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A294909
Numbers k such that (10^k + 47)/3 is prime.
0
1, 3, 5, 6, 9, 13, 120, 220, 568, 640, 666, 1205, 1600, 2479, 2482, 2939, 3077, 8649, 9520, 12407, 33421, 36162, 41190, 65340
OFFSET
1,2
COMMENTS
For k > 1, numbers k such that k-2 occurrences of the digit 3 followed by the digits 49 is prime (see Example section). - Robert Price, Nov 10 2017
a(25) > 3*10^5. - Robert Price, Oct 26 2023
EXAMPLE
3 is in this sequence because (10^3 + 47)/3 = 349 is prime.
Initial terms and associated primes:
a(1) = 1, 19;
a(2) = 3, 349;
a(3) = 5, 33349;
a(4) = 6, 333349;
a(5) = 9, 333333349; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(10^# + 47)/3] &]
PROG
(Magma) [n: n in [0..500] |IsPrime((10^n+47) div 3)];
(PARI) is(n) = ispseudoprime((10^n + 47) / 3); \\ Altug Alkan, Sep 26 2016
CROSSREFS
See also similar sequences listed in A274986.
Sequence in context: A096394 A075472 A072522 * A070111 A070117 A176345
KEYWORD
nonn,more,hard
AUTHOR
Vincenzo Librandi, Sep 26 2016
EXTENSIONS
a(18)-a(24) from Robert Price, Nov 10 2017
STATUS
approved