their independence from decompositions and their relation to constant homology classes.

Contribution

It introduces a new approach to computing disc string operations via simultaneous saddle interactions and characterizes their homology class values.

Findings

Disc string operations are independent of half-pair-of-pants decompositions.

Simultaneous saddle interactions can compute these operations.

Operations take values in homology classes of constant open strings.

Abstract

Previously, we showed that most of the open-closed topological quantum field theory (TQFT) string operations vanish including all the higher genus TQFT operations, and we described a small list of genus zero open-closed TQFT string operations which can be nontrivial. In this paper, we consider open-closed string operations associated to open-closed cobordisms homeomorphic to discs. These operations constitute the main part of genus zero string operations, and they include the saddle string operation of two open strings interacting at their internal points. We show not only that disc string operations are independent of their half-pair-of-pants decompositions but also that these disc string operations can be computed by \emph{simultaneous} saddle interactions of incoming open strings at the same point, and they take values in homology classes of constant open strings on some closed orientable submanifolds, which we will precisely determine. We will also discuss a role played by fundamental constant homology classes in open-closed string topology. Our main tools are saddle interaction diagrams and their deformations.