The Davenport-Lewis-Schinzel problem on the reducibility of $f(X)-g(Y)$

We solve the problem of Davenport–Lewis–Schinzel (DLS), originating in the 1950s, regarding the reducibility of $f(X)-g(Y)\in \mathbb{C}[X,Y]$. This yields an almost-complete solution to the Hilbert–Siegel problem: For a polynomial map $f$ whose composition factors avoid only very specific low-degree polynomials, we explicitly describe over which integers the fibers of $f$ are reducible. We further apply the solution to stability of iterates of $f$ in arithmetic dynamics, and to solving the functional equation $f(X)=g(Y)$ in $X,Y\in \mathbb{C}(z)$.