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%I #8 Mar 30 2012 18:37:42
%S 2,3,5,7,17,13,0,31,37,113,0,61,0,193,401,211,65537,181,0,241,577,
%T 13313,0,421,1297,12289,4357,2113,0,1009,0,1321,25601,2424833,
%U 752734097,1801,0,786433,495617,2161,0,4801,0,15361,7057,155189249,0
%N Smallest prime p such that (p-1) has n divisors, or 0 if no such prime exists.
%C The only primes p for which p-1 has a prime number of divisors are Fermat primes A019434.
%e a(17)=65537 because DivisorSigma[0,65536]=17.
%t it=Table[ p=Prime[ n ]; DivisorSigma[ 0, p-1 ], {n, 400000} ]; Flatten[ Position[ it, #, 1, 1 ]&/@Range[ 100 ]/.{}- > 0 ]
%Y Cf. A004249, A007516, A066529.
%K nonn
%O 1,1
%A _Wouter Meeussen_, Jan 20 2002
%E Comment clarified by _T. D. Noe_, Nov 06 2009
%E Edited by _Max Alekseyev_, Nov 10 2009
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