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!)
A271645 Numbers k such that (23*10^k + 91)/3 is prime. 0
1, 2, 4, 15, 16, 19, 20, 26, 38, 47, 52, 75, 122, 191, 246, 257, 294, 305, 374, 592, 682, 729, 1092, 2053, 2997, 4065, 13936, 17214, 19059, 37433, 142105, 214633, 242909 (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 6 followed by the digits 97 is prime (see Example section).
a(34) > 3*10^5.
LINKS
Makoto Kamada, Search for 76w97.
EXAMPLE
4 is in this sequence because (23*10^4 + 91)/3 = 76697 is prime.
Initial terms and primes associated:
a(1) = 1, 107;
a(2) = 2, 797;
a(3) = 4, 76697;
a(4) = 15, 7666666666666697;
a(5) = 16, 76666666666666697, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(23*10^# + 91)/3] &]
PROG
(PARI) is(n)=ispseudoprime((23*10^n + 91)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
Sequence in context: A196239 A019543 A304568 * A265483 A357692 A154906
KEYWORD
nonn,more
AUTHOR
Robert Price, Apr 11 2016
EXTENSIONS
a(31) from Robert Price, Aug 11 2019
a(32)-a(33) from Robert Price, May 31 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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified February 29 23:21 EST 2024. Contains 370428 sequences. (Running on oeis4.)