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 A253715 #8 Apr 10 2019 15:11:22
%S A253715 1,2160,34417,139317984,2220346081,8987960385360,143243407002961,
%T A253715 579849276161764800,9241205157168647617,37408396193312133889584,
%U A253715 596187109366334725327921,2413365271435489729590825120,38462415164418513312636815521,155695847083980788221510357869840
%N A253715 Indices of centered heptagonal numbers (A069099) which are also hexagonal numbers (A000384).
%C A253715 Also positive integers y in the solutions to 4*x^2-7*y^2-2*x+7*y-2 = 0, the corresponding values of x being A253714.
%H A253715 Colin Barker, <a href="/A253715/b253715.txt">Table of n, a(n) for n = 1..416</a>
%H A253715 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,64514,-64514,-1,1).
%F A253715 a(n) = a(n-1)+64514*a(n-2)-64514*a(n-3)-a(n-4)+a(n-5).
%F A253715 G.f.: x*(2159*x^3+32257*x^2-2159*x-1) / ((x-1)*(x^2-254*x+1)*(x^2+254*x+1)).
%e A253715 2160 is in the sequence because the 2160th centered heptagonal number is 16322041, which is also the 2857th hexagonal number.
%t A253715 LinearRecurrence[{1,64514,-64514,-1,1},{1,2160,34417,139317984,2220346081},20] (* _Harvey P. Dale_, Apr 10 2019 *)
%o A253715 (PARI) Vec(x*(2159*x^3+32257*x^2-2159*x-1)/((x-1)*(x^2-254*x+1)*(x^2+254*x+1)) + O(x^100))
%Y A253715 Cf. A000384, A069099, A253714, A253716.
%K A253715 nonn,easy
%O A253715 1,2
%A A253715 _Colin Barker_, Jan 10 2015