A350374 Numbers with exactly 7 semiprime divisors.

420, 630, 660, 780, 840, 924, 990, 1020, 1050, 1092, 1140, 1170, 1320, 1380, 1386, 1428, 1470, 1530, 1540, 1560, 1596, 1638, 1650, 1680, 1710, 1716, 1740, 1820, 1848, 1860, 1890, 1932, 1950, 2040, 2070, 2142, 2184, 2220, 2244, 2280, 2380, 2394, 2436, 2460, 2508, 2550

Offset

1,1

Links

Table of n, a(n) for n=1..46.

Mathematica

q[n_] := DivisorSum[n, 1 &, PrimeOmega[#] == 2 &] == 7; Select[Range[2500], q] (* Amiram Eldar, Dec 28 2021 *)

Prog

(PARI) isok(k) = sumdiv(k, d, bigomega(d)==2) == 7; \\ Michel Marcus, Dec 28 2021

Crossrefs

Numbers with exactly k semiprime divisors: A346041 (k=1), A345381 (k=2), A345382 (k=3), A350371 (k=4), A350372 (k=5), A350373 (k=6), this sequence (k=7), A350375 (k=8).

Sequence in context: A024410 A200521 A380432 * A189982 A376374 A070237

Adjacent sequences: A350371 A350372 A350373 * A350375 A350376 A350377

Keyword

nonn

Author

Wesley Ivan Hurt, Dec 27 2021

Status

approved