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 A056664 Numbers n such that 80*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 3
 2, 18, 78, 138, 222, 462, 543, 1095, 1418, 3246, 3876, 4416, 9506, 11090, 14601, 27810, 29187 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also numbers n such that (8*10^(n+1)-71)/9 is prime. a(18) > 10^5. - Robert Price, Nov 01 2014 LINKS Makoto Kamada, Prime numbers of the form 88...881. FORMULA a(n) = A099421(n+1) - 1. - Robert Price, Nov 01 2014 MATHEMATICA Do[ If[ PrimeQ[ 80*(10^n - 1)/9 + 1 ], Print[n]], {n, 15000}] CROSSREFS Cf. A002275, A092675 (corresponding primes), A099421. Sequence in context: A197093 A034473 A024171 * A213830 A064057 A176496 Adjacent sequences:  A056661 A056662 A056663 * A056665 A056666 A056667 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 09 2000 EXTENSIONS 1415 (giving a probable prime) from Rick L. Shepherd, Mar 02 2004. There are no other terms <= 2500. a(16)-a(17) derived from A099421 by Robert Price, Nov 01 2014 STATUS approved

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