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A178142
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Sum over the divisors d = 2 and/or 3 of n.
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5
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0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3, 2, 0, 5, 0, 2, 3
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OFFSET
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1,2
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COMMENTS
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Periodic with period {0,2,3,2,0,5}.
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LINKS
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FORMULA
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a(n) = sum_{d|n, d=2 or d=3} d.
a(n+6) = a(n).
a(n) = -a(n-1)+a(n-3)+a(n-4). G.f.: -x*(2+5*x+5*x^2) / ( (x-1)*(1+x)*(1+x+x^2) ).
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MATHEMATICA
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Table[Total@ Select[Divisors@ n, 2 <= # <= 3 &], {n, 120}] (* or *)
Table[Total[Divisors@ n /. {d_ /; d < 2 -> Nothing, d_ /; d > 3 -> Nothing} ], {n, 120}] (* Michael De Vlieger, Feb 07 2016 *)
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PROG
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(PARI) a(n) = sumdiv(n, d, d*((d==2) || (d==3))); \\ Michel Marcus, Feb 07 2016
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CROSSREFS
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KEYWORD
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nonn,easy,less
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AUTHOR
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EXTENSIONS
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Replaced recurrence by a shorter one; added keyword:less - R. J. Mathar, May 28 2010
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STATUS
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approved
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