Originally [posted](https://math.stackexchange.com/questions/3268304/continuity-of-a-differential-of-a-banach-valued-holomorphic-map) on MSE. 


Let $U$ be an open set in $\mathbb{C}^{n}$ let $F$ be a Banach space (in my case even a dual Banach space), and let $\varphi:U\to F$ be a holomorphic map. I seem to be able to prove that the differential map $D\varphi:U\times\mathbb{C}^{n}\to F$ defined by $$D\varphi (z,v)= \lim\limits\_{t\to 0}\frac{\varphi(z+tv)-\varphi(z)}{t}$$ is holomorphic. 



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> Is there a reference for this assertion? (Or at least for continuity)
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I tried to look into some sources on infinite-dimensional holomorphicity and could not find such a statement, but some of those sources are rather complicated, and so it is likely I missed it.



Continuity of a differential of a Banach-valued holomorphic map
I don‘t know a reference but the result follows immediately from Hartogs‘ theorem (the fact that the image space is Banach is a bit of a red herring since a mapping into such a space is holomorphic if and only if is is weakly holomorphic).


