A286916 -id:A286916 - cp's OEIS frontend

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

5, 13, 41, 107, 3049, 52727

Offset: 1

Mikk Heidemaa, May 19 2017

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).

			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).

Cf. A286916, A210873.

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

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

A287061 Numbers k such that (10^(k+3)*6319 + 863)/9 is prime (k > 0).

Original entry on oeis.org

11, 19, 23, 31, 73, 257, 613, 5327, 62359

Offset: 1

Mikk Heidemaa, May 19 2017

The generated prime numbers are of the form '702'||...'1'...||'207' ('1' concatenated k times to which the prefix '702' and the suffix '207' are concatenated once).

a(1)..a(7) themselves are primes. a(10) > 80000, if it exists.

			11 is a term as (10^(11+3)*6319 + 863)/9 = 70211111111111207 is prime.
23 is a term because (10^(23+3)*6319 + 863)/9 = 70211111111111111111111111207 is prime.

Cf. A032674, A260405, A286916.

ParallelMap[ If[ PrimeQ[ (10^(#+3)*6319+863)/9], #, Nothing]&, Range[1000]]

is(n) = ispseudoprime((10^(n+3)*6319+863)/9); \\ Altug Alkan, Jul 23 2017

cp's OEIS Frontend

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

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