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 A329756 #10 Feb 16 2025 08:33:58
%S A329756 0,1,456,14976,181780,1273970,6293756,24395756,79119496,223821235,
%T A329756 568280240,1321714636,2858876956,5817509516,11237224740,20751835560,
%U A329756 36849296016,63215722181,105182448536,170297734920,269047574180,415753060646,629674964556,936359517556
%N A329756 Doubly heptagonal pyramidal numbers.
%H A329756 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/HeptagonalPyramidalNumber.html">Heptagonal Pyramidal Number</a>
%H A329756 <a href="/index/Ps#pyramidal_numbers">Index to sequences related to pyramidal numbers</a>
%H A329756 <a href="/index/Rec#order_10">Index entries for linear recurrences with constant coefficients</a>, signature (10,-45,120,-210,252,-210,120,-45,10,-1).
%F A329756 G.f.: x*(1 + 446*x + 10461*x^2 + 52420*x^3 + 75580*x^4 + 32544*x^5 + 3504*x^6 + 44*x^7)/(1 - x)^10.
%F A329756 a(n) = A002413(A002413(n)).
%F A329756 a(n) = Sum_{k=0..A002413(n)} A000566(k).
%F A329756 a(n) = n *(5*n-2) *(n+1) *(5*n^3+3*n^2-2*n+6) *(25*n^3+15*n^2-10*n-12)/1296. - _R. J. Mathar_, Nov 28 2019
%t A329756 A002413[n_] := n (n + 1) (5 n - 2)/6; a[n_] := A002413[A002413[n]]; Table[a[n], {n, 0, 25}]
%t A329756 Table[Sum[k (5 k - 3)/2, {k, 0, n (n + 1) (5 n - 2)/6}], {n, 0, 25}]
%t A329756 nmax = 25; CoefficientList[Series[x (1 + 446 x + 10461 x^2 + 52420 x^3 + 75580 x^4 + 32544 x^5 + 3504 x^6 + 44 x^7)/(1 - x)^10, {x, 0, nmax}], x]
%t A329756 LinearRecurrence[{10, -45, 120, -210, 252, -210, 120, -45, 10, -1}, {0, 1, 456, 14976, 181780, 1273970, 6293756, 24395756, 79119496, 223821235}, 26]
%Y A329756 Cf. A000566, A002413, A140236, A264891, A329753, A329754, A329755, A329757.
%K A329756 nonn,easy
%O A329756 0,3
%A A329756 _Ilya Gutkovskiy_, Nov 20 2019