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A280431
Numbers k such that (2*10^k - 41)/3 is prime.
0
2, 3, 4, 5, 8, 9, 27, 32, 39, 44, 101, 140, 268, 386, 695, 741, 1155, 1564, 1611, 4076, 6083, 11672, 26685, 50603, 134720
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 6 followed by the digits 53 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
4 is in this sequence because (2*10^4 - 41) / 3 = 6653 is prime.
Initial terms and associated primes:
a(1) = 2, 53;
a(2) = 3, 653;
a(3) = 4, 6653;
a(4) = 5, 66653;
a(5) = 8, 66666653; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(2*10^# - 41) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((2*10^n-41)/3) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 02 2017
EXTENSIONS
a(25) from Robert Price, Feb 14 2018
STATUS
approved