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 A056655 Numbers n such that 10*R_n +7 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
 0, 1, 3, 4, 7, 22, 28, 39, 130, 135, 214, 610, 766, 2152, 2575, 22972, 42688, 85711, 85863 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers n such that (10^(n+1)+53)/9 is prime. 2575 also produces a probable prime. a(20) > 10^5. - Robert Price, Jan 13 2015 LINKS Makoto Kamada, Prime numbers of the form 11...117. FORMULA a(n) = A097684(n) - 1 for all n >= 0. - Rick L. Shepherd, Aug 23 2004 MATHEMATICA Do[ m = n; If[ primeQ[ 10*(10^n - 1)/9 + 7 ], Print[ n ] ], {n, 1, 1250} ] CROSSREFS Cf. A093139 (corresponding primes), A097684. Sequence in context: A241660 A030724 A124082 * A332971 A292033 A288501 Adjacent sequences:  A056652 A056653 A056654 * A056656 A056657 A056658 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 09 2000 EXTENSIONS 2152 (giving a probable prime) from Rick L. Shepherd, Mar 23 2004 2575 from Rick L. Shepherd, Aug 23 2004 a(16)-a(19) derived from A097684 by Robert Price, Jan 13 2015 STATUS approved

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Last modified May 26 02:56 EDT 2020. Contains 334613 sequences. (Running on oeis4.)