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

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A286936 Numbers k such that (71*10^k - 287)/9 is prime. 0
1, 2, 4, 8, 26, 28, 43, 70, 92, 128, 331, 364, 478, 532, 689, 778, 895, 1210, 5081, 7855, 17852, 20864, 42598, 56858, 120703, 173854 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
For k>1, numbers such that the digit 7 followed by k-2 occurrences of the digit 8 followed by the digits 57 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Search for 78w57.
EXAMPLE
4 is in this sequence because (71*10^4 - 287)/9 = 78857 is prime.
Initial terms and primes associated:
a(1) = 1, 47;
a(2) = 2, 757;
a(3) = 4, 78857;
a(4) = 8, 788888857;
a(5) = 26, 788888888888888888888888857; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(71*10^# - 287)/9] &]
CROSSREFS
Sequence in context: A191700 A000643 A261489 * A323948 A196265 A112285
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 16 2017
EXTENSIONS
a(25)-a(26) from Robert Price, Sep 06 2019
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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 14:33 EDT 2024. Contains 371254 sequences. (Running on oeis4.)