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A289942
Numbers k such that (85*10^k - 1)/3 is prime.
0
1, 2, 5, 9, 64, 69, 118, 377, 409, 616, 959, 1093, 1726, 3034, 5929, 8140, 11197, 15293, 15381, 27811, 58091, 63757, 72025, 83921, 122585, 139913, 166297, 256301
OFFSET
1,2
COMMENTS
Numbers k such that the digits 28 followed by k occurrences of the digit 3 is prime (see Example section).
a(29) > 3*10^5.
EXAMPLE
2 is in this sequence because (85*10^2 - 1)/3 = 2833 is prime.
Initial terms and associated primes:
a(1) = 1, 283;
a(2) = 2, 2833;
a(3) = 5, 2833333;
a(4) = 9, 28333333333;
a(5) = 64, 283333333333333333333333333333333333333333333333333333333333333333; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(85*10^# - 1)/3] &]
PROG
(PARI) isok(k) = isprime((85*10^k - 1)/3); \\ Michel Marcus, Jul 16 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jul 15 2017
EXTENSIONS
a(25)-a(27) from Robert Price, Feb 03 2020
a(28) from Robert Price, Jul 02 2023
STATUS
approved