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 A274587 #13 Jan 18 2021 17:20:59
%S A274587 1,23,176,5968,888778,30192278,233944673,7947232183,1183597668523,
%T A274587 40207478867501,311547395822378,10583440358908726,1576213585538112676,
%U A274587 53544862512524597468,414892028679967914251,14094115694115827467213,2099065698850118586101173
%N A274587 Values of n such that 2*n-1 and 4*n-1 are both triangular numbers.
%H A274587 Colin Barker, <a href="/A274587/b274587.txt">Table of n, a(n) for n = 1..650</a>
%H A274587 <a href="/index/Rec#order_07">Index entries for linear recurrences with constant coefficients</a>, signature (35,-1189,40391,-40391,1189,-35,1).
%F A274587 Intersection of A174114 and A213399.
%F A274587 G.f.: x*(1-12*x+560*x^2-13236*x^3+560*x^4-12*x^5+x^6) / ((1-x)*(1-34*x+x^2)*(1+1154*x^2+x^4)).
%e A274587 23 is in the sequence because 2*23-1 = 45, 4*23-1 = 91, and 45 and 91 are both triangular numbers.
%t A274587 Rest@ CoefficientList[Series[x (1 - 12 x + 560 x^2 - 13236 x^3 + 560 x^4 - 12 x^5 + x^6)/((1 - x) (1 - 34 x + x^2) (1 + 1154 x^2 + x^4)), {x, 0, 17}], x] (* _Michael De Vlieger_, Jun 30 2016 *)
%t A274587 LinearRecurrence[{35,-1189,40391,-40391,1189,-35,1},{1,23,176,5968,888778,30192278,233944673},20] (* _Harvey P. Dale_, Jan 18 2021 *)
%o A274587 (PARI) isok(n) = ispolygonal(2*n-1, 3) && ispolygonal(4*n-1, 3)
%o A274587 (PARI) Vec(x*(1-12*x+560*x^2-13236*x^3+560*x^4-12*x^5+x^6)/((1-x)*(1-34*x+x^2)*(1+1154*x^2+x^4)) + O(x^20))
%Y A274587 Cf. A174114, A213399, A274588.
%K A274587 nonn,easy
%O A274587 1,2
%A A274587 _Colin Barker_, Jun 29 2016