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A294908
Numbers k such that (13*10^k + 107)/3 is prime.
0
1, 5, 7, 9, 16, 39, 87, 127, 129, 132, 226, 228, 327, 772, 1191, 1689, 4953, 5143, 25121, 26327, 52861, 60966
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 3 followed by the digits 69 is prime (see Example section).
a(23) > 2*10^5.
EXAMPLE
5 is in this sequence because (13*10^5 + 107)/3 = 433369 is prime.
Initial terms and associated primes:
a(1) = 1, 79;
a(2) = 5, 433369;
a(3) = 7, 43333369;
a(4) = 9, 4333333369;
a(5) = 16, 43333333333333369; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(13*10^# + 107)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 10 2017
STATUS
approved