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

Thanks to everyone who made a donation during our annual appeal!
To see the list of donors, or make a donation, see the OEIS Foundation home page.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A257034 Numbers k such that 7*R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
 0, 1, 3, 9, 16, 30, 45, 123, 171, 295, 324, 2785, 2791, 3910, 4015, 4050, 6319, 6415, 14670 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also, numbers k such that (718*10^n-7)/9 is prime. Terms from Kamada. a(20) > 10^5. LINKS Makoto Kamada, Near-repdigit numbers of the form ABAA...AA. Makoto Kamada, Prime numbers of the form 7977...77. EXAMPLE For k=3, 7*R_5 + 2*10^3 = 77777 + 2000 = 79777 which is prime. MATHEMATICA Select[Range[0, 30000], PrimeQ[(718*10^#-7)/9 ] &] PROG (MAGMA) [n: n in [0..400] | IsPrime((718*10^n-1) div 9)]; // Vincenzo Librandi, Apr 15 2015 CROSSREFS Cf. A002275. Sequence in context: A244475 A212870 A306447 * A232167 A058538 A197531 Adjacent sequences:  A257031 A257032 A257033 * A257035 A257036 A257037 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 14 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 January 17 15:12 EST 2020. Contains 330958 sequences. (Running on oeis4.)