Source: wikibot/doubly-triangular-number
= Doubly triangular number
{wiki=Doubly_triangular_number}
A doubly triangular number is a figurate number that represents a triangular pyramid. In mathematical terms, a doubly triangular number can be derived by summing triangular numbers. The \\(n\\)-th triangular number \\(T_n\\) is given by the formula: \\\[ T_n = \\frac\{n(n + 1)\}\{2\} \\\] Doubly triangular numbers can also be expressed in a closed formula.