Sharp isoperimetric inequalities on the Hamming cube near the critical exponent

Abstract

An isoperimetric inequality on the Hamming cube for exponents $\beta \ge 0.50057$ is proved, achieving equality on any subcube. This was previously known for $\beta \ge {\log }_2(3/2)\approx 0.585$. Improved bounds are also obtained at the critical exponent $\beta =0.5$, including a bound that is asymptotically sharp for small subsets. A key ingredient is a new Bellman-type function involving the Gaussian isoperimetric profile which appears to be a good approximation of the true envelope function. Verification uses computer-assisted proofs and interval arithmetic. Applications include progress towards a conjecture of Kahn and Park as well as sharp Poincaré inequalities for Boolean-valued functions near $L^1$.

46 pages, 8 figures; final version to appear in Adv. Math.; accompanying code at https://github.com/roos-j/dir24-isoperim