This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A061213 #18 Jul 02 2025 16:02:01
%S A061213 2,4,19,181,2701,56701,1587601,57153601,2571912001,141455160001,
%T A061213 9336040560001,728211163680001,66267215894880001,6958057668962400001,
%U A061213 834966920275488000001,113555501157466368000001
%N A061213 a(n) = 1 + product of first n positive triangular numbers (A000217).
%H A061213 Harry J. Smith, <a href="/A061213/b061213.txt">Table of n, a(n) for n=1..100</a>
%F A061213 a(n) = A006472(n+1) + 1.
%F A061213 a(n) = (n*(n+1)/2)*(a(n-1)-1) + 1.
%e A061213 a(6) = 1 * 3 * 6 * 10 *15 * 21 + 1 = 56701.
%p A061213 for n from 1 to 30 do printf(`%d,`,1+product(j*(j+1)/2, j=1..n)) od:
%o A061213 (PARI) a(n) = { 1 + prod(k=1, n, k*(k + 1)/2) } \\ _Harry J. Smith_, Jul 19 2009
%Y A061213 Cf. A000217, A006472.
%K A061213 nonn
%O A061213 1,1
%A A061213 _Amarnath Murthy_, Apr 21 2001
%E A061213 More terms from _James Sellers_, Apr 24 2001
%E A061213 Offset changed from 0 to 1 and example corrected by _Harry J. Smith_, Jul 19 2009