mean_value_problem_of_roots_same_norm

The following tighter bound depending on the degree $d$ of the polynomial $p$, in the case of $p$ all roots having the same norm has been shown by Tischler. $|p(z) - p(c)|/|z-c| \le (d-1)/d \cdot |p'(z)|$.

lemma mean_value_problem_of_roots_same_norm (p : Polynomial ℂ) (hp : 2 ≤ p.natDegree)
    (h : ∀ x y : ℂ, p.IsRoot x ∧ p.IsRoot y → ‖x‖=‖y‖) (z : ℂ) (K : ℝ) :
    ∃ c : ℂ, p.derivative.eval c = 0 ∧
      ‖p.eval z - p.eval c‖ / ‖z - c‖ ≤ (p.natDegree - 1)/ p.natDegree * ‖p.derivative.eval z‖

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