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A283185
Numbers k such that (86*10^k + 121)/9 is prime.
0
0, 1, 4, 6, 24, 52, 60, 76, 94, 106, 136, 196, 283, 319, 486, 1560, 1615, 1747, 2188, 2359, 5004, 5608, 9301, 22086, 30391, 76128, 90588, 139315
OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 69 is prime (see Example section).
a(29) > 2*10^5.
EXAMPLE
4 is in this sequence because (86*10^4 + 121)/9 = 95569 is prime.
Initial terms and associated primes:
a(1) = 0, 23;
a(2) = 1, 109;
a(3) = 4, 95569;
a(4) = 6, 9555569;
a(5) = 24, 9555555555555555555555569; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(86*10^# + 121)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Mar 02 2017
EXTENSIONS
a(28) from Robert Price, Aug 30 2019
STATUS
approved