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Discrete Mathematics University of Kentucky CS 275 Spring ... - MGNet

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We can compound logical operators to make complicated propositions. In<br />

general, using parentheses makes the expressions clearer, even though more<br />

symbols are used. However, there is a well defined operator precedence<br />

accepted in the field. Lower numbered operators take precedence over higher<br />

numbered operators.<br />

Examples:<br />

• ¬p!q = (¬p) !q<br />

• p!q"r = (p!q) "r<br />

Operator Precedence<br />

¬ 1<br />

! 2<br />

" 3<br />

% 4<br />

& 5<br />

9<br />

Definition: A compound proposition that is always true is a tautology. One that<br />

is always false is a contradiction. One that is neither is a contingency.<br />

Example:<br />

p ¬p p!¬p p"¬p<br />

T F F T<br />

F T F T<br />

contigencies contradiction tautology<br />

Definition: Compound propositions p and q are logically equivalent if p&q is a<br />

tautology and is denoted p'q (sometimes written as p!q instead).<br />

10

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