<p>A mixed equation in a group <i>G</i> is given by a non-trivial element <i>w</i>(<i>x</i>) of the free product <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(G *\mathbb {Z}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>G</mi> <mrow /> <mo>∗</mo> <mi mathvariant="double-struck">Z</mi> </mrow> </math></EquationSource> </InlineEquation>, and a solution is some <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(g\in G\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>g</mi> <mo>∈</mo> <mi>G</mi> </mrow> </math></EquationSource> </InlineEquation> such that <i>w</i>(<i>g</i>) is the identity. For <i>G</i> acylindrically hyperbolic with trivial finite radical (e.g. torsion-free), we show that any mixed equation of length <i>n</i> has a non-solution of length comparable to <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\log (n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>log</mo> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>, which is the best possible bound. Similarly, we show that there is a common non-solution of length <i>O</i>(<i>n</i>) to all mixed equations of length <i>n</i>, again the best possible bound. In fact, in both cases, we show that a random walk of appropriate length yields a non-solution with positive probability.</p>

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Non-solutions to mixed equations in acylindrically hyperbolic groups coming from random walks

  • Henry Bradford,
  • Alessandro Sisto

摘要

A mixed equation in a group G is given by a non-trivial element w(x) of the free product \(G *\mathbb {Z}\) G Z , and a solution is some \(g\in G\) g G such that w(g) is the identity. For G acylindrically hyperbolic with trivial finite radical (e.g. torsion-free), we show that any mixed equation of length n has a non-solution of length comparable to \(\log (n)\) log ( n ) , which is the best possible bound. Similarly, we show that there is a common non-solution of length O(n) to all mixed equations of length n, again the best possible bound. In fact, in both cases, we show that a random walk of appropriate length yields a non-solution with positive probability.