James Anderson (computer scientist): Difference between revisions

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{| class="wikitable" align=right
|- bgcolor="#ddeeff" align="center"
|colspan=3|'''Variable inputs'''
|colspan=2| '''Function values'''
|- bgcolor="#ddeeff" align="center"
| ''x'' || ''y'' || ''z'' || <math>xy \vee \bar{x}z \vee yz</math> || <math>xy \vee \bar{x}z</math>
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| 0 || 0 || 0 || 0 || 0
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In [[Boolean algebra (logic)|Boolean algebra]], the '''consensus theorem''' or '''rule of consensus'''<ref>Frank Markham Brown, ''Boolean Reasoning: The Logic of Boolean Equations'', 2nd edition 2003, p. 44</ref> is the identity:
 
:<math>xy \vee \bar{x}z \vee yz = xy \vee \bar{x}z</math>
 
The '''consensus''' or '''resolvent''' of the terms <math>xy</math> and <math>\bar{x}z</math> is <math>yz</math>. It is the conjunction of all the unique literals of the terms, excluding the literal which appears unnegated in one term and negated in the other.
 
The conjunctive [[De Morgan's laws|dual]] of this equation is:
 
:<math>(x \vee y)(\bar{x} \vee z)(y \vee z) = (x \vee y)(\bar{x} \vee z)</math>
==Proof==
 
    [[Sides of an equation|LHS]] = <math>xy \vee \bar{x}z \vee (x \vee \bar{x})yz</math>
        = <math>xy \vee \bar{x}z \vee xyz \vee \bar{x}yz</math>
        = <math>xy \vee xyz \vee \bar{x}z \vee \bar{x}yz</math>
        = <math>xy(1 \vee z) \vee \bar{x}z(1 \vee y)</math>
        = <math>xy \vee \bar{x}z</math>
        = RHS
 
==Consensus==
{{anchor|Consensus}}{{anchor|Opposition}}
The '''consensus''' or '''consensus term''' of two conjunctive terms of a disjunction is defined when one term contains the literal <math>a</math> and the other the literal <math>\bar{a}</math>, an '''opposition'''. The consensus is the conjunction of the two terms, omitting both <math>a</math> and <math>\bar{a}</math>, and repeated literals; the consensus is undefined if there is more than one opposition. For example, the consensus of <math>\bar{x}yz</math> and <math>w\bar{y}z</math> is <math>w\bar{x}z</math>.<ref>Frank Markham Brown, ''Boolean Reasoning: The Logic of Boolean Equations'', 2nd edition 2003, p. 81</ref>
 
The consensus can be derived from <math>(x\vee y)</math> and <math>(\bar{x} \vee z)</math> through the [[resolution (logic)|resolution]] [[inference rule]].  This shows that the LHS is derivable from the RHS (if ''A'' &rarr; ''B'' then ''A'' &rarr; ''AB''; replacing ''A'' with RHS and ''B'' with (''y'' ∨ ''z'') ).  The RHS can be derived from the LHS simply through the [[conjunction elimination]] inference rule.  Since RHS &rarr; LHS and LHS &rarr; RHS (in [[propositional calculus]]), then LHS = RHS (in Boolean algebra).
 
==Digital logic circuitry==
 
In [[digital logic]], including the consensus term in a circuit can eliminate [[race hazard]]s.
 
==History==
 
The concept of consensus was introduced by Archie Blake in 1937.<ref name="blake">"Canonical expressions in Boolean algebra", Dissertation, Dept. of Mathematics, U. of Chicago, 1937, reviewed in J. C. C. McKinsey, ''The Journal of Symbolic Logic'' '''3''':2:93 (June 1938) {{DOI|10.2307/2267634}} {{jstor|2267634}}</ref> It was rediscovered by Samson and Mills in 1954<ref>Edward W. Samson, Burton E. Mills, Air Force Cambridge Research Center Technical Report 54-21, April 1954</ref> and by [[Willard van Orman Quine|Quine]] in 1955.<ref>W.V. Quine, "The problem of simplifying truth functions", ''American Mathematical Monthly'' '''59''':521-531, 1952</ref> Quine coined the term 'consensus'. Robinson used it for clauses in 1965 as the basis of his "[[resolution (logic)|resolution principle]]".<ref>J. Alan Robinson, "A Machine-Oriented Logic Based on the Resolution Principle", ''Journal of the ACM'' '''12''':1: 23–41.</ref><ref>D.E. Knuth, ''The Art of Computer Programming'' '''4A''': ''Combinatorial Algorithms'', part 1, p. 539</ref>
 
==Notes==
{{Reflist|1}}
 
== References ==
* Roth, Charles H. Jr. and Kinney, Larry L. (2004, 2010). "Fundamentals of Logic Design", 6th Ed., p.&nbsp;66ff.
 
{{DEFAULTSORT:Consensus Theorem}}
[[Category:Boolean algebra]]
[[Category:Theorems in propositional logic]]

Latest revision as of 00:38, 18 November 2014

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