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| ==Initial definition== | | ==Initial definition== |
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− | The '''minimal negation operator''' <math>\nu~\!</math> is a [[multigrade operator]] <math>(\nu_k)_{k \in \mathbb{N}}~\!</math> where each <math>\nu_k~\!</math> is a <math>k~\!</math>-ary [[boolean function]] defined in such a way that <math>\nu_k (x_1, \ldots , x_k) = 1~\!</math> in just those cases where exactly one of the arguments <math>x_j~\!</math> is <math>0.~\!</math> | + | The '''minimal negation operator''' <math>\nu~\!</math> is a [[multigrade operator]] <math>(\nu_k)_{k \in \mathbb{N}}~\!</math> where each <math>\nu_k~\!</math> is a <math>k~\!</math>-ary [[boolean function]] defined by the rule that <math>\nu_k (x_1, \ldots , x_k) = 1~\!</math> if and only if exactly one of the arguments <math>x_j~\!</math> is <math>0.~\!</math> |
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− | In contexts where the initial letter <math>\nu~\!</math> is understood, the minimal negation operators can be indicated by argument lists in parentheses. In the following text a distinctive typeface will be used for logical expressions based on minimal negation operators, for example, <math>\texttt{(x, y, z)}~\!</math> = <math>\nu (x, y, z).~\!</math> | + | In contexts where the initial letter <math>\nu~\!</math> is understood, the minimal negation operators can be indicated by argument lists in parentheses. In the following text a distinctive typeface will be used for logical expressions based on minimal negation operators, for example, <math>\texttt{(x, y, z)} = \nu (x, y, z).~\!</math> |
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− | 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. | + | The first four members of this family of operators are shown below. The third and fourth columns give paraphrases in two other notations, where tildes and primes, respectively, indicate logical negation. |
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| To express the general case of <math>\nu_k~\!</math> in terms of familiar operations, it helps to introduce an intermediary concept: | | To express the general case of <math>\nu_k~\!</math> in terms of familiar operations, it helps to introduce an intermediary concept: |
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− | '''Definition.''' Let the function <math>\lnot_j : \mathbb{B}^k \to \mathbb{B}~\!</math> be defined for each integer <math>j~\!</math> in the interval <math>[1, k]~\!</math> by the following equation: | + | '''Definition.''' Let the function <math>\lnot_j : \mathbb{B}^k \to \mathbb{B}~\!</math> be defined for each integer <math>j~\!</math> in the interval <math>[1, k]~\!</math> by the following equation: |
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− | If we think of the point <math>x = (x_1, \ldots, x_k) \in \mathbb{B}^k~\!</math> as indicated by the boolean product <math>x_1 \cdot \ldots \cdot x_k~\!</math> or the logical conjunction <math>x_1 \land \ldots \land x_k,~\!</math> then the minimal negation <math>\texttt{(} x_1, \ldots, x_k \texttt{)}~\!</math> indicates the set of points in <math>\mathbb{B}^k~\!</math> that differ from <math>x~\!</math> in exactly one coordinate. This makes <math>\texttt{(} x_1, \ldots, x_k \texttt{)}~\!</math> a discrete functional analogue of a ''point omitted neighborhood'' in analysis, more exactly, a ''point omitted distance one neighborhood''. In this light, the minimal negation operator can be recognized as a differential construction, an observation that opens a very wide field. It also serves to explain a variety of other names for the same concept, for example, ''logical boundary operator'', ''limen operator'', ''least action operator'', or ''hedge operator'', to name but a few. The rationale for these names is visible in the venn diagrams of the corresponding operations on sets. | + | If we take the boolean product <math>x_1 \cdot \ldots \cdot x_k~\!</math> or the logical conjunction <math>x_1 \land \ldots \land x_k~\!</math> to indicate the point <math>x = (x_1, \ldots, x_k)~\!</math> in the space <math>\mathbb{B}^k~\!</math> then the minimal negation <math>\texttt{(} x_1 \texttt{,} \ldots \texttt{,} x_k \texttt{)}~\!</math> indicates the set of points in <math>\mathbb{B}^k~\!</math> that differ from <math>x~\!</math> in exactly one coordinate. This makes <math>\texttt{(} x_1 \texttt{,} \ldots \texttt{,} x_k \texttt{)}~\!</math> a discrete functional analogue of a point-omitted neighborhood in ordinary real analysis, more exactly, a point-omitted distance-one neighborhood. In this light, the minimal negation operator can be recognized as a differential construction, an observation that opens a very wide field. |
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− | The remainder of this discussion proceeds on the ''algebraic boolean convention'' that the plus sign <math>(+)~\!</math> and the summation symbol <math>(\textstyle\sum)~\!</math> both refer to addition modulo 2. Unless otherwise noted, the boolean domain <math>\mathbb{B} = \{ 0, 1 \}~\!</math> is interpreted so that <math>0 = \mathrm{false}~\!</math> and <math>1 = \mathrm{true}.~\!</math> This has the following consequences: | + | The remainder of this discussion proceeds on the algebraic convention that the plus sign <math>(+)~\!</math> and the summation symbol <math>(\textstyle\sum)~\!</math> both refer to addition mod 2. Unless otherwise noted, the boolean domain <math>\mathbb{B} = \{ 0, 1 \}~\!</math> is interpreted for logic in such a way that <math>0 = \mathrm{false}~\!</math> and <math>1 = \mathrm{true}.~\!</math> This has the following consequences: |
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| | valign="top" | <big>•</big> | | | valign="top" | <big>•</big> |
− | | The inclusive disjunctions indicated for the <math>\nu_k~\!</math> of more than one argument may be replaced with exclusive disjunctions without affecting the meaning, since the terms disjoined are already disjoint. | + | | The inclusive disjunctions indicated for the <math>\nu_k~\!</math> of more than one argument may be replaced with exclusive disjunctions without affecting the meaning since the terms in disjunction are already disjoint. |
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