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A283186
Numbers k such that 89*10^k - 7 is prime.
0
1, 2, 3, 19, 33, 42, 73, 259, 271, 297, 303, 324, 392, 623, 1218, 2801, 4273, 6071, 6383, 8699, 8706, 15139, 17163, 40491, 54518, 83976, 89687, 105284, 168614
OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 88 followed by k-1 occurrences of the digit 9 followed by the digit 3 is prime (see Example section).
a(30) > 2*10^5.
EXAMPLE
3 is in this sequence because 89*10^3 - 7 = 88993 is prime.
Initial terms and associated primes:
a(1) = 1, 883;
a(2) = 2, 8893;
a(3) = 3, 88993;
a(4) = 19, 889999999999999999993;
a(5) = 33, 88999999999999999999999999999999993; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[89*10^# - 7] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Mar 02 2017
EXTENSIONS
a(28)-a(29) from Robert Price, Apr 23 2020
STATUS
approved