A245379 Refactorable numbers with exactly two distinct prime divisors.

12, 18, 24, 36, 40, 56, 72, 80, 88, 96, 104, 108, 136, 152, 184, 225, 232, 248, 288, 296, 328, 344, 376, 384, 424, 441, 448, 472, 488, 536, 568, 584, 632, 640, 664, 712, 776, 808, 824, 856, 864, 872, 896, 904, 972, 1016, 1048, 1089, 1096, 1112, 1152

Offset

1,1

Comments

Numbers n such that d(n) | n and omega(n) = 2, where d(n) is the number of divisors of n and omega(n) is the number of distinct prime factors of n.

Intersection of A033950 and A007774.

Links

Jens Kruse Andersen, Table of n, a(n) for n = 1..10000

Eric Weisstein's World of Mathematics, Refactorable Number

Maple

with(numtheory): A245379:=n->`if`((n mod tau(n))=0 and nops(factorset(n))=2, n, NULL): seq(A245379(n), n=1..2000);

Mathematica

Select[Range[1200], Divisible[#, DivisorSigma[0, #]]&&PrimeNu[#]==2&] (* Harvey P. Dale, Jul 03 2021 *)

Prog

(Sage)
def is_A245379(n):
    a = sloane.A000005
    b = sloane.A001221
    return a(n).divides(n) and b(n) == 2
A245379_list = lambda up_to: filter(is_A245379, (1..up_to))
A245379_list(1152) # Peter Luschny, Jul 20 2014

Crossrefs

Cf. A000005, A001221, A007774, A033950.

Sequence in context: A284342 A362844 A198470 * A363814 A224218 A076485

Adjacent sequences: A245376 A245377 A245378 * A245380 A245381 A245382

Keyword

nonn

Author

Wesley Ivan Hurt, Jul 20 2014

Status

approved