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| {{For|the corresponding concept in [[combinational logic]]|XNOR gate}}
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| [[File:XNOR ANSI Labelled.svg|thumb|118px|XNOR Logic Gate Symbol]]
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| '''Logical equality''' is a [[logical operator]] that corresponds to [[equality (mathematics)|equality]] in [[Boolean algebra (logic)|Boolean algebra]] and to the [[logical biconditional]] in [[propositional calculus]]. It gives the [[function (mathematics)|functional]] value ''[[Truth|true]]'' if both functional arguments have the same [[logical value]], and ''[[False (logic)|false]]'' if they are different.
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| It is customary practice in various applications, if not always technically precise, to indicate the operation of '''logical equality''' on the logical operands ''x'' and ''y'' by any of the following forms:
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| <math>\begin{matrix}
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| x \leftrightarrow y & \quad & \quad & x \Leftrightarrow y & \quad & Exy \\
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| x \ \mbox{EQ} \ y & \quad & \quad & x = y
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| \end{matrix}</math>
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| Some logicians, however, draw a firm distinction between a ''functional form'', like those in the lefthand column, which they interpret as an application of a function to a pair of arguments — and thus a mere indication that the value of the compound expression depends on the values of the component expressions — and an ''equational form'', like those in the righthand column, which they interpret as an assertion that the arguments have equal values, in other words, that the functional value of the compound expression is ''true''.
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| In [[mathematics]], the plus sign "+" almost invariably indicates an operation that satisfies the axioms assigned to addition in the type of [[algebraic structure]] that is known as a ''[[field (mathematics)|field]]''. For boolean algebra, this means that the logical operation signified by "+" is not the same as the [[inclusive disjunction]] signified by "∨" but is actually equivalent to the logical inequality operator signified by "≠", or what amounts to the same thing, the [[exclusive disjunction]] signified by "XOR". Naturally, these variations in usage have caused some failures to communicate between mathematicians and switching engineers over the years. At any rate, one has the following array of corresponding forms for the symbols associated with logical inequality:
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| : <math>\begin{matrix}
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| x + y & \quad & \quad & x \not\equiv y & \quad & Jxy \\
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| x \ \mbox{XOR} \ y & \quad & \quad & x \ne y
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| \end{matrix}</math>
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| This explains why "EQ" is often called "[[XNOR gate|XNOR]]" in the [[combinational logic]] of circuit engineers, since it is the ''Negation'' of the ''[[XOR]]'' operation; '''NXOR''' is a less commonly used alternative.{{cn|date=September 2012}} Another rationalization of the admittedly circuitous name "XNOR" is that one begins with the "both false" operator NOR and then adds the eXception, "or both true".
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| ==Definition==
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| '''Logical equality''' is an [[logical operation|operation]] on two [[logical value]]s, typically the values of two [[proposition]]s, that produces a value of ''true'' if and only if both operands are false or both operands are true.
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| The [[truth table]] of '''p EQ q''' (also written as '''p = q''', '''p ↔ q''', or '''p ≡ q''', or '''p == q''') is as follows:
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| {| align="center" border="1" cellpadding="8" cellspacing="0" style="background:lightcyan; font-weight:bold; text-align:center; width:45%"
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| |+ '''Logical Equality'''
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| |- style="background:paleturquoise"
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| ! style="width:15%" | p
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| ! style="width:15%" | q
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| ! style="width:15%" | p = q
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| |-
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| | 0 || 0 || 1
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| |-
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| | 0 || 1 || 0
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| |-
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| | 1 || 0 || 0
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| |-
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| | 1 || 1 || 1
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| |}
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| [[File:Venn1001.svg|thumb|left|150px|The [[Venn diagram]] of A EQ B (red part is true)]]
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| {{Clear}}
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| ==Alternative descriptions==
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| The form (''x'' = ''y'') is equivalent to the form (''x'' ∧ ''y'') ∨ (¬''x'' ∧ ¬''y'').
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| <math>(x = y) = \lnot(x \oplus y) = \lnot x \oplus y = x \oplus \lnot y = (x \land y) \lor (\lnot x \land \lnot y) = (\lnot x \lor y) \land (x \lor \lnot y)</math>
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| For the operands ''x'' and ''y'', the [[truth table]] of the logical equality operator is as follows: | |
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| {| class="wikitable"
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| ! colspan="2" rowspan="2" | <math>x \leftrightarrow y </math> !! colspan="2" | y
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| |-
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| ! T !! F
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| |-
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| ! rowspan="2" | x !! T
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| | style="padding: 1em;" | T
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| | style="padding: 1em;" | F
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| |-
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| ! F
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| | style="padding: 1em;" | F
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| | style="padding: 1em;" | T
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| |}
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| == See also ==
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| {{Portal|Thinking}}
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| * [[Boolean function]]
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| * [[If and only if]]
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| * [[Logical equivalence]]
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| * [[Logical biconditional]]
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| * [[Propositional calculus]]
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| ==External links==
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| * Mathworld, [http://mathworld.wolfram.com/XNOR.html XNOR]
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| {{Logical connectives}}
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| {{DEFAULTSORT:Logical Equality}}
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| [[Category:Logical connectives]]
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| [[Category:Logic gates]]
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