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A321516
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Number of composite divisors of n that are < n.
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3
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0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 2, 0, 0, 0, 2, 0, 2, 0, 2, 0, 0, 0, 4, 0, 0, 1, 2, 0, 3, 0, 3, 0, 0, 0, 5, 0, 0, 0, 4, 0, 3, 0, 2, 2, 0, 0, 6, 0, 2, 0, 2, 0, 4, 0, 4, 0, 0, 0, 7, 0, 0, 2, 4, 0, 3, 0, 2, 0, 3, 0, 8, 0, 0, 2, 2, 0, 3, 0, 6, 2, 0, 0, 7, 0, 0, 0
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OFFSET
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1,12
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COMMENTS
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a(n) > 0 iff n is a term of A033942.
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LINKS
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EXAMPLE
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For n = 24: The divisors of 24 are 1, 2, 3, 4, 6, 8, 12, 24. Four of those divisors, namely 4, 6, 8 and 12 are composite and < 24, so a(24) = 4.
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MATHEMATICA
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a[n_] := Length[Select[Most[Divisors[n]], CompositeQ]]; Array[a, 87] (* Amiram Eldar, Nov 12 2018 *)
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PROG
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(PARI) a(n) = my(d=divisors(n), i=0); for(k=2, #d-1, if(!ispseudoprime(d[k]), i++)); i
(PARI) a(n) = sumdiv(n, d, (d<n) && (d>1) && !isprime(d)); \\ Michel Marcus, Nov 12 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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