A297121 Self-describing sequence: sequence starts with a(1) = 2 and a(n) is chosen to be the smallest positive number not already in the sequence such that the assertion "sequence gives the positions of the even digits when the sequence is read as a string of digits" is true.

2, 4, 1, 6, 7, 8, 20, 10, 12, 14, 16, 18, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100, 21, 19, 23, 25, 37, 39, 41, 43, 45, 57, 59, 61, 63, 65, 77, 79, 81, 83, 85, 99, 104, 106, 112

Offset

1,1

Comments

Inspired by Sloane's sequence A125132.

Links

Jean-Marc Falcoz, Table of n, a(n) for n = 1..2001

Example

Here are the digits strung together (the even digits occur at positions that are indexed by terms of the sequence):
-241678201
0121416182
2242628303
2343638404...
Explanation: a(2) must be even, so a(2)=4; a(3)=1; a(4) must be even, so a(4)=6; a(5) cannot be 3 or 5 (contradiction) thus a(5)=7. And so on.

Crossrefs

Sequence in context: A019680 A249144 A080032 * A105357 A346246 A167546

Adjacent sequences: A297118 A297119 A297120 * A297122 A297123 A297124

Keyword

base,nonn

Author

Eric Angelini and Jean-Marc Falcoz, Dec 26 2017

Status

approved