OFFSET
1,1
COMMENTS
The only primes p for which p-1 has a prime number of divisors are Fermat primes A019434.
EXAMPLE
a(17)=65537 because DivisorSigma[0,65536]=17.
MATHEMATICA
it=Table[ p=Prime[ n ]; DivisorSigma[ 0, p-1 ], {n, 400000} ]; Flatten[ Position[ it, #, 1, 1 ]&/@Range[ 100 ]/.{}- > 0 ]
CROSSREFS
KEYWORD
nonn
AUTHOR
Wouter Meeussen, Jan 20 2002
EXTENSIONS
Comment clarified by T. D. Noe, Nov 06 2009
Edited by Max Alekseyev, Nov 10 2009
STATUS
approved