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A087627
Count ...n,2n,2n...
0
1, 2, 2, 2, 4, 4, 3, 6, 6, 4, 8, 8, 5, 10, 10, 6, 12, 12, 7, 14, 14, 8, 16, 16, 9, 18, 18, 10, 20, 20, 11, 22, 22, 12, 24, 24, 13, 26, 26, 14, 28, 28, 15, 30, 30, 16, 32, 32, 17, 34, 34, 18, 36, 36, 19, 38, 38, 20, 40, 40, 21, 42, 42, 22, 44, 44, 23, 46, 46, 24, 48, 48, 25, 50, 50
OFFSET
0,2
FORMULA
a(n) = #{0<=k<=n: 3 divides k(n-k)}.
G.f.: ( 1+2*x+2*x^2 ) / ( (x-1)^2*(1+x+x^2)^2 ). - R. J. Mathar, Feb 14 2015
CROSSREFS
Sequence in context: A101909 A215676 A130872 * A195051 A219654 A096491
KEYWORD
easy,nonn
AUTHOR
Paul Barry, Sep 14 2003
STATUS
approved