A056658 Numbers k such that 60*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 2, 3, 9, 17, 20, 21, 27, 42, 65, 120, 132, 177, 240, 453, 552, 1599, 2174, 2977, 3648, 7707, 8315, 10391, 12457, 21056, 26222, 48296, 64040, 84903, 92975, 95071

Offset

1,2

Comments

Also numbers k such that (2*10^(k+1)-17)/3 is prime.

Links

Table of n, a(n) for n=1..31.

Makoto Kamada, Prime numbers of the form 66...661.

Index entries for primes involving repunits

Formula

a(n) = A098088(n) - 1. - Robert Price, Aug 19 2014

Mathematica

Do[ If[ PrimeQ[ 60*(10^n - 1)/9 + 1], Print[n]], {n, 7000}]

Crossrefs

Cf. A002275, A092571 (corresponding primes), A098088.

Sequence in context: A143890 A270339 A272057 * A302164 A295965 A034467

Adjacent sequences: A056655 A056656 A056657 * A056659 A056660 A056661

Keyword

hard,nonn

Author

Robert G. Wilson v, Aug 09 2000

Extensions

1599 and 2174 (corresponding to probable primes) from Rick L. Shepherd, Feb 28 2004

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

a(23)-a(31) derived from A098088 by Robert Price, Aug 19 2014

Status

approved