A326894 a(1) = 1; thereafter, a(n) is the reversal of the next prime after a(n - 1) in base 3 if n is prime, and the reversal of the next composite after a(n - 1) in base 3 if n is composite.

1, 2, 1, 4, 7, 8, 19, 20, 5, 2, 1, 4, 7, 8, 1, 4, 7, 8, 19, 20, 5, 2, 1, 4, 2, 4, 2, 4, 7, 8, 19, 20, 5, 2, 4, 2, 1, 4, 2, 4, 7, 8, 19, 20, 5, 2, 1, 4, 2, 4, 2, 4, 7, 8, 1, 4, 2, 4, 7, 8, 19, 20, 5, 2, 4, 2, 1, 4, 2, 4, 7, 8, 19, 20, 5, 2, 4, 2, 1, 4, 2, 4, 7, 8, 1, 4, 2, 4, 7, 8, 1, 4, 2

Offset

1,2

Comments

The sequence is written in base 10.

Rémy Sigrist's argument from A326344 gives an upper bound of 23, although the true maximum value is 20, as confirmed by Andrew Weimholt's argument from A326344.

Links

N. J. A. Sloane, Table of n, a(n) for n = 1..20000

Robert Dougherty-Bliss, Proof that A326894 is bounded with maximum value 20

Example

By definition, a(1) = 1. The next composite after a(3) = 1 is 4, or 11 in base 3. Reversed this is still 11 in base 3, or 4 in base 10. Thus a(4) = 4 since 4 is composite.

Crossrefs

Cf. A326344, A326892, A151800, A113646.

Sequence in context: A358566 A139769 A357470 * A378343 A275778 A007839

Adjacent sequences: A326891 A326892 A326893 * A326895 A326896 A326897

Keyword

nonn,base

Author

Robert Dougherty-Bliss, Sep 13 2019

Extensions

Corrected by N. J. A. Sloane, Sep 13 2019

Status

approved