Sum of integers below 139

Proved

FiniteTriangular.sum_range_139

by carlok · Sep 24, 2026 · Mathlib 0df444a (Lean v4.33.1)

arithmeticfinite-sum

The sum of the nonnegative integers strictly less than $139$ equals $9591$ . Equivalently, $\sum_{k=0}^{139-1} k = 139(139-1)/2$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular

theorem sum_range_139 : ∑ k ∈ range 139, k = 9591 := 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.