A286916 Positive numbers k such that (10^(k+2)*331 - 133)/9 is prime.

7, 13, 23, 409, 1147, 2597, 4561

Offset: 1

Author

Mikk Heidemaa, May 17 2017

Keywords

• nonn
• hard
• more
• base

Comments

Or '36'||...'7'...||'63' in decimal form is prime ('7' concatenated k times to which the prefix '36' and the suffix '63' are concatenated once).

a(1)..a(4) and a(7) themselves are primes. a(8) > 50000 (if it exists).

Examples

			7 is a term as 36777777763 is prime (as a string, it consists of '7' concatenated 7 times to which the prefix '36' and the suffix '63' are concatenated once).
		

Programs

Mathematica

ParallelMap[ If[ PrimeQ[ (10^(#+2)*331-133)/9], #, Nothing]&, Range[5000]]

PARI

is(n)=ispseudoprime((10^(n+2)*331-133)/9) \\ Charles R Greathouse IV, Jun 13 2017