A056708 Numbers k such that 4*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 4, 13, 25, 36, 357, 373, 1041, 1089, 1093, 1297, 8274, 10732, 15972, 18114, 21823, 34519, 36096, 75498

Offset

1,2

Comments

Also numbers k such that (43*10^k - 7)/9 is prime.

a(20) > 10^5. - Robert Price, Dec 22 2014

Links

Table of n, a(n) for n=1..19.

Makoto Kamada, Prime numbers of the form 477...77.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 4*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 6000}]

Crossrefs

Cf. A002275, A093940.

Sequence in context: A272734 A273414 A154820 * A307271 A333297 A092484

Adjacent sequences: A056705 A056706 A056707 * A056709 A056710 A056711

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

Extensions

a(12) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(13)-a(19) from Robert Price, Dec 22 2014

Status

approved