English
Differentiability within insert x s at x is equivalent to differentiability within s at x.
Русский
Дифференцируемость внутри вставки x s в x эквивалентна дифференцируемости внутри s в x.
LaTeX
$$$\mathrm{DifferentiableWithinAt } 𝕜 f (\mathrm{Set.insert} x\, s) x \iff \mathrm{DifferentiableWithinAt } 𝕜 f s x$$$
Lean4
theorem fderivWithin_of_mem_nhdsWithin (st : t ∈ 𝓝[s] x) (ht : UniqueDiffWithinAt 𝕜 s x)
(h : DifferentiableWithinAt 𝕜 f t x) : fderivWithin 𝕜 f s x = fderivWithin 𝕜 f t x :=
((DifferentiableWithinAt.hasFDerivWithinAt h).mono_of_mem_nhdsWithin st).fderivWithin ht