OFFSET
1,1
COMMENTS
This sequence and A000977 are identical through their first 32 terms, but A000977(33) = 210. [Comment edited by Jon E. Schoenfield, Dec 30 2014]
LINKS
T. D. Noe, Table of n, a(n) for n = 1..1000
Hans Montanus and Ron Westdijk, Cellular Automation and Binomials, Green Blue Mathematics (2022), p. 90.
FORMULA
omega(a(n)) = A001221(a(n)) = 3. - Jonathan Vos Post, Sep 20 2005
a(n) ~ 2n log n / (log log n)^2. - Charles R Greathouse IV, Jul 28 2016
EXAMPLE
220 = 2*2*5*11 is here but 210 = 2*3*5*7 is not; compare A000977.
MAPLE
A033992 := proc(n)
if (nops(numtheory[factorset](n)) = 3) then
RETURN(n)
fi: end: seq(A033992(n), n=1..500); # Jani Melik, Feb 24 2011
MATHEMATICA
Select[Range[300], PrimeNu[#]==3&] (* Harvey P. Dale, May 01 2013 *)
PROG
(Haskell)
a033992 n = a033992_list !! (n-1)
a033992_list = filter ((== 3) . a001221) [1..]
-- Reinhard Zumkeller, May 03 2013
(PARI) is(n)=omega(n)==3 \\ Charles R Greathouse IV, Apr 28 2015
(PARI) A246655(lim)=my(v=List(primes([2, lim\=1]))); for(e=2, logint(lim, 2), forprime(p=2, sqrtnint(lim, e), listput(v, p^e))); Set(v)
list(lim, pr=3)=if(pr==1, return(A246655(lim))); my(v=List(), pr1=pr-1, mx=prod(i=1, pr1, prime(i))); forprime(p=prime(pr), lim\mx, my(u=list(lim\p, pr1)); for(i=1, #u, listput(v, p*u[i]))); Set(v) \\ Charles R Greathouse IV, Feb 03 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved