A056717 Numbers k such that 6*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 5, 7, 25, 31, 112, 199, 533, 616, 718, 787, 1357, 2779, 3889, 4192, 7537, 7945, 23938, 32632, 49169, 56453, 61097, 90211

Offset

1,2

Comments

Also numbers k such that (55*10^k - 1)/9 is prime.

Links

Table of n, a(n) for n=1..23.

Makoto Kamada, Prime numbers of the form 611...11.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 6*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]

Crossrefs

Cf. A002275, A093631.

Sequence in context: A166660 A354202 A037374 * A072370 A057490 A057253

Adjacent sequences: A056714 A056715 A056716 * A056718 A056719 A056720

Keyword

nonn

Author

Robert G. Wilson v, Aug 11 2000

Extensions

2779, 3889, 4192, 7537, 7945 from Hugo Pfoertner, Oct 19 2004

23938 from Ray Chandler, Sep 25 2010

32632 and 49169 from Ray Chandler, Feb 08 2012

56453 from Serge Batalov, Mar 12 2009 confirmed as next term by Ray Chandler, Feb 09 2012

61097 from Ray Chandler, Feb 09 2012

90211 from Ray Chandler, Feb 10 2012

Status

approved