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 A081065 #26 Mar 14 2016 20:02:48
%S A081065 2,24,330,4592,63954,890760,12406682,172802784,2406832290,33522849272,
%T A081065 466913057514,6503259955920,90578726325362,1261598908599144,
%U A081065 17571805994062650,244743685008277952,3408839784121828674,47479013292697323480,661297346313640700042
%N A081065 Numbers n such that n^2 = (1/3)*(n+floor(sqrt(3)*n*floor(sqrt(3)*n))).
%C A081065 a(n)/2 gives indices of pentagonal numbers which are also triangular.
%C A081065 a(n) itself gives x-values solving the Diophantine equation 2*x^2 + (x-1)^2 = y^2.
%H A081065 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (15,-15,1).
%F A081065 a(n) = 15*a(n-1) - 15*a(n-2) + a(n-3).
%F A081065 a(n) = 14*a(n-1) - a(n-2) - 4. [_Sture Sjöstedt_, May 02 2011]
%F A081065 G.f.: 2*(1-3*x)/((1-x)*(1-14*x+x^2)). - _Bruno Berselli_, Nov 11 2011
%t A081065 LinearRecurrence[{15,-15,1},{2,24,330},20] (* _Harvey P. Dale_, Mar 14 2016 *)
%o A081065 (PARI) Vec(2*(1-3*x)/((1-x)*(1-14*x+x^2)) + O(x^40)) \\ _Michel Marcus_, Nov 17 2014
%Y A081065 Cf. A046090, A046174, A046175.
%K A081065 nonn,easy
%O A081065 1,1
%A A081065 _Benoit Cloitre_, Apr 15 2003