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 A254855 Indices of octagonal numbers (A000567) that are also centered heptagonal numbers (A069099). 3
 1, 2, 16, 43, 407, 1108, 10558, 28757, 274093, 746566, 7115852, 19381951, 184738051, 503184152, 4796073466, 13063405993, 124513172057, 339145371658, 3232546400008, 8804716257107, 83921693228143, 228583477313116, 2178731477531702, 5934365693883901 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also positive integers x in the solutions to 6*x^2 - 7*y^2 - 4*x + 7*y - 2 = 0, the corresponding values of y being A254856. 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^4+x^3-12*x^2+x+1) / ((x-1)*(x^4-26*x^2+1)). EXAMPLE 16 is in the sequence because the 16th octagonal number is 736, which is also the 15th centered heptagonal number. MATHEMATICA LinearRecurrence[{1, 26, -26, -1, 1}, {1, 2, 16, 43, 407}, 30] (* Harvey P. Dale, Aug 31 2021 *) PROG (PARI) Vec(-x*(x^4+x^3-12*x^2+x+1)/((x-1)*(x^4-26*x^2+1)) + O(x^100)) CROSSREFS Cf. A000567, A069099, A254856, A254857. Sequence in context: A211620 A023638 A139267 * A181340 A275032 A295533 Adjacent sequences: A254852 A254853 A254854 * A254856 A254857 A254858 KEYWORD nonn,easy AUTHOR Colin Barker, Feb 08 2015 STATUS approved

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Last modified August 6 18:55 EDT 2024. Contains 374981 sequences. (Running on oeis4.)