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

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Definition: A proposition is a statement <strong>of</strong> a true or false fact (but not both).<br />

Examples:<br />

• 2+2 = 4 is a proposition because this is a fact.<br />

• x+1 = 2 is not a proposition unless a specific value <strong>of</strong> x is stated.<br />

Definition: The negation <strong>of</strong> a proposition p, denoted by ¬p and pronounced not<br />

p, means that, “it is not the case that p.” The truth values for ¬p are the opposite<br />

for p.<br />

Examples:<br />

• p: Today is Thursay, ¬p: Today is not Thursday.<br />

• p: At least a foot <strong>of</strong> snow falls in Boulder on Fridays. ¬p: Less than a foot<br />

<strong>of</strong> snow falls in Boulder on Fridays.<br />

5<br />

Definition: The conjunction <strong>of</strong> propositions p and q, denoted p!q, is true if<br />

both p and q are true, otherwise false.<br />

Definition: The disjunction <strong>of</strong> propositions p and q, denoted p"q, is true if<br />

either p or q is true, otherwise false.<br />

Definition: The exclusive or <strong>of</strong> propositions p and q, denoted p#q, is true if<br />

only one <strong>of</strong> p and q is true, otherwise false.<br />

Truth tables:<br />

p ¬p q p!q p"q p#q<br />

T F T T T F<br />

T * F * F F T T<br />

F * T * T F T T<br />

F T F F F F<br />

* The truth table for p and ¬p is really a 2$2 table.<br />

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