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A288483
Numbers k such that (782*10^k - 71)/9 is prime.
0
0, 2, 9, 14, 15, 30, 48, 57, 75, 98, 125, 483, 792, 930, 1133, 1764, 3123, 3212, 3429, 3989, 5595, 17262, 37287, 47033, 49148, 87809
OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 86 followed by k-1 occurrences of the digit 8 followed by the digit 1 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
2 is in this sequence because (782*10^2 - 71)/9 = 8681 is prime.
Initial terms and associated primes:
a(1) = 0, 79;
a(2) = 2, 8681;
a(3) = 9, 86888888881;
a(4) = 14, 8688888888888881;
a(5) = 15, 86888888888888881; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(782*10^# - 71)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 09 2017
STATUS
approved