login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A014613 Numbers that are products of 4 primes. 143
16, 24, 36, 40, 54, 56, 60, 81, 84, 88, 90, 100, 104, 126, 132, 135, 136, 140, 150, 152, 156, 184, 189, 196, 198, 204, 210, 220, 225, 228, 232, 234, 248, 250, 260, 276, 294, 296, 297, 306, 308, 315, 328, 330, 340, 342, 344, 348, 350, 351, 364, 372, 375, 376 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

J. H. Conway, Heiko Dietrich and E. A. O'Brien, Counting groups: gnus, moas and other exotica, Math. Intell., Vol. 30, No. 2, Spring 2008.

Eric Weisstein's World of Mathematics, Almost Prime.

FORMULA

Product p_i^e_i with Sum e_i = 4.

a(n) ~ 6n log n / (log log n)^3. - Charles R Greathouse IV, May 04 2013

a(n) = A078840(4,n). - R. J. Mathar, Jan 30 2019

MATHEMATICA

Select[Range[200], Plus @@ Last /@ FactorInteger[ # ] == 4 &] (* Vladimir Joseph Stephan Orlovsky, Apr 23 2008 *)

Select[Range[400], PrimeOmega[#] == 4&] (* Jean-Fran├žois Alcover, Jan 17 2014 *)

PROG

(PARI) isA014613(n) = bigomega(n)==4 \\ Michael B. Porter, Dec 13 2009

(Python)

from sympy import factorint

def ok(n): return sum(factorint(n).values()) == 4

print([k for k in range(377) if ok(k)]) # Michael S. Branicky, Nov 19 2021

CROSSREFS

Cf. A033987, A114106 (number of 4-almost primes <= 10^n).

Sequences listing r-almost primes, that is, the n such that A001222(n) = r: A000040 (r = 1), A001358 (r = 2), A014612 (r = 3), this sequence (r = 4), A014614 (r = 5), A046306 (r = 6), A046308 (r = 7), A046310 (r = 8), A046312 (r = 9), A046314 (r = 10), A069272 (r = 11), A069273 (r = 12), A069274 (r = 13), A069275 (r = 14), A069276 (r = 15), A069277 (r = 16), A069278 (r = 17), A069279 (r = 18), A069280 (r = 19), A069281 (r = 20). - Jason Kimberley, Oct 02 2011

Sequence in context: A036328 A067028 A110893 * A323350 A307341 A046370

Adjacent sequences:  A014610 A014611 A014612 * A014614 A014615 A014616

KEYWORD

nonn

AUTHOR

Eric W. Weisstein

EXTENSIONS

More terms from Patrick De Geest, Jun 15 1998

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified October 1 16:19 EDT 2022. Contains 357149 sequences. (Running on oeis4.)