English
There exists a thickness δ>0 such that the thickening of K sits inside U and derivatives behave nicely on it.
Русский
Существуют δ>0 такие, что ct thickening(K) ⊆ U и производные сходятся равномерно на этом множестве.
LaTeX
$$$\\exists δ>0\\; (\\text{cthickening } δ K) \\subseteq U \\text{ and } TendstoUniformlyOn (deriv \\circ F) (cderiv δ f) φ K$$$
Lean4
theorem cderiv_eq_deriv [CompleteSpace E] (hU : IsOpen U) (hf : DifferentiableOn ℂ f U) (hr : 0 < r)
(hzr : closedBall z r ⊆ U) : cderiv r f z = deriv f z :=
two_pi_I_inv_smul_circleIntegral_sub_sq_inv_smul_of_differentiable hU hzr hf (mem_ball_self hr)