Minimal negation operator

In logic and mathematics, the minimal negation operator $$\nu\!$$ is a multigrade operator $$(\nu_{k})_{k \in \mathbb{N}}$$ where each $$\nu_{k}\!$$ is a k-ary boolean function defined in such a way that $$\nu_{k}(x_1, \ldots, x_k) = 1$$ if and only if exactly one of the arguments $$x_{j}$$ is 0.

In contexts where the initial letter $$\nu\!$$ is understood, the minimal negation operators can be indicated by argument lists in parentheses. The first four members of this family of operators are shown below, with paraphrases in a couple of other notations, where tildes and primes, respectively, indicate logical negation.


 * $$\begin{matrix}

(\ )     & = & 0 & = & \mbox{false} \\ (x)      & = & \tilde{x} & = & x' \\ (x, y)   & = & \tilde{x}y \lor x\tilde{y} & = & x'y \lor xy' \\ (x, y, z) & = & \tilde{x}yz \lor x\tilde{y}z \lor xy\tilde{z} & = & x'yz \lor xy'z \lor xyz' \end{matrix}$$

It may also be noted that $$(x, y)\!$$ is the same function as $$x + y\!$$ and $$x \ne y$$, and that the inclusive disjunctions indicated for $$(x, y)\!$$ and for $$(x, y, z)\!$$ may be replaced with exclusive disjunctions without affecting the meaning, because the terms disjoined are already disjoint. However, the function $$(x, y, z)\!$$ is not the same thing as the function $$x + y + z\!$$.

The minimal negation operator (mno) has a legion of aliases: logical boundary operator, limen operator, threshold operator, or least action operator, to name but a few. The rationale for these names is visible in the venn diagrams of the corresponding operations on sets.

The next section discusses two ways of visualizing the operation of minimal negation operators. A few bits of terminology will be needed as a language for talking about the pictures, but the formal details are tedious reading, and may already be familiar to many. As a result, the full definitions of the terms marked in bold are relegated to a Glossary at the end of the article.

Truth tables
Table 1 is a truth table for the sixteen boolean functions of type f : B3 &rarr; B, each of which is either a boundary of a point in B3 or the complement of such a boundary.

Charts and graphs
Two common ways of visualizing the space Bk of 2k points are the hypercube picture and the venn diagram picture. Depending on how literally or figuratively one regards these pictures, each point of Bk is either identified with or represented by a point of the k-cube and also by a cell of the venn diagram on k "circles".

In addition, each point of Bk is the unique point in the fiber of truth $$[|s|]$$ of a singular proposition s : Bk → B, and thus it is the unique point where a singular conjunction of k literals is 1.

For example, consider two cases at opposite vertices of the cube:


 * The point whose coordinates are all 1 is the unique point where the conjunction of all posited variables $$x_1\ x_2\ \ldots\ x_{n-1}\ x_n$$ is 1.


 * The point whose coordinates are all 0 is the unique point where the conjunction of all negated variables $$(x_1)(x_2)\ldots(x_{n-1})(x_n)$$ is 1.

To pass from these limiting examples to the general case, observe that a singular proposition s : Bk → B can be given canonical expression as a conjunction of literals, $$s = e_1 e_2 \ldots e_{k-1} e_k$$. Then the proposition $$\nu (e_1, e_2, \ldots, e_{k-1}, e_k)$$ is 1 on the points adjacent to the point where s is 1, and 0 everywhere else on the cube.

For example, consider the case where k = 3. Then the minimal negation operation $$\nu (p, q, r)\!$$, when there is no risk of confusion written more simply as $$(p, q, r)\!$$, has the following Venn diagram:


 * [[Image:Minimal negation operator 1.png|thumb|none|Figure 1. (p, q, r) shaded]]

For a contrasting example, the boolean function expressed by the form $$((p),(q),(r))\!$$ has the following Venn diagram:


 * [[Image:Minimal negation operator 2.png|thumb|none|Figure 2. ((p),(q),(r)) shaded]]

Glossary of basic terms

 * A boolean domain B is a generic 2-element set, say, B = {0, 1}, whose elements are interpreted as logical values, usually but not invariably with 0 = false and 1 = true.


 * A boolean variable x is a variable that takes its value from a boolean domain, as x &isin; B.


 * In situations where boolean values are interpreted as logical values, a boolean-valued function f : X → B or a boolean function g : Bk → B is frequently called a proposition.


 * Given a sequence of k boolean variables, x1, …, xk, each variable xj may be treated either as a basis element of the space Bk or as a coordinate projection xj : Bk → B.


 * This means that the k objects xj for j = 1 to k are just so many boolean functions xj : Bk → B, subject to logical interpretation as a set of basic propositions that generate the complete set of $$2^{2^k}$$ propositions over Bk.


 * A literal is one of the 2k propositions x1, …, xk, (x1), …, (xk), in other words, either a posited basic proposition xj or a negated basic proposition (xj), for some j = 1 to k.


 * In mathematics generally, the fiber of a point y under a function f : X → Y is defined as the inverse image $$f^{-1}(y)$$.


 * In the case of a boolean function f : Bk → B, there are just two fibers:
 * The fiber of 0 under f, defined as $$f^{-1}(0)$$, is the set of points where f is 0.
 * The fiber of 1 under f, defined as $$f^{-1}(1)$$, is the set of points where f is 1.


 * When 1 is interpreted as the logical value true, then $$f^{-1}(1)$$ is called the fiber of truth in the proposition f. Frequent mention of this fiber makes it useful to have a shorter way of referring to it.  This leads to the definition of the notation $$[|f|] = f^{-1}(1)\!$$ for the fiber of truth in the proposition f.


 * A singular boolean function s : Bk → B is a boolean function whose fiber of 1 is a single point of Bk.


 * In the interpretation where 1 equals true, a singular boolean function is called a singular proposition.


 * Singular boolean functions and singular propositions serve as functional or logical representatives of the points in Bk.


 * A singular conjunction in Bk → B is a conjunction of k literals that includes just one conjunct of the pair $$\{ x_j,\ \nu (x_j) \}$$ for each j = 1 to k.


 * A singular proposition s : Bk → B can be expressed as a singular conjunction:


 * $$s\ \ =\ e_1 e_2 \ldots e_{k-1} e_k$$,
 * where
 * $$e_j\ =\ x_j\!$$
 * or
 * $$e_j\ =\ \nu (x_j)\!$$,
 * for
 * $$j\ \ =\ 1\ \mbox{to}\ k$$.
 * }
 * for
 * $$j\ \ =\ 1\ \mbox{to}\ k$$.
 * }
 * }