This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A138304 #4 Mar 30 2012 17:22:47
%S A138304 0,1,2,2,2,3,4,3,5,4,4,5,6,4,9,6,8,6,7,9,8,8,11,12,11,12,7,12,9,16,11,
%T A138304 11,17,9,18,6,11,17,23,18,20,13,20,16,19,13,12,15,24,20,28,24,17,23,
%U A138304 28,32,29,15,24,23,13,31,20,32,23,28,15,21,32,22,28,42,27,29,21,43,40,27
%N A138304 Number of prime primitive roots of prime(n).
%H A138304 T. D. Noe, <a href="/A138304/b138304.txt">Table of n, a(n) for n=1..10000</a>
%e A138304 a(5)=2 because the primitive roots of 11 are 2, 6, 7 and 8, two of which are prime.
%t A138304 Table[p=Prime[n]; g=Select[Prime[Range[n-1]], MultiplicativeOrder[ #,p]==p-1&]; Length[g], {n,100}]
%Y A138304 Cf. A060749 (primitive roots), A138304 (prime primitive roots).
%K A138304 nonn
%O A138304 1,3
%A A138304 _T. D. Noe_, Mar 14 2008