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 A135833 #5 Jan 29 2014 17:17:31
%S A135833 1,2,2,4,5,7,9,13,18,21,28,43,56,62,72,94,133,142,187,241,313,376,436,
%T A135833 517,709,858,982,1271,1561,1814,2192,2658,3184,3853,4601,5648,6881,
%U A135833 8009,9535,11651,13712,16325,19381,23323,27097,31782,37924,44673,52695,62147
%N A135833 Number of Section I primes between 2^n and 2^(n+1). See A135832.
%C A135833 Comparing these numbers with A036378, the number of primes between 2^n and 2^(n+1), leads one to conjecture that the density of Section I primes is 0.
%H A135833 T. D. Noe, <a href="http://www.sspectra.com/math/IteratedPhi2.pdf">Computing Numbers in Section I of the Totient Iteration</a>
%e A135833 3; 5, 7; 11, 13; 17, 23, 29, 31; 41, 47, 53, 59, 61; 83,...
%t A135833 class[ n_ ] := Length[ NestWhileList[ EulerPhi,n,#>2& ] ]-1; k=2; Table[ cnt=0; While[ p=Prime[ k ]; p<2^(n+1), If[ class[ p ]==n, cnt++ ]; k++ ]; cnt, {n,20} ] (* _T. D. Noe_, Aug 04 2008 *)
%Y A135833 Cf. A092878 (number of odd numbers in Section I).
%K A135833 nonn
%O A135833 1,2
%A A135833 _T. D. Noe_, Nov 30 2007
%E A135833 More terms from _T. D. Noe_, Aug 04 2008
%E A135833 Extension. _T. D. Noe_, Nov 18 2008