English

Circular sorting in the alternating group

Combinatorics 2026-08-06 v1

Abstract

The symmetric group SnS_n is generated by transpositions, and problems of sorting permutations using transpositions are well studied. In recent work, Adin, Alon, and Roichman studied the related problem of sorting nn points on a circle, and gave a formula for the maximum number of adjacent swaps required. This is equivalent to the number of adjacent transpositions required to transform any permutation into a power of the cyclic permutation (1,2,,n)(1,2,\ldots, n). The focus of this work is an analogous question in the alternating group AnA_n, which is generated by 33-cycles. That is, using 3-cycles instead of transpositions, what is the maximum number of steps required to transform an even permutation into a power of (1,2,,n)(1,2,\ldots, n) in the alternating group? We determine this number exactly for even nn and n1(mod4)n \equiv 1 \pmod{4}. For n3(mod4)n \equiv 3 \pmod{4}, we show that the sorting number can take one of two possible values and give explicit constructions demonstrating that the larger value occurs infinitely often.

Cite

@article{arxiv.2608.06338,
  title  = {Circular sorting in the alternating group},
  author = {Melanie Ferreri and Eric Swartz and Nicholas J. Werner},
  journal= {arXiv preprint arXiv:2608.06338},
  year   = {2026}
}

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20 pages