This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A256426 Numbers k such that 7*R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
 2, 3, 5, 17, 44, 47, 78, 504, 533, 768, 915, 1155, 1313, 7128, 8088, 15717, 33074 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also, numbers k such that (7*10^k + 173)/9 is prime. Terms from Kamada. a(18) > 33074. LINKS Makoto Kamada, Near-repdigit numbers of the form AA...AABA. Makoto Kamada, Prime numbers of the form 77...7797. EXAMPLE For k=3, 7*R_11 + 20 = 777 + 20 = 797 which is prime. MATHEMATICA Select[Range[0, 30000], PrimeQ[(7*10^# + 173)/9] &] PROG (MAGMA) [n: n in [0..400] | IsPrime((7*10^n + 173) div 9)]; // Vincenzo Librandi, Apr 13 2015 CROSSREFS Cf. A002275. Sequence in context: A211972 A076706 A243441 * A019350 A235630 A235636 Adjacent sequences:  A256423 A256424 A256425 * A256427 A256428 A256429 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 12 2015 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 March 20 13:18 EDT 2019. Contains 321345 sequences. (Running on oeis4.)