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 A276915 #23 Dec 17 2016 10:41:40
%S A276915 0,1,10,143,1988,27693,385710,5372251,74825800,1042188953,14515819538,
%T A276915 202179284583,2815994164620,39221739020101,546288352116790,
%U A276915 7608815190614963,105977124316492688,1476070925240282673,20559015829047464730,286350150681424223551
%N A276915 Indices of triangular numbers in A276914 which are also pentagonal.
%C A276915 A276914(a(n)) = A014979(n + 1). All numbers which are both triangular and pentagonal can be found in sequence A276914.
%H A276915 Daniel Poveda Parrilla, <a href="/A276915/b276915.txt">Table of n, a(n) for n = 0..200</a>
%H A276915 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (13,13,-1).
%F A276915 a(n) = 14*a(n-1) - a(n-2) - 4*(-1)^n for n>1, a(0)=0, a(1)=1.
%F A276915 a(n) = (A046175(n) + (A046175(n) mod 2))/2.
%F A276915 From _Colin Barker_, Sep 23 2016: (Start)
%F A276915 G.f.: x*(1 - 3*x) / ((1 + x)*(1 - 14*x + x^2)).
%F A276915 a(n) = 13*a(n-1) + 13*a(n-2) - a(n-3) for n>2.
%F A276915 a(n) = ( -6*(-1)^n + (3+sqrt(3))*(7-4*sqrt(3))^n - (-3+sqrt(3))*(7+4*sqrt(3))^n )/24. (End)
%t A276915 RecurrenceTable[{a[n] == 14 a[n - 1] - a[n - 2] - 4 (-1)^n, a[0] == 0, a[1] == 1}, a, {n, 19}] (* _Michael De Vlieger_, Sep 23 2016 *)
%o A276915 (PARI) concat(0, Vec(x*(1-3*x)/((1+x)*(1-14*x+x^2)) + O(x^30))) \\ _Colin Barker_, Sep 23 2016
%Y A276915 Cf. A014979, A046175, A276914.
%K A276915 nonn,easy
%O A276915 0,3
%A A276915 _Daniel Poveda Parrilla_, Sep 22 2016