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A009087
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Numbers n such that the number of divisors of n is prime (i.e. numbers of the form p^(q-1) for primes p,q).
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9
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2, 3, 4, 5, 7, 9, 11, 13, 16, 17, 19, 23, 25, 29, 31, 37, 41, 43, 47, 49, 53, 59, 61, 64, 67, 71, 73, 79, 81, 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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Invented by the HR Automatic Concept Formation Program. If the sum of divisors is prime, then the number of divisors is prime, i.e. this is a supersequence of A023194.
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REFERENCES
| S. Colton, Automated Theory Formation in Pure Mathematics. New York: Springer (2002)
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..1000
S. Colton, Refactorable Numbers - A Machine Invention, J. Integer Sequences, Vol. 2, 1999, #2.
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FORMULA
| p^(q-1), p, q primes
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EXAMPLE
| tau(16)=5 and 5 is prime.
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MATHEMATICA
| Select[Range[250], PrimeQ[DivisorSigma[0, #]]&] (* From Harvey P. Dale, Sep 28 2011 *)
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CROSSREFS
| Cf. A023194, A036454
Sequence in context: A066724 A079851 A089237 * A026477 A079852 A084400
Adjacent sequences: A009084 A009085 A009086 * A009088 A009089 A009090
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KEYWORD
| nice,nonn,easy
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AUTHOR
| Simon Colton (simonco(AT)cs.york.ac.uk)
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