

A177891


Numbers n such that sum of proper (or aliquot) divisors of n is a semiprime.


1



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

1,1


COMMENTS

This is to A037020 as semiprimes A001358 are to primes A000040. The first four values are themselves semiprime.


LINKS



FORMULA



EXAMPLE

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.


MAPLE

filter:= proc(n) uses numtheory;
bigomega(sigma(n)n) = 2
end proc:


MATHEMATICA

semiPrimeQ[x_] := Plus @@ Last /@ FactorInteger@ x == 2; fQ[n_] := semiPrimeQ[ DivisorSigma[1, n]  n]; Select[ Range@ 200, fQ]


PROG

(PARI) isok(n) = bigomega(sigma(n)n) == 2; \\ Michel Marcus, Apr 05 2015


CROSSREFS



KEYWORD

nonn,easy


AUTHOR



STATUS

approved



