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 A225839 #27 Jan 16 2019 15:15:28
%S A225839 0,378,17766,39209940,1842032556,4065365016846,190985619471570,
%T A225839 421505175637435176,19801770996209306328,43702499616375188919330,
%U A225839 2053087220237987679246270,4531162564803507161896556028,212868189148913267563402477956,469799997000254729943383533193910
%N A225839 Triangular numbers representable as triangular(m) + triangular(2m).
%C A225839 Triangular numbers of the sequence A147875: a(n) = A147875(A225785(n)) - see also _Ralf Stephan_ in Program lines. [_Bruno Berselli_, May 20 2013]
%H A225839 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,103682,-103682,-1,1).
%F A225839 G.f.: 378*x*(1+46*x+x^2)/((1-x)*(1-322*x+x^2)*(1+322*x+x^2)). [_Bruno Berselli_, May 20 2013]
%t A225839 CoefficientList[Series[378 x (1 + 46 x + x^2)/((1 - x) (1 - 322 x + x^2) (1 + 322 x + x^2)), {x, 0, 20}], x] (* _Bruno Berselli_, May 20 2013 *)
%t A225839 LinearRecurrence[{1,103682,-103682,-1,1},{0,378,17766,39209940,1842032556},20] (* _Harvey P. Dale_, Jan 16 2019 *)
%o A225839 (PARI) for(n=1,10^9,t=n*(5*n+3)/2;x=sqrtint(2*t);if(t==x*(x+1)/2,print(t))) \\ _Ralf Stephan_, May 17 2013
%Y A225839 Cf. A000217, A147875.
%Y A225839 Cf. A108281 (triangular numbers representable as triangular(m) + m^2).
%Y A225839 Cf. A225785 (numbers n such that triangular(n) + triangular(2n) is a triangular number).
%K A225839 nonn,easy
%O A225839 1,2
%A A225839 _Alex Ratushnyak_, May 17 2013
%E A225839 More terms from _Bruno Berselli_, May 20 2013