The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A084428 Numbers n such that n 7's followed by n is prime. 7
 1, 23, 209, 1171, 6239, 7543 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(7) > 15000. - Derek Orr, Jul 29 2014 a(7) > 10^5. - Robert Price, Apr 05 2019 LINKS EXAMPLE 23 is in the sequence because 7777777777777777777777723(23 7's followed by 23) is prime. MATHEMATICA Do[ If[GCD[n, 30] == 1 && PrimeQ[ FromDigits[ Join[ Table[7, {n}], IntegerDigits[n]]]], Print[n]], {n, 1, 6500}] PROG (PARI) for(n=1, 15000, p=""; for(k=1, n, p=concat(p, Str(7))); p=concat(p, Str(n)); if(ispseudoprime(eval(p)), print1(n, ", "))) \\ Derek Orr, Jul 29 2014 CROSSREFS Cf. A070746, A068817. Sequence in context: A022683 A042020 A263521 * A322287 A327918 A327919 Adjacent sequences:  A084425 A084426 A084427 * A084429 A084430 A084431 KEYWORD nonn,base,more,hard AUTHOR Farideh Firoozbakht, Jun 27 2003 EXTENSIONS a(6) from Derek Orr, Jul 29 2014 STATUS approved

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

Last modified May 12 07:28 EDT 2021. Contains 343821 sequences. (Running on oeis4.)