login
A178212
Nonsquarefree numbers divisible by exactly three distinct primes.
7
60, 84, 90, 120, 126, 132, 140, 150, 156, 168, 180, 198, 204, 220, 228, 234, 240, 252, 260, 264, 270, 276, 280, 294, 300, 306, 308, 312, 315, 336, 340, 342, 348, 350, 360, 364, 372, 378, 380, 396, 408, 414, 440, 444, 450, 456, 460, 468, 476, 480, 490, 492
OFFSET
1,1
LINKS
FORMULA
A001221(a(n)) = 3; A001222(a(n)) > 3; A000005(n) >= 12;
a(n) = A123712(n) for n <= 52, possibly more.
EXAMPLE
60 is in the sequence because it is not squarefree and it is divisible by three distinct primes: 2, 3, 5.
72 is not in the sequence, because although it is not squarefree, it is divisible by only two distinct primes: 2 and 3.
MATHEMATICA
nsD3Q[n_] := Block[{fi = FactorInteger@ n}, Length@ fi == 3 && Plus @@ Last /@ fi > 3]; Select[ Range@ 494, nsD3Q] (* Robert G. Wilson v, Feb 09 2012 *)
Select[Range[500], PrimeNu[#] == 3 && PrimeOmega[#] > 3 &] (* Alonso del Arte, Mar 23 2015, based on a comment from Robert G. Wilson v, Feb 09 2012; requires Mathematica 7.0+ *)
PROG
(PARI) is_A178212(n)={ omega(n)==3 & bigomega(n)>3 }
for(n=1, 999, is_A178212(n) & print1(n", ")) \\ M. F. Hasler, Feb 09 2012
(Haskell)
a178212 n = a178212_list !! (n-1)
a178212_list = filter f [1..] where
f x = length (a027748_row x) == 3 && any (> 1) (a124010_row x)
-- Reinhard Zumkeller, Apr 03 2015
CROSSREFS
A subsequence of A033987.
A085987 is a subsequence.
Sequence in context: A217740 A375055 A123712 * A182855 A350371 A009129
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, May 24 2010
STATUS
approved