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 A099409 Numbers n such that 2*R_n + 5 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3
 0, 1, 3, 9, 15, 28, 64, 1168, 1695, 2362 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers n such that (2*10^n+43)/9 is prime. a(11) > 50000. - Robert Price, Oct 27 2014 LINKS Makoto Kamada, Prime numbers of the form 22...227. FORMULA a(n) = A056677(n-1) + 1. MATHEMATICA Do[ If[ PrimeQ[ 2(10^n - 1)/9 + 5], Print[n]], {n, 0, 5000}] PROG (MAGMA) [n: n in [0..1000] | IsPrime( 2*(10^n - 1) div 9 + 5)]; // Vincenzo Librandi, Oct 28 2014 CROSSREFS Cf. A002275, A056677, A093167. Sequence in context: A280963 A122819 A056287 * A002127 A061810 A048701 Adjacent sequences:  A099406 A099407 A099408 * A099410 A099411 A099412 KEYWORD nonn AUTHOR Robert G. Wilson v, Oct 14 2004 EXTENSIONS Added a(1)=0, adapted Mathematica program, Vincenzo Librandi, Oct 28 2014 STATUS approved

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Last modified August 15 18:27 EDT 2018. Contains 313779 sequences. (Running on oeis4.)