login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A346041 Numbers with exactly 1 semiprime divisor. 6
4, 6, 8, 9, 10, 14, 15, 16, 21, 22, 25, 26, 27, 32, 33, 34, 35, 38, 39, 46, 49, 51, 55, 57, 58, 62, 64, 65, 69, 74, 77, 81, 82, 85, 86, 87, 91, 93, 94, 95, 106, 111, 115, 118, 119, 121, 122, 123, 125, 128, 129, 133, 134, 141, 142, 143, 145, 146, 155, 158, 159, 161, 166, 169 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Numbers of the form p*q or p^k, where p and q are prime and k >= 2.
LINKS
EXAMPLE
6 is in the sequence since it has exactly 1 semiprime divisor, 6.
16 is in the sequence since it has exactly 1 semiprime divisor, 4.
MATHEMATICA
Select[Range@200, Length@Select[Divisors@#, PrimeOmega@#==2&]==1&] (* Giorgos Kalogeropoulos, Jul 03 2021 *)
PROG
(PARI) isok(k) = sumdiv(k, d, bigomega(d)==2) == 1; \\ Michel Marcus, Jul 03 2021
(Python)
from sympy import factorint
def ok(n):
f = factorint(n); w = len(f); W = sum(f.values())
return (w == 1 and W >= 2) or (w == 2 and W == 2)
print(list(filter(ok, range(170)))) # Michael S. Branicky, Jul 03 2021
CROSSREFS
Cf. A001358 (semiprimes), A086971.
Sequence in context: A330991 A180366 A340656 * A341614 A323644 A164702
KEYWORD
nonn
AUTHOR
Wesley Ivan Hurt, Jul 02 2021
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 16:34 EDT 2024. Contains 371254 sequences. (Running on oeis4.)