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A282140
Numbers k such that (49*10^k + 311)/9 is prime.
0
1, 3, 6, 7, 9, 10, 12, 24, 39, 135, 258, 382, 660, 900, 1306, 1528, 3658, 3937, 5157, 7006, 7936, 10956, 15396, 45808, 198403
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 4 followed by the digits 79 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
3 is in this sequence because (49*10^3 + 311)/9 = 5479 is prime.
Initial terms and associated primes:
a(1) = 1, 89;
a(2) = 3, 5479;
a(3) = 6, 5444479;
a(4) = 7, 54444479;
a(5) = 9, 5444444479; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(49*10^# + 311)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Feb 06 2017
EXTENSIONS
a(25) from Robert Price, Jun 13 2019
STATUS
approved