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A290962
Numbers k such that (13*10^k - 43)/3 is prime.
1
1, 2, 4, 5, 8, 12, 55, 125, 136, 221, 224, 668, 1254, 2639, 4745, 5888, 8526, 9139, 13771, 17936, 27713, 38668, 44680, 73891, 135184, 200610, 215592, 247793, 258710, 291721
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 3 followed by the digits 19 is prime (see Example section).
a(31) > 3*10^5.
EXAMPLE
2 is in this sequence because (13*10^2 - 43)/3 = 419 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 419;
a(3) = 4, 43319;
a(4) = 5; 433319;
a(5) = 8, 433333319; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(13*10^# - 43)/3] &]
PROG
(PARI) isok(n) = ispseudoprime((13*10^n - 43)/3) \\ Altug Alkan, Aug 15 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 15 2017
EXTENSIONS
a(25) from Robert Price, Nov 28 2018
a(26)-a(30) from Robert Price, Oct 26 2023
STATUS
approved