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 A279042 #14 Dec 05 2016 05:27:42
%S A279042 4455,30537,461938302,3166172226,47894687058501,328275068740587,
%T A279042 4965816943137597372,34036215673995404100,514865832250497683700195,
%U A279042 3528942913182916419190605,53382319214430283898266055610,365887859090594924500524938502
%N A279042 Numbers k such that 2*k+1 and 10*k+1 are both triangular numbers (A000217).
%H A279042 Colin Barker, <a href="/A279042/b279042.txt">Table of n, a(n) for n = 1..350</a>
%H A279042 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,103682,-103682,-1,1).
%F A279042 a(n) = a(n-1) + 103682*a(n-2) - 103682*a(n-3) - a(n-4) + a(n-5) for n>5.
%F A279042 G.f.: 81*x*(55 + 322*x + 55*x^2) / ((1 - x)*(1 - 322*x + x^2)*(1 + 322*x + x^2)).
%e A279042 4455 is in the sequence because 2*4455+1 = 8911 and 10*4455+1 = 44551 are both triangular numbers.
%t A279042 LinearRecurrence[{1, 103682, -103682, -1, 1}, {4455, 30537, 461938302, 3166172226, 47894687058501}, 20] (* _Vincenzo Librandi_, Dec 05 2016 *)
%o A279042 (PARI) Vec(81*x*(55 + 322*x + 55*x^2) / ((1 - x)*(1 - 322*x + x^2)*(1 + 322*x + x^2)) + O(x^15))
%o A279042 (PARI) isok(k) = ispolygonal(2*k+1, 3) & ispolygonal(10*k+1, 3)
%Y A279042 Cf. A000217, A124174, A274579, A274603, A274680, A274756, A274832.
%K A279042 nonn,easy
%O A279042 1,1
%A A279042 _Colin Barker_, Dec 04 2016