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

5, 13, 41, 107, 3049, 52727

Offset

1,1

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).

Links

Table of n, a(n) for n=1..6.

Example

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).

Mathematica

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

Prog

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

Crossrefs

Cf. A286916, A210873.

Sequence in context: A034735 A305464 A200150 * A229747 A182300 A080925

Adjacent sequences: A287014 A287015 A287016 * A287018 A287019 A287020

Keyword

nonn,hard,more,base

Author

Mikk Heidemaa, May 19 2017

Status

approved