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A199768 Numbers whose greatest prime factor is less than their number of divisors. 0
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

Table of n, a(n) for n=1..59.

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: A310662 A231569 A100390 * A157932 A097619 A113709

Adjacent sequences:  A199765 A199766 A199767 * A199769 A199770 A199771

KEYWORD

nonn,easy

AUTHOR

Franklin T. Adams-Watters, Nov 10 2011

STATUS

approved

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Last modified June 16 13:31 EDT 2019. Contains 324152 sequences. (Running on oeis4.)