Sharp isoperimetric inequalities on the Hamming cube II: The critical exponent

A sharp isoperimetric inequality for the Hamming cube is proved at the critical exponent $\beta =\frac{1}{2}$. This follows up on previous work, where such bounds were established for $\beta$ near $\frac{1}{2}$. As a consequence, this result settles a conjecture of Kahn and Park on cube partitions and yields a sharp $L^1$ Poincaré inequality for Boolean-valued functions. It also confirms a low-noise limit for balanced functions predicted by the Hellinger conjecture on noisy Boolean channels in information theory.