ИСТИНА

We introduce a notion of a key relation, which almost coincides with the notion of a critical relation introduced by Keith A. Kearnes and Ágnes Szendrei. We need only key relations to describe all clones on finite sets. We describe all key relations on 2 elements; these are relations that can be defined as a disjunction of linear equations. Then we show that in general key relations do not have such a nice characterization, but we can get a characterization of key relations preserved by a weak near-unanimity operation. Finally, we show that this characterization can be very helpful in studying of the lattice of clones.