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A287680
Numbers k such that 8*10^k + 81 is prime.
0
0, 2, 3, 9, 11, 26, 39, 42, 58, 75, 105, 167, 279, 561, 2152, 2378, 3774, 5001, 5736, 9470, 13419, 14535, 32291, 89800, 146421
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 0 followed by the digits 81 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
3 is in this sequence because 8*10^3 + 81 = 8081 is prime.
Initial terms and associated primes:
a(1) = 0, 89;
a(2) = 2, 881;
a(3) = 3, 8081;
a(4) = 9, 8000000081;
a(5) = 11, 800000000081; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(8*10^# + 81] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 29 2017
EXTENSIONS
a(25) from Robert Price, Aug 22 2019
STATUS
approved