Better than square-root cancellation and Gaussian behavior on Piatetski-Shapiro Sequences
In this paper, we investigate whether Harper's better than square-root cancellation phenomenon still appears in Piatetski-Shapiro sequences, and when one should expect only square-root size. If $c=c(X)$ satisfies $(c-1)\log X\to 0$ as $X\to\infty$, then for Steinhaus or Rademacher random multiplicative functions we have \[ \mathbb{E}\left|\sum_{n\in\mathcal{N}^{(c)}(X)}f(n)\right|=o\!\left(\sqrt{|\mathcal{N}^{(c)}(X)|}\right). \] For Lebesgue almost every fixed $1<c<\frac{8}{7}$, we prove the multiplicative energy satisfies \[ E^\times(\mathcal{N}^{(c)}(X))=2|\mathcal{N}^{(c)}(X)|^2-|\mathcal{N}^{(c)}(X)|+o\left(|\mathcal{N}^{(c)}(X)|^2\right). \] Together with a largest prime factor estimate, this allows us to apply the criterion of Soundararajan and Xu and obtain a standard complex Gaussian limit for the normalized Steinhaus sum. To the best of our knowledge, this gives the first asymptotic formula for the multiplicative energy of Piatetski-Shapiro sequences. For fixed exponent argument, our proof combines metric arguments with the Van der Corput's $B$-process and a double large sieve estimate with Robert-Sargos spacing theorem.