A clone on a set $A$ is a collection of $m$-ary functions $A^m \to A$ ($m$ depends on the function) that is closed under composition and includes all projection functions. Clones are significant because for any language of possible operations on $A$, the functions definable from those operations is a clone. Let $n = |A|$.

Is it true for all $n \in \mathbb{N}$ that there are only finitely many clones on $A$ containing all constant functions? That is, for each $a \in A$ the clone contains a function $f: A \to A$, such that $f(x) = a$ for all $x$.


The classic case is where $A = \{0,1\}$; a set of functions is "expressively adequate" if it generates the clone of all functions. It is a famous and fundamental result that sets such as $\{\lor, \lnot\}$ and $\{\to, \bot\}$ are expressively adequate. A more powerful and incredibly beautiful result is Post's lattice which completely characterizes ALL countably many possible clones on $\{0,1\}$. Wikipedia states: "the lattice of clones on a three-element (or larger) set [has] the cardinality of the continuum, and a complicated inner structure."

But I have an alternate definition of expressive adequacy in mind where we wish to always allow constants $0$ and $1$. Thus we restrict to those clones containing the constant functions, and we find that there are only finitely many clones on $\{0,1\}$. In fact, there are 7 clones:

  • $UM$, which contains just constant functions and projections
  • $\Lambda$, the set of conjunctive functions
  • $V$, the set of disjunctive functions
  • $U$, the set of essentially unary functions (depend only one one coordinate)
  • $A$, the set of affine functions
  • $M$, the set of monotone functions
  • $\top$, all functions.

Thus I am interested in whether there are still finitely many such clones for a set of size greater than $2$.


The answer to the question in your title is yes.

Ágoston, I.; Demetrovics, J.; Hannák, L. On the number of clones containing all constants (a problem of R. McKenzie). Lectures in universal algebra (Szeged, 1983), 21–25, Colloq. Math. Soc. János Bolyai, 43, North-Holland, Amsterdam, 1986.

''Answering a question raised by McKenzie, the authors prove that over any finite set with at least three elements there are continuously many clones containing all constants. Here a clone over some set A is a set of finitary functions over A containing all projections and closed with respect to composition.''

Images of the article from Google Books:

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  • $\begingroup$ Thanks! Seems like it answers it. I'll try to find the article but leave this up for a while in case there is an available proof. $\endgroup$ – 6005 Aug 3 '17 at 20:57
  • 2
    $\begingroup$ @6005 The entire article is available on Google books. $\endgroup$ – Noah Schweber Aug 3 '17 at 21:23
  • $\begingroup$ @NoahSchweber and it's short :D $\endgroup$ – 6005 Aug 3 '17 at 21:39

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