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 A099413 Numbers n such that 4*R_n + 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
 0, 1, 2, 4, 10, 20, 26, 722, 1310, 3170, 28934, 66284, 67796 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers n such that (4*10^n+23)/9 is prime. a(14) > 10^5. - Robert Price, Nov 26 2014 LINKS Makoto Kamada, Prime numbers of the form 44...447. FORMULA a(n) = A056682(n-1) + 1. - Robert Price, Nov 26 2014 MATHEMATICA Do[ If[ PrimeQ[ 4(10^n - 1)/9 + 3], Print[n]], {n, 5000}] CROSSREFS Cf. A002275, A056682. Sequence in context: A083844 A026554 A259132 * A271624 A283090 A283143 Adjacent sequences:  A099410 A099411 A099412 * A099414 A099415 A099416 KEYWORD nonn,hard,more AUTHOR Robert G. Wilson v, Oct 14 2004 EXTENSIONS a(11)-a(13) from Robert Price, Nov 26 2014 STATUS approved

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Last modified December 11 02:48 EST 2018. Contains 318049 sequences. (Running on oeis4.)