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A293398
Numbers k such that (728*10^k - 71)/9 is prime.
0
0, 2, 5, 6, 11, 14, 41, 47, 51, 84, 164, 228, 293, 1178, 1212, 1292, 2099, 2694, 4446, 7382, 13638, 39428, 40971
OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 80 followed by k-1 occurrences of the digit 8 followed by the digit 1 is prime (see Example section).
a(24) > 2*10^5.
EXAMPLE
2 is in this sequence because (728*10^2 - 71)/9 = 8081 is prime.
Initial terms and associated primes:
a(1) = 0, 73;
a(2) = 2, 8081;
a(3) = 5, 8088881;
a(4) = 6, 80888881;
a(5) = 11, 8088888888881; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(728*10^# - 71)/9] &]
PROG
(PARI) isok(k) = isprime((728*10^k - 71)/9); \\ Altug Alkan, Oct 08 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Oct 08 2017
STATUS
approved