A203620 A self-describing sequence: when the sequence is read as a string of digits, a(n) says the position of the digits that are prime.

2, 3, 5, 1, 7, 8, 22, 11, 20, 15, 21, 14, 23, 25, 26, 27, 29, 31, 32, 19, 35, 36, 37, 39, 40, 41, 49, 51, 52, 53, 54, 55, 57, 58, 59, 60, 61, 63, 70, 71, 73, 75, 76, 77, 78, 79, 81, 82, 83, 85, 90, 92, 94, 98, 105, 109, 200, 115, 201, 114, 122, 123, 125, 126

Offset

1,1

Comments

Digits in position a(n) are prime, namely 2, 3, 5 or 7. Any step chooses the minimum integer not yet present in the sequence and not leading to a contradiction.

Links

Table of n, a(n) for n=1..64.

Index to sequences related to say what you see

Example

The sequence cannot start with 1 because the first digit, 1 itself, is not prime. Then let us put 2. The next digit must be prime: 3. Even the third must be prime: 5. No specific indications for the fourth digit. We can choose 1 because the first digit, 2, is prime. The fifth must be prime: 7. And so on.

Crossrefs

Cf. A114315 and A121053.

Sequence in context: A191795 A330612 A121053 * A249070 A249069 A191308

Adjacent sequences: A203617 A203618 A203619 * A203621 A203622 A203623

Keyword

nonn,base

Author

Paolo P. Lava, Feb 15 2012

Status

approved