Section Plus.
Context {K : AbsRing} {V : NormedModule K}.
End Plus.

Lemma Derive_sum_n (f : nat -> R -> R) (n : nat) (x : R) :
(forall k, (k <= n)%nat -> ex_derive (f k) x) ->
Derive (fun y => sum_n (fun k => f k y) n) x = sum_n (fun k => Derive (f k) x) n.