∀ {E : Type u_1} [inst : NormedAddCommGroup E] [inst_1 : NormedSpace ℂ E] {F : Type u_2} [inst_2 : NormedAddCommGroup F]
  [inst_3 : NormedSpace ℂ F] {H : Type u_3} [inst_4 : TopologicalSpace H] {I : ModelWithCorners ℂ E H} [I.Boundaryless]
  {M : Type u_4} [inst_6 : TopologicalSpace M] [inst_7 : ChartedSpace H M] [IsManifold I 1 M] [StrictConvexSpace ℝ F]
  {f : M → F} {U : Set M} {c : M},
  MDifferentiableOn I (modelWithCornersSelf ℂ F) f U →
    IsPreconnected U → IsOpen U → c ∈ U → IsMaxOn (norm ∘ f) U c → Set.EqOn f (Function.const M (f c)) U