login
A294569
Numbers k such that (43*10^k + 101)/9 is prime.
0
1, 3, 6, 9, 12, 27, 33, 45, 63, 81, 351, 357, 540, 582, 1998, 2401, 4140, 4675, 6963, 7873, 19749, 138648, 173826, 190351
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 7 followed by the digits 89 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
3 is in this sequence because (43*10^3 + 101)/9 = 4789 is prime.
Initial terms and associated primes:
a(1) = 1, 59;
a(2) = 3, 4789;
a(3) = 6, 4777789;
a(4) = 9, 4777777789;
a(5) = 12, 4777777777789; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(43*10^# + 101)/9] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 02 2017
EXTENSIONS
a(22)-a(24) from Robert Price, Nov 30 2018
STATUS
approved