OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 2 followed by the digits 09 is prime (see Example section).
a(27) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 42w09.
EXAMPLE
4 is in this sequence because (38*10^4 - 119) / 9 = 42209 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 409;
a(3) = 4, 42209;
a(4) = 5, 422209;
a(5) = 8, 422222209; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(38*10^# - 119) / 9] &]
PROG
(PARI) isok(n) = isprime((38*10^n - 119) / 9); \\ Michel Marcus, Nov 27 2016
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 26 2016
STATUS
approved