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

1,2

Comments

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

Links

Table of n, a(n) for n=1..21.

Makoto Kamada, Prime numbers of the form 211...11.

Index entries for primes involving repunits

Mathematica

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

Crossrefs

Cf. A002275, A068814.

Sequence in context: A073452 A112976 A100570 * A299547 A140989 A281086

Adjacent sequences: A056697 A056698 A056699 * A056701 A056702 A056703

Keyword

nonn,hard,more

Author

Robert G. Wilson v, Aug 10 2000

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

Status

approved