

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
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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(n1)+26*a(n2)26*a(n3)a(n4)+a(n5).
G.f.: x*(x^3+13*x^2x1) / ((x1)*(x^426*x^2+1)).


EXAMPLE

15 is in the sequence because the 15th centered heptagonal number is 736, which is also the 16th octagonal number.


PROG

(PARI) Vec(x*(x^3+13*x^2x1)/((x1)*(x^426*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



