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

 Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 59th year, we have over 358,000 sequences, and we’ve crossed 10,300 citations (which often say “discovered thanks to the OEIS”). Other ways to Give
 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A180922 Smallest n-digit number that is divisible by exactly 3 primes (counted with multiplicity). 3
 8, 12, 102, 1001, 10002, 100006, 1000002, 10000005, 100000006, 1000000003, 10000000001, 100000000006, 1000000000001, 10000000000001, 100000000000018, 1000000000000002, 10000000000000006, 100000000000000007, 1000000000000000001, 10000000000000000007 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This is to 3 as smallest n-digit semiprime A098449 is to 2, and as smallest n-digit prime A003617 is to 1. Smallest n-digit triprime. Smallest n-digit 3-almost prime. LINKS EXAMPLE a(1) = 8 because 8=2^3 is the smallest (only) 1-digit number divisible by exactly 3 primes (counted with multiplicity). a(2) = 12 because 12 = 2^2 * 3 is the smallest of the (21) 2-digit numbers divisible by exactly 3 primes (counted with multiplicity). a(3) = 102 because 102 = 2 * 3 * 17 is the smallest 3-digit numbers divisible by exactly 3 primes (counted with multiplicity). PROG (PARI) A180922(n)=for(n=10^(n-1), 10^n-1, bigomega(n)==3&return(n)) \\ M. F. Hasler, Jan 23 2011 (Python) from sympy import factorint def triprimes(n): f = factorint(n); return sum(f[p] for p in f) == 3 def a(n): an = max(1, 10**(n-1)) while not triprimes(an): an += 1 return an print([a(n) for n in range(1, 21)]) # Michael S. Branicky, Apr 10 2021 CROSSREFS Cf. A003617, A014612, A098449. Sequence in context: A098664 A266800 A230545 * A083128 A196077 A067923 Adjacent sequences: A180919 A180920 A180921 * A180923 A180924 A180925 KEYWORD nonn,base,easy AUTHOR Jonathan Vos Post, Jan 23 2011 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.

Last modified December 6 14:43 EST 2022. Contains 358644 sequences. (Running on oeis4.)