More precisely, given a holomorphic function $f$ on a simply connected domain $\Omega \subset \mathbb{C}$ where for any $z \in \Omega$, $z+1$ is also in $\Omega$, does there exist a holomorphic function $g(z)$ such that $g(z+1) = f(z) + g(z)$? Obviously, it won't be unique, but I'm wondering if existence is guaranteed. It seems like it should be but I don't know how to prove it.
I am aware of the Euler-Maclaurin formula which gives an asymptotic expression for $g(z)$ but it doesn't seem obvious to me how this would give a holomorphic function here.
Here is a simple proof when $\Omega = \mathbb{C}$. Here is a sophisticated proof for any $\Omega$. More precisely, the latter link proves the result whenever $\Omega/\mathbb{Z}$ is Stein but, since the map $z \mapsto \exp(2 \pi i z)$ identifies $\Omega/\mathbb{Z}$ with an open subset of $\mathbb{C}^{\ast}$, it will always be Stein.
I'm thinking about if I can remove the sophistication.