Problem: Let $f_1, f_2$ be two entire functions not constant. Prove that $e^{f_1} + e^{f_2} \equiv 1$ is impossible.
I tried to apply the Picard's theorem to investigate the left side of $e^{f_1} + e^{f_2} \equiv 1$, but fail to gain any progress. If we try to represent $f_1$ or $f_2$ by the other one, the multiplicity is inevitable.