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

Author

Paolo P. Lava, Feb 15 2012

Keywords

• nonn
• base

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.

Examples

			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.
		

Links

• Index to sequences related to say what you see

Crossrefs

Cf. A114315 and A121053.