feat: port Archive.Wiedijk100Theorems.InverseTriangleSum (#5172)

Archive.lean

@@ -13,3 +13,4 @@ import Archive.Imo.Imo2011Q5
 import Archive.Imo.Imo2019Q4
 import Archive.Imo.Imo2020Q2
 import Archive.Wiedijk100Theorems.AreaOfACircle
+import Archive.Wiedijk100Theorems.InverseTriangleSum

Archive/Wiedijk100Theorems/InverseTriangleSum.lean

@@ -0,0 +1,43 @@
+/-
+Copyright (c) 2020. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Jalex Stark, Yury Kudryashov
+
+! This file was ported from Lean 3 source module wiedijk_100_theorems.inverse_triangle_sum
+! leanprover-community/mathlib commit 5563b1b49e86e135e8c7b556da5ad2f5ff881cad
+! Please do not edit these lines, except to modify the commit id
+! if you have ported upstream changes.
+-/
+import Mathlib.Algebra.BigOperators.Basic
+import Mathlib.Data.Real.Basic
+
+/-!
+# Sum of the Reciprocals of the Triangular Numbers
+
+This file proves Theorem 42 from the [100 Theorems List](https://www.cs.ru.nl/~freek/100/).
+
+We interpret “triangular numbers” as naturals of the form $\frac{k(k+1)}{2}$ for natural `k`.
+We prove that the sum of the reciprocals of the first `n` triangular numbers is $2 - \frac2n$.
+
+## Tags
+
+discrete_sum
+-/
+
+
+open scoped BigOperators
+
+open Finset
+
+/-- **Sum of the Reciprocals of the Triangular Numbers** -/
+theorem Theorem100.inverse_triangle_sum :
+    ∀ n, ∑ k in range n, (2 : ℚ) / (k * (k + 1)) = if n = 0 then 0 else 2 - (2 : ℚ) / n := by
+  refine' sum_range_induction _ _ (if_pos rfl) _
+  rintro (_ | n)
+  · rw [if_neg, if_pos] <;> norm_num
+  simp_rw [if_neg (Nat.succ_ne_zero _), Nat.succ_eq_add_one]
+  have A : (n + 1 + 1 : ℚ) ≠ 0 := by norm_cast; norm_num
+  push_cast
+  field_simp [Nat.cast_add_one_ne_zero]
+  ring
+#align theorem_100.inverse_triangle_sum Theorem100.inverse_triangle_sum