A056700 Numbers k such that 2*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 2, 3, 12, 18, 23, 57, 128, 543, 584, 833, 2450, 2810, 2873, 3671, 6384, 10296, 16704, 53049, 56544, 74253

Offset: 1

Author

Robert G. Wilson v, Aug 10 2000

Keywords

• nonn
• hard
• more

Comments

Also numbers k such that (19*10^k-1)/9 is prime.

Links

• Makoto Kamada, Prime numbers of the form 211...11.
• Index entries for primes involving repunits

Crossrefs

Cf. A002275, A068814.

Programs

Mathematica

Do[ If[ PrimeQ[ 2*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]

Extensions

6384 from Hugo Pfoertner, Oct 16 2004

10296 and 16704 from Ray Chandler, Sep 2010

0 added by Arkadiusz Wesolowski, Mar 10 2011

53049 and 56544 from Serge Batalov, Mar 2009 confirmed as next terms by Ray Chandler, Feb 03 2012

74253 from Ray Chandler, Feb 03 2012