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A280060
Numbers k such that (2*10^k + 49)/3 is prime.
0
0, 1, 2, 3, 5, 6, 7, 8, 13, 21, 22, 25, 26, 38, 200, 395, 442, 561, 908, 1295, 5541, 7795, 8600, 19157, 22536, 45636
OFFSET
1,3
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 6 followed by the digits 83 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
3 is in this sequence because (2*10^3 + 49) / 3 = 683 is prime.
Initial terms and associated primes:
a(1) = 0, 17;
a(2) = 1, 23;
a(3) = 2, 83;
a(4) = 3, 683;
a(5) = 5, 66683; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(2*10^# + 49) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((2*10^n + 49)/3) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 04 2017
STATUS
approved