A338745 When a(n) is even, a(n) is the number of prime digits present so far in the sequence, a(n) included.

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, 22, 37, 39, 26, 41, 43, 28, 45, 30, 32, 47, 34, 49, 51, 36, 53, 55, 40, 57, 59, 61, 63, 44, 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

Offset: 1

Author

Eric Angelini and Carole Dubois, Nov 07 2020

Keywords

• base
• nonn

Comments

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).

Examples

			The first even term is a(3) = 2 and there are indeed 2 prime digits so far in the sequence (3 and 2 itself);
The next even term is a(6) = 4 and there are now 4 prime digits so far (3, 2, 5 and 7);
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.
		

Crossrefs

Cf. A338741, A338742, A338743, A338744, A338746 (variants on the same idea).