A350371 Numbers with exactly 4 semiprime divisors.

60, 84, 90, 120, 126, 132, 140, 150, 156, 168, 198, 204, 220, 228, 234, 240, 260, 264, 270, 276, 280, 294, 306, 308, 312, 315, 336, 340, 342, 348, 350, 364, 372, 378, 380, 408, 414, 440, 444, 456, 460, 476, 480, 490, 492, 495, 516, 520, 522, 525, 528, 532, 550, 552, 558

Offset: 1

Author

Wesley Ivan Hurt, Dec 27 2021

Keywords

• nonn

Crossrefs

Numbers with exactly k semiprime divisors: A346041 (k=1), A345381 (k=2), A345382 (k=3), this sequence (k=4), A350372 (k=5), A350373 (k=6), A350374 (k=7), A350375 (k=8).

Programs

Mathematica

q[n_] := DivisorSum[n, 1 &, PrimeOmega[#] == 2 &] == 4; Select[Range[600], q] (* Amiram Eldar, Dec 28 2021 *)
spd4Q[n_]:=Count[Divisors[n],?(PrimeOmega[#]==2&)]==4; Select[Range[600],spd4Q] (* _Harvey P. Dale, Apr 30 2023 *)

PARI

isok(k) = sumdiv(k, d, bigomega(d)==2) == 4; \\ Michel Marcus, Dec 28 2021