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A287210
Numbers k such that 78*10^k - 7 is prime.
0
0, 1, 2, 4, 11, 14, 15, 16, 18, 21, 28, 44, 55, 286, 304, 494, 970, 1611, 2051, 2756, 6452, 16758, 29291, 98413, 113954, 197702
OFFSET
1,3
COMMENTS
For k > 1, numbers k such that the digits 77 followed by k-1 occurrences of the digit 9 followed by the digit 3 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
2 is in this sequence because 78*10^2 - 7 = 7793 is prime.
Initial terms and associated primes:
a(1) = 0, 71;
a(2) = 1, 773;
a(3) = 2, 7793;
a(4) = 4, 779993;
a(5) = 11, 7799999999993; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[78*10^# - 7] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, May 21 2017
EXTENSIONS
a(25)-a(26) from Robert Price, Oct 08 2020
STATUS
approved