A criterion for positive completeness in ternary logic

The operator of positive closure is considered on the set $P_k$ of functions of $k$-valued logic. Some positive complete systems of functions are defined. It is proved that every positive complete class of functions from $P_k$ is positive generated by the set of all functions depending on at most $k$ variables. For each $k\geqslant 3$, the three families of positive precomplete classes are defined. It is shown that, for $k=3$, the 10 classes of these families constitute a criterion system.