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 A056656 Numbers n such that 20*R_n + 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
 0, 1, 2, 7, 10, 35, 94, 100, 127, 259, 350, 466, 644, 1010, 1177, 1216, 2441, 3760, 3805, 15616, 26458, 63116, 88544, 93496 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers n such that (2*10^(n+1)+7)/9 is prime. LINKS Makoto Kamada, Prime numbers of the form 22...223. FORMULA a(n) = A096506(n) - 1. MATHEMATICA Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 3 ], Print[n]], {n, 7000}] CROSSREFS Cf. A093162 (corresponding primes), A096056. Sequence in context: A215407 A084184 A015963 * A151346 A247166 A110739 Adjacent sequences:  A056653 A056654 A056655 * A056657 A056658 A056659 KEYWORD nonn AUTHOR Robert G. Wilson v, Aug 09 2000 EXTENSIONS 2441 from Rick L. Shepherd, Mar 27 2004 15616 and 26458 from Erik Branger, Jan 31 2010 63116, 88544 and 93496 from Erik Branger, Mar 14 2011; confirmed as next terms by Ray Chandler, Feb 17 2012 STATUS approved

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