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 A253673 Indices of centered triangular numbers (A005448) that are also centered octagonal numbers (A016754). 5
 1, 16, 65, 1520, 6321, 148896, 619345, 14590240, 60689441, 1429694576, 5946945825, 140095478160, 582740001361, 13727927165056, 57102573187505, 1345196766697280, 5595469432374081, 131815555209168336, 548298901799472385, 12916579213731799600 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also positive integers x in the solutions to 3*x^2 - 8*y^2 - 3*x + 8*y = 0, the corresponding values of y being A253674. Also indices of centered square numbers (A001844) that are also octagonal numbers (A000567). - Colin Barker, Feb 10 2015 LINKS Colin Barker, Table of n, a(n) for n = 1..1000 Index entries for linear recurrences with constant coefficients, signature (1,98,-98,-1,1). FORMULA a(n) = a(n-1)+98*a(n-2)-98*a(n-3)-a(n-4)+a(n-5). G.f.: x*(3*x-1)*(5*x^2+18*x+1) / ((x-1)*(x^2-10*x+1)*(x^2+10*x+1)). EXAMPLE 16 is in the sequence because the 16th centered triangular number is 361, which is also the 10th centered octagonal number. PROG (PARI) Vec(x*(3*x-1)*(5*x^2+18*x+1)/((x-1)*(x^2-10*x+1)*(x^2+10*x+1)) + O(x^100)) CROSSREFS Cf. A005448, A016754, A253674, A253675. Cf. A000567, A001844, A254895, A254896. Sequence in context: A034720 A232050 A119285 * A041494 A057689 A045024 Adjacent sequences:  A253670 A253671 A253672 * A253674 A253675 A253676 KEYWORD nonn,easy AUTHOR Colin Barker, Jan 08 2015 STATUS approved

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Last modified August 9 02:14 EDT 2020. Contains 336310 sequences. (Running on oeis4.)