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A289133
a(n) is the number of odd integers divisible by 9 in ]2*(n-1)^2, 2*n^2[.
5
0, 0, 0, 1, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7, 7, 8, 8, 8, 8, 9, 9, 9, 9, 9, 10, 10, 10, 10, 11, 11, 11, 11, 11, 12, 12, 12, 12, 13, 13, 13, 13, 13, 14, 14, 14, 14, 15, 15, 15, 15, 15, 16, 16, 16, 16, 17, 17, 17, 17, 17, 18
OFFSET
0,9
COMMENTS
This sequence has the form (0+2k,0+2k,0+2k,1+2k,1+2k,1+2k,1+2k,1+2k,2+2k) for k>=0.
FORMULA
a(n + 9*k) = a(n) + 2*k.
G.f.: (x^8+x^3)/(x^10-x^9-x+1). - Alois P. Heinz, Jun 26 2017
MATHEMATICA
Table[Count[Mod[Table[2((n-1)^2 +k)-1, {k, 1, 2 n-1}], 9], 0], {n, 0, 50}]
PROG
(PARI) a(n) = sum(k=2*(n-1)^2, 2*n^2, ((k % 2) && ((k % 9) == 0))); \\ Michel Marcus, Jun 26 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Ralf Steiner, Jun 25 2017
EXTENSIONS
More terms from Michel Marcus, Jun 26 2017
STATUS
approved