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A278334
Numbers k such that (856*10^k - 1) / 9 is prime.
0
2, 3, 5, 8, 9, 15, 20, 24, 41, 63, 66, 99, 281, 300, 462, 686, 726, 1196, 1574, 2543, 3023, 5322, 12161, 13677, 33797, 137633
OFFSET
1,1
COMMENTS
For k >= 0, numbers k such that the digits 95 followed by k occurrences of the digit 1 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
3 is in this sequence because (856*10^3 - 1) / 9 = 95111 is prime.
Initial terms and associated primes:
a(1) = 2, 9511;
a(2) = 3, 95111;
a(3) = 5, 9511111;
a(4) = 8, 9511111111;
a(5) = 9, 95111111111; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(856*10^# - 1) / 9] &]
PROG
(PARI) is(n)=ispseudoprime((856*10^n - 1)/9) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 18 2016
EXTENSIONS
a(26) from Robert Price, Mar 30 2020
STATUS
approved