A087627 Count ...n,2n,2n...

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

Links

Table of n, a(n) for n=0..74.

Index entries for linear recurrences with constant coefficients, signature (0,0,2,0,0,-1).

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

Adjacent sequences: A087624 A087625 A087626 * A087628 A087629 A087630

Keyword

easy,nonn

Author

Paul Barry, Sep 14 2003

Status

approved