A061309 a(n) is the smallest number required to make the concatenation of a(n) and 2n+1 a prime, or -1 if no such number exists.

1, 0, 0, 0, 1, 0, 0, -1, 0, 0, 4, 0, -1, 1, 0, 0, 2, -1, 0, 1, 0, 0, -1, 0, 1, 1, 0, -1, 1, 0, 0, 1, -1, 0, 2, 0, 0, -1, 2, 0, 1, 0, -1, 4, 0, 1, 1, -1, 0, 1, 0, 0, -1, 0, 0, 2, 0, -1, 1, 3, 3, 1, -1, 0, 1, 0, 4, -1, 0, 0, 2, 2, -1, 5, 0, 0, 1, -1, 0, 4, 2, 0, -1, 0, 3, 1, 0, -1, 4, 0, 0, 13

Offset: 0

Author

Amarnath Murthy, Apr 26 2001

Keywords

• sign
• base

Comments

a(0) = 0 for a prime.

Examples

			a(9) = 0 as 19 itself is a prime; a(10) = 4 as 421 is a prime but 121, 221, 321 are not.
		

Extensions

More terms from Frank Ellermann, Jun 15 2001