OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 6 followed by the digits 19 is prime (see Example section).
a(31) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 16w19.
EXAMPLE
4 is in this sequence because (5*10^4-143)/3 = 16619 is prime.
Initial terms and associated primes:
a(1) = 3, 1619;
a(2) = 4, 16619;
a(3) = 5, 166619;
a(4) = 6, 1666619;
a(5) = 10, 16666666619, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(5*10^#-143)/3] &]
PROG
(PARI) lista(nn) = for(n=1, nn, if(ispseudoprime((5*10^n-143)/3), print1(n, ", "))); \\ Altug Alkan, Apr 14 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert Price, Apr 14 2016
STATUS
approved