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

1,1

Links

Table of n, a(n) for n=1..55.

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 *)

Prog

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

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).

Sequence in context: A123712 A178212 A182855 * A009129 A174292 A085987

Adjacent sequences: A350368 A350369 A350370 * A350372 A350373 A350374

Keyword

nonn

Author

Wesley Ivan Hurt, Dec 27 2021

Status

approved