OFFSET
1,1
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.
REFERENCES
S. Colton, Automated Theory Formation in Pure Mathematics. New York: Springer (2002)
LINKS
Indranil Ghosh, Table of n, a(n) for n = 1..12546 (terms 1..1000 from T. D. Noe)
S. Colton, Refactorable Numbers - A Machine Invention, J. Integer Sequences, Vol. 2, 1999, #2.
FORMULA
p^(q-1), p, q primes.
EXAMPLE
tau(16)=5 and 5 is prime.
MATHEMATICA
Select[Range[250], PrimeQ[DivisorSigma[0, #]]&] (* Harvey P. Dale, Sep 28 2011 *)
PROG
(Haskell)
a009087 n = a009087_list !! (n-1)
a009087_list = filter ((== 1) . a010051 . (+ 1) . a100995) a000961_list
-- Reinhard Zumkeller, Jun 05 2013
(PARI) is(n)=isprime(isprimepower(n)+1) \\ Charles R Greathouse IV, Sep 16 2015
CROSSREFS
KEYWORD
nice,nonn,easy
AUTHOR
Simon Colton (simonco(AT)cs.york.ac.uk)
STATUS
approved