A287017 Positive numbers k such that (10^(k+2)*109 + 89)/9 is prime.

5, 13, 41, 107, 3049, 52727

Offset: 1

Author

Mikk Heidemaa, May 19 2017

Keywords

• nonn
• hard
• more
• base

Comments

Or '12'||...'1'...||'21' in decimal form is prime ('1' concatenated k times to which the prefix '12' and the suffix '21' are concatenated once).

a(1)...a(6) themselves are primes. a(7) > 70000 (if it exists).

Examples

			5 is a term as 121111121 is prime (as a string, it consists of '1' concatenated 5 times to which the prefix '12' and the suffix '21' are concatenated once).
		

Crossrefs

Cf. A286916, A210873.

Programs

Mathematica

ParallelMap[ If[ PrimeQ[ (10^(#+2)*109+89)/9], #, Nothing]&, Range[3500]]

PARI

is(n)=ispseudoprime((10^(n+2)*109+89)/9) \\ Charles R Greathouse IV, Jun 13 2017