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!)
A199768 Numbers whose greatest prime factor is less than their number of divisors. 2
4, 6, 8, 12, 16, 18, 20, 24, 27, 30, 32, 36, 40, 42, 45, 48, 50, 54, 56, 60, 64, 70, 72, 75, 80, 81, 84, 90, 96, 100, 105, 108, 112, 120, 126, 128, 132, 135, 140, 144, 150, 160, 162, 168, 180, 189, 192, 196, 198, 200, 210, 216, 220, 224, 225, 240, 243, 250, 252 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
The greatest prime factor equals the number of divisors only for 1 (as defined in A006530) and numbers of the form p^(p-1) for p a prime.
LINKS
EXAMPLE
4 has 2 as its greatest prime factor, and it has 3 factors (1, 2, 4), so it is in the sequence.
10 has 5 as its greatest prime factor, but it has only 4 factors (1, 2, 5, 10), so it is not in the sequence.
MATHEMATICA
Select[Range[300], FactorInteger[#][[-1, 1]]<DivisorSigma[0, #]&] (* Harvey P. Dale, Nov 19 2011 *)
CROSSREFS
Cf. A006530 (greatest prime factor), A000005 (number of divisors), A036878 (p^(p-1)).
Sequence in context: A231569 A337812 A100390 * A157932 A097619 A113709
KEYWORD
nonn,easy
AUTHOR
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 June 22 07:58 EDT 2024. Contains 373565 sequences. (Running on oeis4.)