Sum of integers below 786

Proved

FiniteTriangular.sum_range_786

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

arithmeticfinite-sum

The sum of the nonnegative integers strictly less than $786$ equals $308505$ . Equivalently, $\\sum_{k=0}^{786-1} k = 786(786-1)/2$ .

Preamble

import Mathlib
open Finset

Formal statement

namespace FiniteTriangular

theorem sum_range_786 : ∑ k ∈ range 786, k = 308505 := 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.