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A166684 Numbers n such that d(n)<4. 12
1, 2, 3, 4, 5, 7, 9, 11, 13, 17, 19, 23, 25, 29, 31, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 121, 127, 131, 137, 139, 149, 151, 157, 163, 167, 169, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
1 together with primes and squares of primes.
Numbers n such that A229964(n) = 0. - Eric M. Schmidt, Oct 05 2013
Numbers that cannot be written as a product of 2 distinct nonunits. - Peter Munn, May 26 2023
LINKS
FORMULA
a(n) = A000430(n-1), n>1. - R. J. Mathar, May 21 2010
MATHEMATICA
Select[Range[300], DivisorSigma[0, #]<4&] (* or *) Select[With[ {prs = Prime[Range[200]]}, Union[Join[{1}, prs, prs^2]]], #<301&] (* Harvey P. Dale, Jan 04 2012 *)
PROG
(PARI) is(n)=isprime(n) || (issquare(n, &n) && isprime(n)) || n==1 \\ Charles R Greathouse IV, Dec 23 2022
CROSSREFS
A000430 is the main entry for this sequence.
Sequence in context: A001092 A327782 A000430 * A067126 A274197 A286267
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
Corrected (193 inserted) by R. J. Mathar, May 21 2010
STATUS
approved

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Last modified February 26 11:46 EST 2024. Contains 370352 sequences. (Running on oeis4.)