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Reference.org
Doubly triangular number
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<p>In mathematics, the doubly triangular numbers are the numbers that appear within the <a href="/facts/Integer_sequence/4l3pev7C">sequence</a> of <a href="/facts/Triangular_number/KHonHtQJ">triangular numbers</a>, in positions that are also triangular numbers. That is, if T n = n ( n + 1 ) / 2 {\displaystyle T_{n}=n(n+1)/2} denotes the n {\displaystyle n} th triangular number, then the doubly triangular numbers are the numbers of the form T T n {\displaystyle T_{T_{n}}} . </p>