Asymptotics for smooth numbers in short intervals

Abstract

A number is said to be $y$-smooth if all of its prime factors are less than or equal to $y.$ For all $17/30<θ\leq 1,$ we show that the density of $y$-smooth numbers in the short interval $[x,x+x^θ]$ is asymptotically equal to the density of $y$-smooth numbers in the long interval $[1,x],$ for all $y \geq \exp((\log x)^{2/3+\varepsilon}).$ Assuming the Riemann Hypothesis, we also prove that for all $1/2<θ\leq 1$ there exists a large constant $K$ such that the expected asymptotic result holds for $y\geq (\log x)^{K}.$ Our approach is to count smooth numbers using a Perron integral, shift this to a particular contour left of the saddle point, and employ a zero-density estimate of the Riemann zeta function.