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 A099411 Numbers n such that 3*R_n + 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 4
 1, 2, 3, 6, 46, 394, 978, 2586, 2811, 2968, 3642, 4827, 4918, 5592, 5706, 10683, 12891, 14118, 74350, 88680 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (10^n+11)/3 is prime. a(21) > 10^5. - Robert Price, Nov 02 2014 LINKS Makoto Kamada, Prime numbers of the form 33...337. FORMULA a(n) = A056680(n) + 1. MATHEMATICA Do[ If[ PrimeQ[ 3(10^n - 1)/9 + 4], Print[n]], {n, 10000}] CROSSREFS Cf. A002275, A056680, A093168. Sequence in context: A018359 A274805 A049383 * A018372 A097350 A277374 Adjacent sequences:  A099408 A099409 A099410 * A099412 A099413 A099414 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Oct 14 2004 EXTENSIONS a(16)-a(20) from Robert Price, Nov 02 2014 STATUS approved

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