Sum of integers below 211

Proved

FiniteTriangular.sum_range_211

by carlok · Sep 25, 2026 · Mathlib 0df444a (Lean v4.33.1)

arithmeticfinite-sum

The sum of the nonnegative integers strictly less than $211$ equals $22155$ . Equivalently, $\\sum_{k=0}^{211-1} k = 211(211-1)/2$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular

theorem sum_range_211 : ∑ k ∈ range 211, k = 22155 := by sorry

end FiniteTriangular

Source

Elementary arithmetic series: the sum of the first m positive integers is m(m+1)/2, rewritten on Finset.range.