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 A056677 Numbers n such that 20*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3
 0, 2, 8, 14, 27, 63, 1167, 1694, 2361 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (2*10^(n+1)+43)/9 is prime. LINKS Makoto Kamada, Prime numbers of the form 22...227. FORMULA a(n) = A099409(n+1) - 1. - Robert Price, Jan 30 2015 MATHEMATICA Do[ If[ PrimeQ[20*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}] Flatten[Position[Table[FromDigits[PadLeft[{7}, n, 2]], {n, 5000}], _?PrimeQ]]-1 (* Harvey P. Dale, May 01 2012 *) Select[Range[0, 2500], PrimeQ[20*(10^n - 1)/9 + 7]&] (* Mikk Heidemaa, Nov 28 2015 *) CROSSREFS Cf. A002275, A093167, A099409. Sequence in context: A277276 A324785 A173974 * A053697 A092346 A196227 Adjacent sequences:  A056674 A056675 A056676 * A056678 A056679 A056680 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 STATUS approved

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Last modified May 26 08:39 EDT 2020. Contains 334620 sequences. (Running on oeis4.)