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 A218324 #15 Aug 24 2025 11:35:03
%S A218324 1,115,645,1911,4233,7931,13325,20735,30481,42883,58261,76935,99225,
%T A218324 125451,155933,190991,230945,276115,326821,383383,446121,515355,
%U A218324 591405,674591,765233,863651,970165,1085095,1208761,1341483,1483581,1635375,1797185,1969331
%N A218324 Odd heptagonal pyramidal numbers.
%H A218324 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (4,-6,4,-1).
%F A218324 a(n) = 4*a(n-1) - 6*a(n-2) + 4*a(n-3) - a(n-4).
%F A218324 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) + 320.
%F A218324 a(n) = (2*n-1)*(4*n-3)*(20*n-17)/3.
%F A218324 G.f.: x*(1 + 111*x + 191*x^2 + 17*x^3)/(1-x)^4.
%F A218324 E.g.f.: 17 + exp(x)*(160*x^3 + 144*x^2 + 54*x - 51)/3. - _Elmo R. Oliveira_, Aug 23 2025
%e A218324 The sequence of heptagonal pyramidal numbers A002413 begins 1, 8, 26, 60, 115, 196, 308, 456, 645, 880, ... . As the third odd term is 645, a(3) = 645.
%t A218324 LinearRecurrence[{4,-6,4,-1},{1,115,645,1911},34]
%Y A218324 Cf. A002413, A218325.
%K A218324 nonn,easy,changed
%O A218324 1,2
%A A218324 _Ant King_, Oct 26 2012