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A253446
Indices of centered heptagonal numbers (A069099) which are also centered octagonal numbers (A016754).
3
1, 16, 465, 13920, 417121, 12499696, 374573745, 11224712640, 336366805441, 10079779450576, 302057016711825, 9051630721904160, 271246864640412961, 8128354308490484656, 243579382390074126705, 7299253117393733316480, 218734014139421925367681
OFFSET
1,2
COMMENTS
Also positive integers x in the solutions to 7*x^2 - 8*y^2 - 7*x + 8*y = 0, the corresponding values of y being A253447.
FORMULA
a(n) = 31*a(n-1)-31*a(n-2)+a(n-3).
G.f.: x*(15*x-1) / ((x-1)*(x^2-30*x+1)).
a(n) = sqrt((-2-(15-4*sqrt(14))^n-(15+4*sqrt(14))^n)*(2-(15-4*sqrt(14))^(1+n)-(15+4*sqrt(14))^(1+n)))/(4*sqrt(7)). - Gerry Martens, Jun 04 2015
EXAMPLE
16 is in the sequence because the 16th centered heptagonal number is 841, which is also the 15th centered octagonal number.
MATHEMATICA
LinearRecurrence[{31, -31, 1}, {1, 16, 465}, 20] (* Harvey P. Dale, Oct 04 2023 *)
PROG
(PARI) Vec(x*(15*x-1)/((x-1)*(x^2-30*x+1)) + O(x^100))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Colin Barker, Jan 01 2015
STATUS
approved