---
tag: 34APBKZNAV
kind: concept
name: Doubly triangular number
sources:
  - wikipedia: en:66308569@r1352114900
    locator: Doubly triangular number
    note: adopted
    mention: wikipedia://en/66308569
    sections: [Summary]
same_as:
  wikidata: Q105044211
  wikipedia: Doubly triangular number
inventory: [VVSMQTJB8Y]
---

## Summary

In mathematics, the **doubly triangular numbers** are the numbers that appear within the sequence of triangular numbers, in positions that are also triangular numbers. That is, if $T_n=n(n+1)/2$ denotes the $n$th triangular number, then the doubly triangular numbers are the numbers of the form $T_{T_n}$.