A056707 Numbers k such that 4*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 16, 31, 37, 55, 62, 172, 174, 197, 727, 1246, 1752, 4318, 4328, 4930, 6450, 15206, 24041, 25321, 33068, 41484, 51511

Offset

1,2

Comments

Also numbers k such that (13*10^k - 1)/3 is prime.

a(24) > 10^5. - Robert Price, Dec 21 2014

Links

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

Makoto Kamada, Prime numbers of the form 433...33.

Index entries for primes involving repunits

Example

43 and 433 are prime, so 1 and 2 are terms of the sequence.

Mathematica

Do[ If[ PrimeQ[ 4*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]

Crossrefs

Cf. A002275, A093673.

Sequence in context: A242933 A321308 A109210 * A069256 A344016 A279034

Adjacent sequences: A056704 A056705 A056706 * A056708 A056709 A056710

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 10 2000

Extensions

26044 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

a(18)-a(23) from Robert Price, Dec 21 2014

Status

approved