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Base 41 has Riesel = 8
If k is odd then any n will have a factor 2. This eliminates k = 1, 3, ,5 , 7.
If (k mod 5) == 1 then any n will have a factor 5. This eliminates k = 6.

k n
2 2
4 1
8 Riesel
Covering set {3, 7}