# Cosmetic two-strand twists on fibered knots

## TL;DR

This paper studies when certain modifications called two-strand twists on fibered knots in rational homology spheres produce the same knot, proving restrictions on when such cosmetic changes can occur and introducing the concept of weakly nugatory surgeries.

## Contribution

It establishes that fibered knots do not admit cosmetic generalized crossing changes and characterizes when odd-order two-strand twists are cosmetic, also introducing the notion of weakly nugatory surgeries.

## Key findings

- Fibered knots admit no cosmetic generalized crossing changes.
- Odd-order two-strand twists determined by separating arcs are cosmetic only if order is ±1.
- Every cosmetic band surgery on the unknot is weakly nugatory.

## Abstract

Let $K$ be a knot in a rational homology sphere $M$. This paper investigates the question of when modifying $K$ by adding $m>0$ half-twists to two oppositely-oriented strands, while keeping the rest of $K$ fixed, produces a knot isotopic to $K$. Such a two-strand twist of order $m$, as we define it, is a generalized crossing change when $m$ is even and a non-coherent band surgery when $m=1$. A cosmetic two-strand twist on $K$ is a non-nugatory one that produces an isotopic knot. We prove that fibered knots in $M$ admit no cosmetic generalized crossing changes. Further, we show that if $K$ is fibered, then a two-strand twist of odd order $m$ that is determined by a separating arc in a fiber surface for $K$ can only be cosmetic if $m=\pm 1$. After proving these theorems, we further investigate cosmetic two-strand twists of odd orders. Through two examples, we find that the second theorem above becomes false if `separating' is removed, and that a key technical proposition fails when the order equals 1. A closer look at an order-one example, an instance of cosmetic band surgery on the unknot, reveals it to be nearly trivial in a sense that we name `weakly nugatory'. We correct the technical proposition to obtain a means of using double branched covers to show that certain band surgeries are weakly nugatory. As an application, we prove that every cosmetic band surgery on the unknot is of this type.