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A293684
Numbers k such that (19*10^k + 413)/9 is prime.
0
1, 2, 4, 7, 11, 22, 28, 29, 38, 62, 316, 989, 1532, 2068, 2495, 3431, 4775, 9082, 14935, 15572, 39511, 43283, 66748
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 2 followed by the digits 57 is prime (see Example section).
a(24) > 2*10^5.
EXAMPLE
2 is in this sequence because (19*10^2 + 413)/9 = 257 is prime.
Initial terms and associated primes:
a(1) = 1, 67;
a(2) = 2, 257;
a(3) = 4, 21157;
a(4) = 7, 21111157;
a(5) = 11, 211111111157; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(19*10^# + 413)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 14 2017
STATUS
approved