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A316441
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a(n) = Sum (-1)^k where the sum is over all factorizations of n into factors > 1 and k is the number of factors.
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25
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1, -1, -1, 0, -1, 0, -1, -1, 0, 0, -1, 0, -1, 0, 0, 1, -1, 0, -1, 0, 0, 0, -1, 1, 0, 0, -1, 0, -1, 1, -1, -1, 0, 0, 0, 1, -1, 0, 0, 1, -1, 1, -1, 0, 0, 0, -1, 0, 0, 0, 0, 0, -1, 1, 0, 1, 0, 0, -1, 1, -1, 0, 0, 1, 0, 1, -1, 0, 0, 1, -1, 0, -1, 0, 0, 0, 0, 1, -1
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OFFSET
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1,256
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COMMENTS
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First term greater than 1 in absolute value is a(256) = 2.
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LINKS
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FORMULA
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Dirichlet g.f.: Product_{n > 1} 1/(1 + 1/n^s).
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EXAMPLE
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The factorizations of 24 are (2*2*2*3), (2*2*6), (2*3*4), (2*12), (3*8), (4*6), (24); so a(24) = 1 - 2 + 3 - 1 = 1.
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MATHEMATICA
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facs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[facs[n/d], Min@@#>=d&]], {d, Rest[Divisors[n]]}]];
Table[Sum[(-1)^Length[f], {f, facs[n]}], {n, 200}]
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PROG
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(PARI) A316441(n, m=n, k=0) = if(1==n, (-1)^k, my(s=0); fordiv(n, d, if((d>1)&&(d<=m), s += A316441(n/d, d, k+1))); (s)); \\ Antti Karttunen, Sep 08 2018, after Michael B. Porter's code for A001055
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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