theorem intra_level_exponential_convergence
(V : ℕ → ℝ) (lam : ℝ)
(hlam_pos : 0 < lam) (hlam_le : lam ≤ 1)
(hV_nonneg : ∀ k, 0 ≤ V k)
(hV_contract : ∀ k, V (k + 1) ≤ (1 - lam) * V k) :
∀ k, V k ≤ (1 - lam) ^ k * V 0 := by
intro k
induction k with
| zero => simp
| succ n ih =>
calc V (n + 1) ≤ (1 - lam) * V n := hV_contract n
_ ≤ (1 - lam) * ((1 - lam) ^ n * V 0) := by
apply mul_le_mul_of_nonneg_left ih (by linarith)
_ = (1 - lam) ^ (n + 1) * V 0 := by ringResults 1-7: Relaxation on the CES Potential Landscape