A345212 Numbers with equal numbers of prime and semiprime divisors.

1, 4, 8, 9, 12, 16, 18, 20, 24, 25, 27, 28, 30, 32, 40, 42, 44, 45, 48, 49, 50, 52, 54, 56, 63, 64, 66, 68, 70, 75, 76, 78, 80, 81, 88, 92, 96, 98, 99, 102, 104, 105, 110, 112, 114, 116, 117, 121, 124, 125, 128, 130, 135, 136, 138, 147, 148, 152, 153, 154, 160, 162, 164, 165

Offset

1,2

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

1 has 0 prime divisors and 0 semiprime divisors.
30 is in the sequence since it has 3 prime divisors: {2,3,5} and 3 semiprime divisors {6,10,15}.

Mathematica

pdsdQ[n_]:=Module[{divs=Divisors[n]}, Count[divs, _?(PrimeQ[#]&)]==Count[divs, _?(PrimeOmega[ #]==2&)]]; Select[Range[200], pdsdQ] (* Harvey P. Dale, Apr 03 2024 *)

Crossrefs

Cf. A000040 (primes), A001358 (semiprimes).

Sequence in context: A171598 A259241 A289553 * A013929 A162966 A375142

Adjacent sequences: A345209 A345210 A345211 * A345213 A345214 A345215

Keyword

nonn

Author

Wesley Ivan Hurt, Jun 10 2021

Status

approved