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

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A256906 Numbers k such that 7*R_k + 10 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
 1, 3, 6, 15, 21, 24, 31, 291, 408, 457, 643, 2671, 2676, 10893, 21151, 26445, 77304, 96312 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers k such that (7*10^k + 83)/9 is prime. Terms from Kamada. Note Kamada does not recognize k=1 as 17 is a degenerate case of form AAA..ABA. a(17) > 30000. a(19) > 2*10^5. - Tyler Busby, Feb 01 2023 LINKS Table of n, a(n) for n=1..18. Makoto Kamada, Near-repdigit numbers of the form AA...AABA. Makoto Kamada, Prime numbers of the form 77...7787. Index entries for primes involving repunits. EXAMPLE For k=3, 7*R_11 + 10 = 777 + 10 = 787 which is prime. MATHEMATICA Select[Range[0, 30000], PrimeQ[(7*10^# + 83)/9] &] CROSSREFS Cf. A002275. Sequence in context: A289296 A310126 A298015 * A095869 A292610 A069559 Adjacent sequences: A256903 A256904 A256905 * A256907 A256908 A256909 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 12 2015 EXTENSIONS a(17)-a(18) from Tyler Busby, Feb 01 2023 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 13 23:31 EDT 2024. Contains 374290 sequences. (Running on oeis4.)