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%I #6 Dec 12 2012 12:27:03
%S 1,1,1,1,3,2,3,1,12,9,12,10,60,51,43,50,413,377,346,363,2632,2422,
%T 2253,2231,18960,17923,17221,17038,141594,134894,130276,128814,
%U 1106984,1059947,1021263,1009002,8838825,8485595,8188908,8106052
%N The number of reversible primes (palindromic or emirps) by increasing permissible leading digit and by length.
%C Aside from the first 4 terms here (corresponding to 2, 3, 5 and 7), the counts of 4 successive terms are of primes with leading digit 1, 3, 7 and 9 that are still prime if their decimal representations are reversed. A220248 and A220349 handle palindromic primes and emirps separately.
%e a(5) is the start of the counts for 2-digit primes. 11, 13 and 17 are the only 2-digit primes with leading digit 1 that remain prime read backward. It is not possible for a prime of more than 1 digit to lead with 2 and yield a prime read in reverse, so the following term, a(6), is the count for leading digit 3.
%Y Cf. A220248, A220349.
%K nonn,base
%O 1,5
%A _James G. Merickel_, Dec 11 2012