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A280558
Numbers k such that (13*10^k + 89) / 3 is prime.
0
1, 2, 3, 6, 10, 12, 18, 34, 42, 61, 76, 85, 94, 178, 348, 451, 1123, 1455, 2234, 4519, 7502, 16036, 24216, 156522
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 63 is prime (see Example section).
a(25) > 2*10^5.
EXAMPLE
3 is in this sequence because (13*10^3 + 89) / 3 = 4363 is prime.
Initial terms and associated primes:
a(1) = 1, 73;
a(2) = 2, 463;
a(3) = 3, 4363;
a(4) = 6, 4333363;
a(5) = 10, 43333333363; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(13*10^# + 89) / 3] &]
PROG
(PARI) is(n)=ispseudoprime((13*10^n + 89)/3) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jan 05 2017
EXTENSIONS
a(24) from Robert Price, Sep 10 2018
STATUS
approved