Solutions to holomorphic differential equations are themselves holomorphic functions of time, initial conditions, and parameters.
Poincaré's Holomorphic Lemma
See also
Poincaré's LemmaExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Poincaré's Holomorphic Lemma." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PoincaresHolomorphicLemma.html