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A286916 Positive numbers k such that (10^(k+2)*331 - 133)/9 is prime. 2
7, 13, 23, 409, 1147, 2597, 4561 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Or '36'||...'7'...||'63' in decimal form is prime ('7' concatenated k times to which the prefix '36' and the suffix '63' are concatenated once).
a(1)..a(4) and a(7) themselves are primes. a(8) > 50000 (if it exists).
LINKS
EXAMPLE
7 is a term as 36777777763 is prime (as a string, it consists of '7' concatenated 7 times to which the prefix '36' and the suffix '63' are concatenated once).
MATHEMATICA
ParallelMap[ If[ PrimeQ[ (10^(#+2)*331-133)/9], #, Nothing]&, Range[5000]]
PROG
(PARI) is(n)=ispseudoprime((10^(n+2)*331-133)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
Sequence in context: A316118 A265766 A228727 * A032690 A188557 A356966
KEYWORD
nonn,hard,more,base
AUTHOR
Mikk Heidemaa, May 17 2017
STATUS
approved

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Last modified July 22 12:58 EDT 2024. Contains 374499 sequences. (Running on oeis4.)