A056705 Numbers k such that 3*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 11, 17, 773, 18155

Offset

1,3

Comments

Also numbers k such that (34*10^k - 7)/9 is prime.

Links

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

Makoto Kamada, Prime numbers of the form 377...77.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 3*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 30070}]

Crossrefs

Cf. A002275, A093939.

Sequence in context: A228055 A132092 A226258 * A259744 A065706 A078874

Adjacent sequences: A056702 A056703 A056704 * A056706 A056707 A056708

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

Status

approved