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First page of Palindromic Length in Free Groups: Reflections, Noncrossing Matchings, and Catalan Forms

Palindromic Length in Free Groups: Reflections, Noncrossing Matchings, and Catalan Forms

Junjie Liao

math.GR Sep 15, 2026 · v1
A Lean 4 development verifies the four-palindrome classification for free groups of every finite rank, including the literal five-form conclusion.
Let $F=F(X)$ be a free group of finite rank, with palindromic length taken with respect to the fixed basis $X$. We embed $F$ as the index-two subgroup of the universal Coxeter group $W=F\rtimes_θ\langle t\mid t^2=1\rangle$, where $θ(x)=x^{-1}$ for $x\in X$, and prove $\mathrm{pl}(g)=\min{\ell_T(g),\ell_T(gt)}$. Dyer's deletion theorem then identifies reflection length with the minimum number of unmatched positions in a noncrossing equal-label partial matching on a reduced Coxeter word. This gives an $O(n^3)$-time, $O(n^2)$-space algorithm for palindromic length, together with recovery of an optimal palindromic factorization. The matching model also gives a structural characterization. For every ordered full binary tree with $k$ leaves we define a literal word template whose leaves are palindromes and whose internal vertices carry arbitrary words. A reduced word $w$ represents an element of palindromic length at most $k$ if and only if $w$ is a literal instance of one of these templates. Hence the $C_{k-1}$ ordered binary-tree shapes give a complete finite family for each fixed $k$. For $k=4$ the five templates are exactly the five forms proposed by Frid, proving the completeness of that list. A companion Lean 4 development verifies the four-palindrome classification end to end for every finite rank, including the ordinary reduced-word formulation and the literal five-form conclusion.

Computing the palindromic length of an element of a free group with respect to a fixed basis is an open problem (Bardakov–Shpilrain–Tolstykh Problem 2 / Kourovka 16.9). Frid described lengths at most three and proposed five forms for length four, leaving completeness open.

The free group is embedded as an index-two subgroup of the universal Coxeter group W = F ⋊ ⟨t | t²=1⟩, giving pl(g) = min{ℓ_T(g), ℓ_T(gt)}. Dyer's deletion theorem recasts reflection length as the minimum number of unmatched positions in a noncrossing equal-label matching on a reduced Coxeter word, yielding a cubic-time dynamic program. Optimal matchings are reorganized into ordered full binary trees whose contours reproduce Catalan-form word templates. A Lean 4 development verifies the four-palindrome classification end to end for every finite rank.

An O(n³)-time, O(n²)-space algorithm computes palindromic length and recovers optimal factorizations. For each fixed k the C_{k-1} binary-tree shapes give a complete finite template family; for k=4 the five templates coincide exactly with Frid's proposed forms, proving their completeness, formally verified in Lean 4.