A056701 Numbers k such that 2*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

0, 1, 2, 3, 4, 10, 16, 22, 53, 91, 94, 106, 138, 210, 282, 522, 597, 1049, 2227, 6459, 10582, 18895, 41269, 50702, 53185, 59796, 101395, 116514, 137551, 153116

Offset

1,3

Comments

Also numbers k such that (7*10^k - 1)/3 is prime.

a(31) > 3*10^5. - Robert Price, Oct 19 2014

Links

Table of n, a(n) for n=1..30.

Makoto Kamada, Prime numbers of the form 233...33.

Index entries for primes involving repunits

Mathematica

Do[ If[ PrimeQ[ 2*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 15001}]

Crossrefs

Cf. A002275, A093672.

Sequence in context: A183527 A085934 A220402 * A365352 A285190 A055506

Adjacent sequences: A056698 A056699 A056700 * A056702 A056703 A056704

Keyword

hard,nonn,more

Author

Robert G. Wilson v, Aug 10 2000

Extensions

a(22)-a(26) from Kamada data by Robert Price, Oct 19 2014

a(27)-a(30) from Robert Price, May 31 2023

Status

approved