Example Problems

Example 1

TopologyGraduate

In the setting of 2525-strand braids, let σi\sigma_i for i=1,,24i=1,\dots,24 denote the positive braid generator in which the ii-th strand crosses over the (i+1)(i+1)-st strand.

Consider the braid diagram β\beta obtained by stacking the standard diagrams of the generators σ24σ23σ1σ2σ3σ24\sigma_{24}\sigma_{23}\cdots\sigma_1 \sigma_2\sigma_3\cdots\sigma_{24} from bottom to top.

Construct an admissible system of 2525 curves as follows. Process the crossings of β\beta from top to bottom, and at each crossing push the incoming understrand (the lower branch) along the overstrand (the upper branch) to the outgoing endpoint of the overstrand (the upper endpoint).

This procedure yields a family of 2525 pairwise disjoint simple curves in the upper half-plane connecting the bottom points (i,0)(i,0) to the top points (j,1)(j,1) for i,j=1,,25i,j=1,\dots,25, and hence determines a permutation πS25\pi\in S_{25} defined by π(i)=j\pi(i)=j.

It is given that π\pi consists only of fixed points and disjoint adjacent transpositions. Compute the number of fixed points of π\pi.

23

Model Results

ModelAnswerCorrect?
GPT-5.2-extended25
Gemini 3 Pro23
Gemini 3 Pro (AIStudio+code)1
Grok 4.1 Fast9
Grok 4 Expert1
Claude Sonnet 4.5 (thinking)25
Claude Opus 4.5 (thinking)23

Single-attempt evaluation · Jan 23, 2026

Analysis

The question feels hard because it asks you to read a global permutation from a diagram built out of many local crossings. If one follows the rule literally, this quickly becomes an error-prone tracing problem, since 25 paths must be tracked through many interactions. The simplifying idea is that the construction is stable under braid deformations. The endpoint pairing depends only on the braid class, not on the chosen diagram. One can therefore rearrange the crossings to a simpler equivalent form where the endpoint action is easy to see, and the fixed points can be counted without tracing every curve.

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