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A295129
Numbers k such that (89*10^k - 179)/9 is prime.
0
1, 4, 10, 12, 15, 52, 72, 150, 160, 271, 732, 1566, 2673, 4840, 7818, 9016, 9441, 9448, 11190, 15792, 64504, 80650, 115030, 140008
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 9 followed by k-2 occurrences of the digit 8 followed by the digits 69 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
2 is in this sequence because (89*10^4 - 179)/9 = 98869 is prime.
Initial terms and associated primes:
a(1) = 1, 79;
a(2) = 4, 98869;
a(3) = 10, 98888888869;
a(4) = 12, 9888888888869;
a(5) = 15, 9888888888888869; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(89*10^# - 179)/9] &]
PROG
(PARI) isok(k) = isprime((89*10^k - 179)/9); \\ Michel Marcus, Nov 15 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 15 2017
EXTENSIONS
a(23)-a(24) from Robert Price, Dec 16 2019
STATUS
approved