Sum of integers below 141

Proved

FiniteTriangular.sum_range_141

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

arithmeticfinite-sum

The sum of the nonnegative integers strictly less than $141$ equals $9870$ . Equivalently, $\sum_{k=0}^{141-1} k = 141(141-1)/2$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular

theorem sum_range_141 : ∑ k ∈ range 141, k = 9870 := 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.