An Erdős-Kac theorem for Smooth and Ultra-Smooth integers

Abstract: We prove an Erd\H{o}s-Kac type of theorem for the set $S(x,y)={n\leq x: p|n \Rightarrow p\leq y }$. If $\omega (n)$ is the number of prime factors of $n$, we prove that the distribution of $\omega(n)$ for $n \in S(x,y)$ is Gaussian for a certain range of $y$ using method of moments. The advantage of the present approach is that it recovers classical results for the range $u=o(\log \log x )$ where $u=\frac{\log x}{\log y}$, with a much simpler proof.