A056719 Numbers k such that 7*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 7, 55, 83461, 762811

Offset

1,3

Comments

Also numbers k such that (64*10^k - 1)/9 is prime.

No other terms below 820000. - Serge Batalov, Sep 22 2014

Links

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

Makoto Kamada, Prime numbers of the form 711...11.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 7*10^n + (10^n-1)/9], Print[n]], {n, 0, 35076}]

Crossrefs

Cf. A002275, A093632.

Sequence in context: A197228 A009521 A116120 * A038855 A097254 A043911

Adjacent sequences: A056716 A056717 A056718 * A056720 A056721 A056722

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 11 2000

Extensions

A new PRP term from Serge Batalov, Aug 13 2009

A new PRP term 762811 from Serge Batalov, Sep 22 2014

Status

approved