Problem #459: Let $f(u)$ be the largest $v$ such that no $m\in (u,v)$ is...

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Progress

An equivalent definition is that $v$ is the smallest number $>u$ such that all prime factors of $v$ are factors of $u$. In particular, we trivially have\[u+2\leq f(u)\leq u^2.\]Cambie has noted that when $u=p$ is a prime, $f(p)=p^2$, and if $u$ is even then $f(u)\leq 2^k$, where $2^k$ is the smallest power of $2$ which is $>u$. In particular, $f(u)=u+2$ whenever $u=2^k-2$ for $k\geq 2$.

In particular the function $f(u)$ does not exhibit any regular growth, since there are infinitely many $u$ such that $f(u)=u+2$ and infinitely many $u$ such that $f(u)=u^2$. Cambie can also show that $f(n)=(1+o(1))n$ for almost all $n$. It is not clear what kind of estimates Erdős and Graham had in mind. Since Cambie's observations seem to settle the most obvious questions, I will mark this as solved.