A092308 - OEIS

%I #9 Feb 04 2014 13:08:51

%S 1,2,3,3,3,5,4,4,3,4,5,7,5,4,3,4,3,8,5,4,8,4,3,5,7,5,4,3,8,6,6,4,4,5,

%T 4,6,8,5,3,4,3,11,4,8,5,7,8,4,3,6,5,3,11,4,5,3,4,7,8,8,4,4,6,4,9,4,8,

%U 10,3,7,7,3,4,6,7,3,4,11,8,8,4,13,4,11,4,3,7,7,6,7,3,3,6,5,5,3,4,8,6,14,6,4

%N For p=prime(n), a(n) = the number of primes q such that q-1 divides p-1.

%C For many primes p, there are only 3 primes (2,3,p) such that q-1|p-1. See A092307 for a list of those primes.

%H T. D. Noe, <a href="/A092308/b092308.txt">Table of n, a(n) for n=1..1000</a>

%e a(12)=7 because for prime(12)=37 there are seven primes q={2, 3, 5, 7, 13, 19, 37} such that q-1 divides 36.

%t Table[p=Prime[n]; Length[Select[Divisors[p-1]+1, PrimeQ]], {n, 150}]

%Y Cf. A092307 (primes for which a(n)=3).

%K nonn

%O 1,2

%A _T. D. Noe_, Feb 12 2004