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 A056718 Numbers n such that (61*10^n-7)/9 is prime. 1
 1, 2, 4, 10, 13, 25, 115, 179, 181, 238, 785, 799, 1193, 1730, 1811, 1871, 2116, 2180, 17878, 22093, 30976, 31631, 43271, 52763, 66575 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Or, numbers n such that 6*10^n + 7*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. LINKS Makoto Kamada, Prime numbers of the form 677...77. Maksym Voznyy, Primo certificate for 2116 Maksym Voznyy, Primo certificate for 2180 MATHEMATICA Do[ If[ PrimeQ[ 6*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 5000}] PROG (PARI) is(n)=ispseudoprime((61*10^n-7)/9) \\ Charles R Greathouse IV, Jun 12 2017 CROSSREFS Sequence in context: A174721 A026224 A034233 * A057283 A308987 A107992 Adjacent sequences:  A056715 A056716 A056717 * A056719 A056720 A056721 KEYWORD nonn AUTHOR Robert G. Wilson v, Aug 11 2000 EXTENSIONS The PRP's corresponding to 2116 and 2180 have been proved to be prime by Maksym Voznyy (mvoznyy0526(AT)rogers.com), Jan 05 2008, who has found 2 new PRP's, 17878 and 22093. Definition corrected by N. J. A. Sloane, Jan 05 2008 30976, 31631 and 43271 from Maksym Voznyy, Jan 2008 confirmed as next terms of sequence by Ray Chandler, Feb 06 2012 52763 from Serge Batalov, Feb 27 2009 confirmed as next term of sequence by Ray Chandler, Feb 06 2012 66575 from Ray Chandler, Feb 06 2012 STATUS approved

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Last modified October 15 01:40 EDT 2019. Contains 328025 sequences. (Running on oeis4.)