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 A056657 Numbers k such that 60*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 6
 0, 1, 5, 7, 8, 10, 19, 22, 40, 62, 65, 118, 121, 148, 251, 283, 304, 591, 745, 874, 1203, 1363, 2239, 2402, 5105, 5775, 5812, 12455, 14234, 39605, 55543, 84238, 275921 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers k such that (20*10^k+1)/3 is prime. LINKS Table of n, a(n) for n=1..33. Makoto Kamada, Prime numbers of the form 66...667. Index entries for primes involving repunits FORMULA a(n) = A096507(n) - 1. EXAMPLE 7, 67, 666667, 66666667, 666666667, 66666666667, etc. are primes. MATHEMATICA Do[ If[ PrimeQ[ 60*(10^n - 1)/9 + 7 ], Print[n]], {n, 25556}] CROSSREFS Cf. A002275, A093170, A096507. Sequence in context: A314372 A314373 A192270 * A068312 A082097 A088299 Adjacent sequences: A056654 A056655 A056656 * A056658 A056659 A056660 KEYWORD nonn AUTHOR Robert G. Wilson v, Aug 09 2000 EXTENSIONS More terms from Robert G. Wilson v, Oct 22 2003 2239,2402,5105,5775 from Farideh Firoozbakht, Dec 23 2003 39605 and 55543 from Serge Batalov, Jun 2009 84238 from Serge Batalov, Jul 06 2009 confirmed as next term by Ray Chandler, Feb 23 2012 a(33) derived from A096507 by Robert Price, Jul 07 2024 STATUS approved

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Last modified August 9 05:18 EDT 2024. Contains 375027 sequences. (Running on oeis4.)