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

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A056682 Numbers k such that 40*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 3
 0, 1, 3, 9, 19, 25, 721, 1309, 3169, 28933, 66283, 67795 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers k such that (4*10^(k+1)+23)/9 is prime. a(13) > 10^5. - Robert Price, Nov 30 2014 LINKS Table of n, a(n) for n=1..12. Makoto Kamada, Prime numbers of the form 44...447. Index entries for primes involving repunits FORMULA a(n) = A099413(n+1)-1. - Robert Price, Nov 30 2014 MATHEMATICA Do[ If[ PrimeQ[40*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}] CROSSREFS Cf. A002275, A099413. Sequence in context: A084670 A002091 A056259 * A032670 A068677 A069832 Adjacent sequences: A056679 A056680 A056681 * A056683 A056684 A056685 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS a(10)-a(12) derived from A099413 by Robert Price, Nov 30 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 | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 8 18:41 EDT 2024. Contains 375753 sequences. (Running on oeis4.)