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

Author

Robert G. Wilson v, Aug 09 2000

Keywords

• hard
• nonn

Comments

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

Links

• Makoto Kamada, Prime numbers of the form 66...661.
• Index entries for primes involving repunits

Crossrefs

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

Programs

Mathematica

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

Formula

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

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