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A289081
Numbers k such that (4*10^k - 19)/3 is prime.
0
1, 2, 3, 4, 5, 8, 42, 52, 81, 96, 98, 152, 190, 261, 686, 1930, 2152, 5694, 6341, 14067, 18124, 31678, 58521, 73647, 128244
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 3 followed by the digits 27 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
3 is in this sequence because (4*10^3 - 19)/3 = 1327 is prime.
Initial terms and associated primes:
a(1) = 1, 7;
a(2) = 2, 127;
a(3) = 3, 1327;
a(4) = 4, 13327;
a(5) = 5, 133327; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(4*10^# - 19)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 23 2017
EXTENSIONS
a(25) from Robert Price, Mar 01 2018
STATUS
approved