login
A280272
Numbers k such that (229*10^k - 1)/3 is prime.
0
3, 6, 8, 9, 10, 17, 27, 28, 45, 81, 83, 225, 246, 1732, 2957, 3021, 5459, 5702, 5740, 7924, 15110, 18683, 25149, 46968, 48944, 49571, 70122, 147699, 158339
OFFSET
1,1
COMMENTS
Numbers k such that the number consisting of the digits 76 followed by k occurrences of the digit 3 is prime (see Example section).
a(30) > 2*10^5.
EXAMPLE
3 is in this sequence because (229*10^3 - 1)/3 = 76333 is prime.
Initial terms and associated primes:
a(1) = 3, 76333;
a(2) = 6, 76333333;
a(3) = 8, 7633333333;
a(4) = 9, 76333333333;
a(5) = 10, 763333333333; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(229*10^# - 1)/3] &]
PROG
(PARI) is(n)=ispseudoprime((229*10^n - 1)/3) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 30 2016
EXTENSIONS
a(24)-a(29) from Robert Price, Jul 10 2020
STATUS
approved