 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.