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A294943
Numbers k such that (865*10^k - 1)/9 is prime.
0
7, 13, 25, 40, 58, 133, 229, 232, 310, 358, 778, 977, 1057, 1271, 4417, 4757, 5242, 6413, 11539, 60583, 149737
OFFSET
1,1
COMMENTS
For k > 1, numbers k such that the digits 96 followed by k occurrences of the digit 1 is prime (see Example section).
a(22) > 2*10^5.
EXAMPLE
7 is in this sequence because (865*10^7 - 1)/9 = 961111111 is prime.
Initial terms and associated primes:
a(1) = 7, 961111111;
a(2) = 12, 961111111111111;
a(3) = 23, 961111111111111111111111111;
a(4) = 40, 961111111111111111111111111111111111111111; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(865*10^# - 1)/9] &]
PROG
(PARI) isok(k) = isprime((865*10^k - 1)/9); \\ Michel Marcus, Nov 12 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Nov 11 2017
EXTENSIONS
a(21) from Robert Price, Apr 18 2020
STATUS
approved