OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 61 followed by k-1 occurrences of the digit 6 followed by the digit 9 is prime (see Example section).
a(29) > 2*10^5.
LINKS
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 616w9.
EXAMPLE
5 is in this sequence because (185*10^5 + 7)/3 = 6166669 is prime.
Initial terms and associated primes:
a(1) = 1, 619;
a(2) = 4, 616669;
a(3) = 5, 6166669;
a(4) = 14, 6166666666666669; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(185*10^# + 7)/3] &]
PROG
(PARI) isok(k) = isprime((185*10^k + 7)/3); \\ Michel Marcus, Feb 04 2017
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 01 2017
EXTENSIONS
a(28) from Robert Price, Mar 20 2020
STATUS
approved