OFFSET
1,3
COMMENTS
For k > 0, numbers k such that digits 88 followed by k-1 occurrences of digit 6 followed by the digit 7 is prime (see Example section).
a(43) > 3*10^5.
LINKS
Makoto Kamada, Search for 886w7.
EXAMPLE
6 is in this sequence because (266*10^n+1)/3 = 88666667 is prime.
Initial terms and associated primes:
a(1) = 0, 89;
a(2) = 1, 887;
a(3) = 2, 8867;
a(4) = 3, 88667;
a(5) = 4, 886667;
a(6) = 5, 8866667;
a(7) = 6, 88666667;
a(8) = 8, 8866666667;
a(9) = 10, 886666666667;
a(10) = 13, 886666666666667, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(266*10^#+1)/3] &]
PROG
(Magma) [n: n in [0..220] | IsPrime((266*10^n + 1) div 3)]; // Vincenzo Librandi, Feb 23 2016
(PARI) is(n)=ispseudoprime((266*10^n + 1)/3) \\ Charles R Greathouse IV, Feb 16 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Feb 22 2016
EXTENSIONS
a(39)-a(41) from Robert Price, Apr 22 2020
a(42) from Robert Price, May 31 2023
STATUS
approved