%I #14 Mar 09 2019 05:13:21
%S 0,0,0,1,6,16,47,96,206,406,823,1626,3244,6454,12798,25715,51779,
%T 103315,206649,412883,825395,1650179,3299610,6600347,13201348,26403834
%N Number of irregular primes less than or equal to the 2^n-th prime.
%C Lim_{n->inf.} a(n)/2^n = 1 - e^(-1/2).
%t ip={ the list of irregular primes to 12 million }; Table[ Length[ Select[ip, # <= Prime[2^n] &]], {n, 19}]
%Y Cf. A000928, A033844, A105456, A105460.
%K hard,nonn,more
%O 1,5
%A _Robert G. Wilson v_, Apr 07 2005
%E Data corrected and a(20)-a(26) added by _Amiram Eldar_, Mar 05 2019