http://dcherukhin.i2p/research/research.htm
(статья: pdf ;
рукопись: ps.gz , tex.tgz ) On invariant classes of functions that are repetition-free expressible by formulas //
Diskretn. Anal. Issled. Oper. Ser. 1. 9(4): 57-74 (2002), in Russian [links: mathnet , Zbl 1017.06005 , ams ] Abstract: The main results are as follows: (1) For each complete finite Boolean base $B$,
the cardinality of the sets of invariant classes consisting of functions
expressible by repetition-free formulas over $B$ is...