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A177891
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Numbers n such that sum of proper (or aliquot) divisors of n is a semiprime.
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1
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6, 9, 14, 15, 16, 18, 20, 22, 25, 33, 36, 38, 45, 46, 51, 52, 62, 68, 70, 72, 75, 80, 86, 87, 91, 93, 95, 99, 104, 105, 110, 116, 117, 118, 119, 130, 136, 141, 142, 143, 144, 145, 148, 154, 158, 159, 160, 162, 165, 166, 169, 183, 195, 196, 200
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OFFSET
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1,1
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COMMENTS
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This is to A037020 as semiprimes A001358 are to primes A000040. The first four values are themselves semiprime.
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LINKS
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FORMULA
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EXAMPLE
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a(2) = 9 because the aliquot divisors of 9 are 1 and 3, whose sum is 4 = 2*2, semiprime.
a(5) = 16 because the aliquot divisors of 16 are 1, 2, 4, and 8, whose sum is 15 = 3*5, semiprime.
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MAPLE
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filter:= proc(n) uses numtheory;
bigomega(sigma(n)-n) = 2
end proc:
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MATHEMATICA
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semiPrimeQ[x_] := Plus @@ Last /@ FactorInteger@ x == 2; fQ[n_] := semiPrimeQ[ DivisorSigma[1, n] - n]; Select[ Range@ 200, fQ]
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PROG
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(PARI) isok(n) = bigomega(sigma(n)-n) == 2; \\ Michel Marcus, Apr 05 2015
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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