OFFSET
1,2
COMMENTS
Includes all primes, squares of odd primes, and squarefree semiprimes coprime to 3. - Robert Israel, Dec 16 2019
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
26 has 4 divisors and 4 has 3 divisors. 3 is coprime to 26, so 26 is in the sequence.
MAPLE
filter:= proc(n) uses numtheory;
igcd(tau(tau(n)), n) = 1
end proc:
select(filter, [$1..200]); # Robert Israel, Dec 16 2019
MATHEMATICA
Select[Range[200], GCD[DivisorSigma[0, DivisorSigma[0, # ]], # ] == 1 &] (* Stefan Steinerberger, Jun 05 2008 *)
PROG
(Magma) [k:k in [1..130]|Gcd(k, #Divisors(#Divisors(k))) eq 1]; // Marius A. Burtea, Dec 16 2019
(PARI) is(n) = gcd(numdiv(numdiv(n)), n)==1 \\ Felix Fröhlich, Dec 16 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Leroy Quet, Jun 04 2008
EXTENSIONS
More terms from Stefan Steinerberger, Jun 05 2008
STATUS
approved