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%I #10 Apr 30 2019 13:03:52
%S 1,2,15,40,377,1026,9775,26624,253761,691186,6587999,17944200,
%T 171034201,465858002,4440301215,12094363840,115276797377,313987601826,
%U 2992756430575,8151583283624,77696390397561,211627177772386,2017113393905999,5494155038798400
%N Indices of centered heptagonal numbers (A069099) that are also octagonal numbers (A000567).
%C Also positive integers y in the solutions to 6*x^2 - 7*y^2 - 4*x + 7*y - 2 = 0, the corresponding values of x being A254855.
%H Colin Barker, <a href="/A254856/b254856.txt">Table of n, a(n) for n = 1..1000</a>
%H <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (1,26,-26,-1,1).
%F a(n) = a(n-1)+26*a(n-2)-26*a(n-3)-a(n-4)+a(n-5).
%F G.f.: x*(x^3+13*x^2-x-1) / ((x-1)*(x^4-26*x^2+1)).
%e 15 is in the sequence because the 15th centered heptagonal number is 736, which is also the 16th octagonal number.
%t LinearRecurrence[{1,26,-26,-1,1},{1,2,15,40,377},30] (* _Harvey P. Dale_, Apr 30 2019 *)
%o (PARI) Vec(x*(x^3+13*x^2-x-1)/((x-1)*(x^4-26*x^2+1)) + O(x^100))
%Y Cf. A000567, A069099, A254855, A254857.
%K nonn,easy
%O 1,2
%A _Colin Barker_, Feb 08 2015
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