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A295968
Numbers k such that (44*10^k - 719)/9 is prime.
0
2, 4, 11, 14, 25, 55, 89, 131, 644, 2938, 3338, 4265, 5200, 6178, 7075, 10379, 12830, 26377, 42947, 98284, 121297
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 8 followed by the digits 09 is prime (see Example section).
a(22) > 2*10^5.
EXAMPLE
2 is in this sequence because (44*10^2 - 719)/9 = 409 is prime.
Initial terms and associated primes:
a(1) = 2, 409;
a(2) = 4, 48809;
a(3) = 11, 488888888809;
a(4) = 14, 488888888888809;
a(5) = 25, 48888888888888888888888809; etc.
MATHEMATICA
Select[Range[2, 100000], PrimeQ[(44*10^# - 719)/9] &]
PROG
(PARI) isok(k) = isprime((44*10^k - 719)/9); \\ Michel Marcus, Dec 01 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 30 2017
EXTENSIONS
a(21) from Robert Price, Dec 31 2018
STATUS
approved