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A275020
Numbers k such that (5*10^k + 91) / 3 is prime.
0
1, 2, 3, 10, 19, 35, 43, 80, 107, 143, 199, 218, 255, 304, 353, 560, 904, 996, 1051, 6141, 8075, 9913, 11151, 28469, 75244, 108960, 122592, 178206, 187471, 257431
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 6 followed by the digits 97 is prime (see Example section).
a(31) > 3*10^5.
EXAMPLE
3 is in this sequence because (5*10^3 + 91) / 3 = 1697 is prime.
Initial terms and associated primes:
a(1) = 1, 47;
a(2) = 2, 197;
a(3) = 3, 1697;
a(4) = 10, 16666666697;
a(5) = 19, 16666666666666666697, etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(5*10^# + 91) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((5*10^n + 91)/3) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more
AUTHOR
Robert Price, Nov 12 2016
EXTENSIONS
a(26)-a(29) from Robert Price, Apr 28 2018
a(30) from Robert Price, Oct 25 2023
STATUS
approved