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A056718 Numbers k such that (61*10^k - 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 k such that 6*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
LINKS
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
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, which correspond to 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 March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)