KnowraTriangular numberLinked fromLinked fromThe 5 pages that link to Triangular number, each with the reason it gives.All 5Broader topic 3Related 2Arithmetic progressionBroader topic: Its defining sum is a finite arithmetic progression with common difference one.Hockey-stick identityBroader topic: Setting the lower binomial index to one reduces the identity to a triangular-number sum.Fermat polygonal number theoremBroader topic: The three-sided case is Gauss’s theorem that every nonnegative integer is a sum of three triangular numbers.