OFFSET
1,1
COMMENTS
This is to A037020 as semiprimes A001358 are to primes A000040. The first four values are themselves semiprime.
Contains k^2 if k is in A005383. - Robert Israel, Feb 16 2020
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
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:
select(filter, [$1..1000]); # Robert Israel, Feb 16 2020
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
Jonathan Vos Post, Dec 14 2010
STATUS
approved