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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 (list; graph; refs; listen; history; internal format)
OFFSET

0,2

COMMENTS

a(n)=#{0<=k<=n: 3 divides k(n-k) }

FORMULA

a(n)=sum{k=0..n, if (mod(k(n-k), 3)=0, 1, 0) }

CROSSREFS

Sequence in context: A070705 A101909 A130872 * A195051 A096491 A106160

Adjacent sequences:  A087624 A087625 A087626 * A087628 A087629 A087630

KEYWORD

easy,nonn

AUTHOR

Paul Barry (pbarry(AT)wit.ie), Sep 14 2003

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Last modified February 17 06:20 EST 2012. Contains 205991 sequences.