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 A056727 Numbers n such that 9*10^n + 7*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1
 1, 2, 4, 19, 28, 73, 203, 220, 274, 292, 470, 763, 1891, 3307, 7007, 7306, 9755, 11395, 39452, 78242 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (88*10^n-7)/9 is prime. a(21) > 10^5. - Robert Price, Nov 30 2014 LINKS Makoto Kamada, Prime numbers of the form 977...77. MATHEMATICA Do[ If[ PrimeQ[ 9*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}] CROSSREFS Cf. A002275, A093944 (corresponding primes). Sequence in context: A268338 A171735 A201379 * A154140 A131578 A018273 Adjacent sequences:  A056724 A056725 A056726 * A056728 A056729 A056730 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 11 2000 EXTENSIONS a(18)-a(20) from Robert Price, Nov 30 2014 STATUS approved

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Last modified April 21 14:42 EDT 2021. Contains 343154 sequences. (Running on oeis4.)