OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 47 is prime (see Example section).
a(31) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 95w47.
EXAMPLE
3 is in this sequence because (86*10^3 - 77)/9 = 9547 is prime.
Initial terms and associated primes:
a(1) = 2, 947;
a(2) = 3, 9547;
a(3) = 8, 955555547;
a(4) = 9, 9555555547;
a(5) = 12, 9555555555547, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(86*10^# - 77)/9] &]
PROG
(PARI) is(n)=ispseudoprime((86*10^n - 77)/9) \\ Charles R Greathouse IV, Jun 08 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, May 30 2016
STATUS
approved