sums of zero-free entire functions and its siblings on the disk Can one describe the set $\{e^f+e^g: f, g\in H(C)\}$ in some way?
For example, in unital Banach algebras, every element  has this form.
I am in particular interested in the problem whether the function $u=(1-e^L)^2$ with
$L=(1-z)^{-3}$ is a sum of two exponentials in H(D).
Here $L$ is chosen so that every value is taken by u in D (hopefully).
 A: Describing the set $\{ e^f+e^g:f,g\in H(C)\}$ is difficult, and probably it cannot be described in any reasonable form. However many conditions can be given for
a function $h$ not to be of this form. For example, $h$ cannot be a square (or any power) of an entire function. Proof: Suppose that there are entire functions
$f,g,w$ such that $e^f+e^g=w^n$, and $w$ is not constant. Then $e^{f-g}$ omits zero, and all solutions of $e^{f(z)-g(z)}+1=0$ are multiple. By the Second Main theorem of Nevanlinna, $f-g$ must be constant, so $w$ must be constant, a contradiction.
For functions in the unit disk, this argument will prove that functions
of sufficiently fast growth are not representable in the form $e^f+e^g$.
In particular, this covers your example.
Indeed, using the notation of Nevanlinna theory, we have
$$T(r,u)\geq m(r,u)\geq c(1-r)^{-2},$$
so the conditions of the Second Main Theorem hold, and the above argument shows that $u$ is not a sum of two exponentials.
Reference: Hayman, Meromorphic functions, Chap. 2.
