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A291201
Numbers k such that (13*10^k - 61)/3 is prime.
0
1, 4, 7, 9, 10, 13, 27, 35, 94, 150, 198, 258, 673, 1194, 1492, 2320, 2727, 3767, 6246, 6877, 14481, 34327, 57634, 123137, 190732
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 13 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
4 is in this sequence because (13*10^4 - 61)/3 = 43313 is prime.
Initial terms and associated primes:
a(1) = 1, 23;
a(2) = 4, 43313;
a(3) = 7, 43333313;
a(4) = 9, 4333333313;
a(5) = 10, 43333333313; etc.
MATHEMATICA
Select[Range[1, 100000], PrimeQ[(13*10^# - 61)/3] &]
PROG
(Magma) [n: n in [1..300] |IsPrime((13*10^n - 61) div 3)]; // Vincenzo Librandi, Aug 21 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Aug 20 2017
EXTENSIONS
a(24)-a(25) from Robert Price, Nov 28 2018
STATUS
approved