A338745 - OEIS

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

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OFFSET

1,2

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).

LINKS

Table of n, a(n) for n=1..77.

EXAMPLE

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).

Sequence in context: A105353 A115966 A338744 * A203602 A249559 A182846

Adjacent sequences: A338742 A338743 A338744 * A338746 A338747 A338748

KEYWORD

base,nonn

AUTHOR

Eric Angelini and Carole Dubois, Nov 07 2020

STATUS

approved