A303849 Lexicographically earliest sequence of distinct terms such that what emerges from the mask (right-aligned) is even (see the Comments section for the mask explanation).

1, 20, 2, 21, 3, 22, 4, 23, 5, 24, 6, 25, 7, 26, 8, 27, 9, 28, 200, 10, 201, 11, 202, 12, 203, 13, 204, 14, 205, 15, 206, 16, 207, 17, 208, 18, 209, 19, 210, 29, 211, 30, 212, 31, 213, 32, 214, 33, 215, 34, 216, 35, 217, 36, 218, 37, 219, 38, 220, 39, 221, 40, 222, 41, 223, 42, 224, 43, 225, 44, 226, 45, 227, 46, 228, 47

Offset: 1

Author

Eric Angelini and Jean-Marc Falcoz, May 01 2018

Keywords

• nonn
• base

Comments

For any pair of consecutive terms, one of the terms uses fewer digits than the other. This term is called the mask. Put the mask on the other term, starting from the right. What is not covered by the mask forms an even number on the left.

The sequence starts with a(1) = 1 and is always extended with the smallest integer not yet present that doesn't lead to a contradiction.

This sequence is a permutation of the positive integers, as all integers will appear at some point, either as mask or masked.

Comparing the two b-files (the first 10000 terms), this seems to be a duplicate of A303847. - R. J. Mathar, Jun 23 2018

Examples

			In the pair (1,20), 1 is the mask; 2 emerges and is even;
in the pair (20,2), 2 is the mask; 2 emerges and is even;
in the pair (2,21), 2 is the mask; 2 emerges and is even;
in the pair (21,3), 3 is the mask; 2 emerges and is even;
...
in the pair (117,2018), 117 is the mask; 2 emerges and is even;
etc.
		

Links

• Jean-Marc Falcoz, Table of n, a(n) for n = 1..10001

Crossrefs

Cf. A303784 (same idea, but the mask is left-aligned).