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A281992
Numbers k such that (2*10^k - 143)/3 is prime.
0
2, 3, 4, 9, 12, 19, 48, 54, 90, 97, 104, 174, 349, 385, 1020, 1294, 1737, 2430, 9040, 14173, 15604, 17943, 37447, 149803, 164043
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 6 followed by the digits 19 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
3 is in this sequence because (2*10^3 - 143)/3 = 619 is prime.
Initial terms and associated primes:
a(1) = 2, 19;
a(2) = 3, 619;
a(3) = 4, 6619;
a(4) = 9, 666666619;
a(5) = 12, 666666666619; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(2*10^# - 143)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 04 2017
EXTENSIONS
a(24)-a(25) from Robert Price, Mar 14 2018
STATUS
approved