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arXiv · 2406.12322

Orbit-blocking words and the average-case complexity of Whitehead's problem in the free group of rank 2

Lucy Hyde·Siobhan O'Connor·Vladimir Shpilrain
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Abstract

Let F_2 denote the free group of rank 2. Our main technical result of independent interest is: for any element u of F_2, there is g in F_2 such that no cyclically reduced image of u under an automorphism of F_2 contains g as a subword. We then address computational complexity of the following version of the Whitehead automorphism problem: given a fixed u in F_2, decide, on an input v in F_2 of length n, whether or not v is an automorphic image of u. We show that there is an algorithm that solves this problem and has constant (i.e., independent of n) average-case complexity.

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Lucy Hyde, Siobhan O'Connor, Vladimir Shpilrain. 2024-06-18. Orbit-blocking words and the average-case complexity of Whitehead's problem in the free group of rank 2. https://arxiv.org/abs/2406.12322

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