Sum of integers below 857

Proved

FiniteTriangular.sum_range_857

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

arithmeticfinite-sum

The sum of the nonnegative integers strictly less than $857$ equals $366796$ . Equivalently, $\\sum_{k=0}^{857-1} k = 857(857-1)/2$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular

theorem sum_range_857 : ∑ k ∈ range 857, k = 366796 := 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.