Smooth integers and the Dickman $\rho$ function

We establish an asymptotic formula for $\mathrm{\Psi }(x,y)$ whose shape is $x\rho (\log x\mathrm{/}\log y)$ times correction factors. These factors take into account the contributions of zeta zeros and prime powers and the formula can be regarded as an (approximate) explicit formula for $\mathrm{\Psi }(x,y)$. With this formula at hand we prove oscillation results for $\mathrm{\Psi }(x,y)$, which resolve a question of Hildebrand on the range of validity of $\mathrm{\Psi }(x,y)\asymp x\rho (\log x\mathrm{/}\log y)$. We also address a question of Pomerance on the range of validity of $\mathrm{\Psi }(x,y)\ge x\rho (\log x\mathrm{/}\log y)$. Along the way we improve classical estimates for $\mathrm{\Psi }(x,y)$ and, on the Riemann Hypothesis, uncover an unexpected phase transition of $\mathrm{\Psi }(x,y)$ at $y=(\log x{)}^{3\mathrm{/}2+o(1)}$.