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A293399
Numbers k such that (68*10^k - 11)/3 is prime.
0
0, 1, 4, 6, 9, 20, 57, 64, 88, 196, 265, 421, 620, 654, 729, 1587, 4863, 6628, 12358, 23773, 68798, 119931
OFFSET
1,3
COMMENTS
For k > 0, numbers k such that the digits 22 followed by k-1 occurrences of the digit 6 followed by the digit 3 is prime (see Example section).
a(23) > 2*10^5.
EXAMPLE
1 is in this sequence because (68*10^1 - 11)/3 = 223 is prime.
Initial terms and associated primes:
a(1) = 0, 19;
a(2) = 1, 223;
a(3) = 4, 226663;
a(4) = 6, 22666663;
a(5) = 9, 22666666663; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(68*10^# - 11)/3] &] (* Corrected by Georg Fischer, Jul 22 2019 *)
PROG
(PARI) isok(k) = isprime((68*10^k - 11)/3); \\ Altug Alkan, Oct 08 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 08 2017
EXTENSIONS
a(22) from Robert Price, Jan 07 2020
STATUS
approved