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 A056678 Numbers n such that 20*R_n + 9 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
 1, 2, 4, 13, 175, 415, 2504, 2758, 7924, 9400, 10390, 12104, 19615 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (2*10^(n+1)+61)/9 is prime. a(14) > 10^5. - Robert Price, Nov 01 2014 LINKS Makoto Kamada, Prime numbers of the form 22...229. FORMULA a(n) = A099410(n+1) - 1. - Robert Price, Nov 01 2014 MATHEMATICA Do[ If[ PrimeQ[20*(10^n - 1)/9 + 9], Print[n]], {n, 5000}] CROSSREFS Cf. A002275, A093401 (corresponding primes), A099410. Sequence in context: A055463 A091957 A327443 * A218655 A325117 A193232 Adjacent sequences:  A056675 A056676 A056677 * A056679 A056680 A056681 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS Two more terms from Rick L. Shepherd, Mar 30 2004 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 a(11)-a(13) derived from A099410 by Robert Price, Nov 01 2014 STATUS approved

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Last modified December 12 07:31 EST 2019. Contains 329948 sequences. (Running on oeis4.)