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A281276
Numbers k such that (2*10^k + 457)/9 is prime.
0
1, 2, 4, 5, 7, 8, 11, 13, 20, 22, 38, 47, 76, 269, 299, 527, 724, 1214, 2243, 2327, 12635, 15064, 28921, 34303, 49117, 58151, 89279, 199283
OFFSET
1,2
COMMENTS
For k > 2, numbers k such that k-2 occurrences of the digit 2 followed by the digits 73 is prime (see Example section).
a(29) > 2*10^5.
EXAMPLE
2 is in this sequence because (2*10^2 + 457) / 9 = 73 is prime.
Initial terms and associated primes:
a(1) = 1, 53;
a(2) = 2, 73;
a(3) = 4, 2273;
a(4) = 5, 22273;
a(5) = 7, 2222273; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(2*10^# + 457) / 9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 18 2017
EXTENSIONS
a(28) from Robert Price, Jan 08 2018
STATUS
approved