The OEIS is supported by the many generous donors to the OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 Table of n, a(n) for n=1..7. 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 Adjacent sequences: A286913 A286914 A286915 * A286917 A286918 A286919 KEYWORD nonn,hard,more,base AUTHOR Mikk Heidemaa, May 17 2017 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

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