login
A279467
Numbers k such that (14*10^k - 71) / 3 is prime.
0
1, 2, 3, 4, 6, 10, 14, 22, 25, 31, 43, 63, 123, 430, 508, 1457, 1701, 3371, 3429, 3548, 4582, 7463, 8852, 10594, 30621, 143662
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 6 followed by the digits 43 is prime (see Example section).
a(27) > 2*10^5.
EXAMPLE
4 is in this sequence because (14*10^4 - 71) / 3 = 46643 is prime.
Initial terms and associated primes:
a(1) = 1, 23;
a(2) = 2, 443;
a(3) = 3, 4643;
a(4) = 4, 46643;
a(5) = 6, 4666643; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(14*10^# - 71) / 3] &]
PROG
(Magma) [n: n in [1..500] |IsPrime((14*10^n - 71) div 3)]; // Vincenzo Librandi Dec 14 2016
(PARI) is(n)=ispseudoprime((14*10^n - 71)/3) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Dec 12 2016
EXTENSIONS
a(26) from Robert Price, Dec 28 2018
STATUS
approved