OFFSET
1,1
COMMENTS
Numbers with exactly four distinct prime divisors (cf. A033993), one of which has multiplicity 1 and the others at least 2. - David A. Corneth, Jun 10 2022
LINKS
David A. Corneth, Table of n, a(n) for n = 1..10000
EXAMPLE
6300 is in the sequence as 4, 6, 9, 10, 14, 15, 21, 25, 35 are the exactly 9 of its semiprime divisors. - David A. Corneth, Jun 10 2022
MATHEMATICA
q[n_] := DivisorSum[n, 1 &, PrimeOmega[#] == 2 &] == 9; Select[Range[40000], q] (* Amiram Eldar, Dec 30 2021 *)
spd9Q[n_]:=Count[Divisors[n], _?(PrimeOmega[#]==2&)]==9; Select[Range[ 40000], spd9Q] (* Harvey P. Dale, Jun 09 2022 *)
PROG
(PARI) isok(k) = sumdiv(k, d, bigomega(d)==2) == 9; \\ Michel Marcus, Dec 30 2021
(PARI) is(n)= if(n==1, return(0)); my(f = vecsort(factor(n)[, 2])); #f == 4 && f[1] == 1 && f[2]>=2 \\ David A. Corneth, Jun 10 2022
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wesley Ivan Hurt, Dec 29 2021
STATUS
approved