login
A066814
Smallest prime p such that (p-1) has n divisors, or 0 if no such prime exists.
4
2, 3, 5, 7, 17, 13, 0, 31, 37, 113, 0, 61, 0, 193, 401, 211, 65537, 181, 0, 241, 577, 13313, 0, 421, 1297, 12289, 4357, 2113, 0, 1009, 0, 1321, 25601, 2424833, 752734097, 1801, 0, 786433, 495617, 2161, 0, 4801, 0, 15361, 7057, 155189249, 0
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