%I #5 Nov 15 2020 12:55:49
%S 1,3,2,5,7,4,9,11,13,15,6,17,19,21,8,23,10,25,27,14,29,31,16,33,18,35,
%T 22,37,39,26,41,43,28,45,30,32,47,34,49,51,36,53,55,40,57,59,61,63,44,
%U 65,67,46,69,71,73,50,52,75,77,79,58,81,83,85,60,87,62,89,91,93,95,64,97,99,101,103,66
%N When a(n) is even, a(n) is the number of prime digits present so far in the sequence, a(n) included.
%C The prime digits are 2, 3, 5 and 7. The prime numbers appear in their natural order in the sequence [except for the switch a(2)<->a(3)]. Some nonprimes will never appear (12 for instance).
%e The first even term is a(3) = 2 and there are indeed 2 prime digits so far in the sequence (3 and 2 itself);
%e The next even term is a(6) = 4 and there are now 4 prime digits so far (3, 2, 5 and 7);
%e The next even term is a(11) = 6 and there are now 6 prime digits so far (3, 2, 5, 7, 3 and 5); etc.
%Y Cf. A338741, A338742, A338743, A338744, A338746 (variants on the same idea).
%K base,nonn
%O 1,2
%A _Eric Angelini_ and _Carole Dubois_, Nov 07 2020