%I #32 Aug 12 2015 02:37:22
%N Similar to A152396, but here the requirement is for finding any n primes, not necessarily from the shortest concatenations.
%C Tentatively, as of Dec 2012, the likely value of a(6) is 20968. A noteworthy fact, perhaps, is that were this sequence to limit itself to non-titanic primes (ones under 10^999), then it would look the same to the point shown and have the stated tentative value for a(6) as its a(5), despite there being a number of smaller values eventually reaching a 5th prime. - _James G. Merickel_, Dec 06 2012
%C a(5)=8338 has not been determined with complete certainty, but is likely correct (See A232657). a(6)=20968 has fairly convincing support, but even finding a good upper bound for a(7) is hard. - _James G. Merickel_, Jun 14 2014
%e 21, 32, and 321 are all composite, and 43 is prime. So a(1)=4. Then the first stem resulting in 2 primes is 10, with 109 and 10987 both prime. So a(2)=10. 73 produces 4 primes in this way if improper concatenation (including 73 itself) is included, but it is not. Since stem values from 11 through 72 never produce more than 2 primes properly, a(3)=73.
%Y Cf. A152396, A232657.
%A _James G. Merickel_, Oct 20 2009
%E a(5) added by _James G. Merickel_, Feb 06 2014