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A293034
Numbers k such that (203*10^k + 1)/3 is prime.
0
1, 5, 12, 17, 21, 23, 25, 29, 109, 168, 195, 327, 417, 461, 1991, 3512, 3955, 4239, 6228, 24317, 91613, 131457, 152143, 159381, 162689
OFFSET
1,2
COMMENTS
For k > 0, numbers k such that the digits 67 followed by k-1 occurrences of the digit 6 followed by the digit 7 is prime (see Example section).
a(26) > 2*10^5.
EXAMPLE
5 is in this sequence because (203*10^5 + 1)/3 = 6766667 is prime.
Initial terms and associated primes:
a(1) = 1, 677;
a(2) = 5, 6766667;
a(3) = 12, 67666666666667;
a(4) = 17, 6766666666666666667;
a(5) = 21, 67666666666666666666667; etc.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(203*10^# + 1)/3] &]
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Sep 28 2017
EXTENSIONS
a(22)-a(25) from Robert Price, Mar 22 2020
STATUS
approved