login
A056658
Numbers k such that 60*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
3
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.
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
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