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 A254856 Indices of centered heptagonal numbers (A069099) that are also octagonal numbers (A000567). 3
 1, 2, 15, 40, 377, 1026, 9775, 26624, 253761, 691186, 6587999, 17944200, 171034201, 465858002, 4440301215, 12094363840, 115276797377, 313987601826, 2992756430575, 8151583283624, 77696390397561, 211627177772386, 2017113393905999, 5494155038798400 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS 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. LINKS Colin Barker, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (1,26,-26,-1,1). FORMULA a(n) = a(n-1)+26*a(n-2)-26*a(n-3)-a(n-4)+a(n-5). G.f.: x*(x^3+13*x^2-x-1) / ((x-1)*(x^4-26*x^2+1)). EXAMPLE 15 is in the sequence because the 15th centered heptagonal number is 736, which is also the 16th octagonal number. MATHEMATICA LinearRecurrence[{1, 26, -26, -1, 1}, {1, 2, 15, 40, 377}, 30] (* Harvey P. Dale, Apr 30 2019 *) PROG (PARI) Vec(x*(x^3+13*x^2-x-1)/((x-1)*(x^4-26*x^2+1)) + O(x^100)) CROSSREFS Cf. A000567, A069099, A254855, A254857. Sequence in context: A200156 A249997 A032016 * A001007 A300393 A152015 Adjacent sequences:  A254853 A254854 A254855 * A254857 A254858 A254859 KEYWORD nonn,easy AUTHOR Colin Barker, Feb 08 2015 STATUS approved

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Last modified June 12 10:54 EDT 2021. Contains 344947 sequences. (Running on oeis4.)