OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 57 is prime (see Example section).
a(31) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 26w57.
EXAMPLE
3 is in this sequence because (8*10^3 - 29)/3 = 2657 is prime.
Initial terms and associated primes:
a(1) = 1, 17;
a(2) = 2, 257;
a(3) = 3, 2657;
a(4) = 8, 266666657;
a(5) = 9, 2666666657, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(8*10^# - 29)/3] &]
PROG
(PARI) is(n)=ispseudoprime((8*10^n-29)/3) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, May 07 2016
EXTENSIONS
a(29)-a(30) from Robert Price, Jul 03 2018
STATUS
approved