A066814 Smallest prime p such that (p-1) has n divisors, or 0 if no such prime exists.

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.

Links

Table of n, a(n) for n=1..47.

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

Cf. A004249, A007516, A066529.

Sequence in context: A372407 A059471 A059496 * A358047 A072885 A178209

Adjacent sequences: A066811 A066812 A066813 * A066815 A066816 A066817

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