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 A056658 Numbers n such that 60*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (2*10^(n+1)-17)/3 is prime. LINKS Makoto Kamada, Prime numbers of the form 66...661. 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

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Last modified June 18 18:14 EDT 2021. Contains 345120 sequences. (Running on oeis4.)