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A293272
Numbers k such that (10^k - 19)/9 is prime.
0
3, 4, 6, 9, 34, 36, 126, 141, 160, 286, 678, 1236, 3757, 4681, 4881, 6360, 6588, 7056, 7752, 41925, 43423, 43503, 47565, 52608, 55084, 76069, 199248
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 1 followed by the digits 09 is prime (see Example section).
a(28) > 2*10^5.
EXAMPLE
4 is in this sequence because (10^4 - 19)/9 = 1109 is prime.
Initial terms and associated primes:
a(1) = 3, 109;
a(2) = 4, 1109;
a(3) = 6, 111109;
a(4) = 9, 111111109;
a(5) = 34, 1111111111111111111111111111111109; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[10^# - 19)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 06 2017
EXTENSIONS
a(27) from Robert Price, Jan 02 2018
STATUS
approved