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Tautology and contradiction

A compound statement that is always true, regardless of what

trułh values are assigned to its statement variables, is called a

tautology or a logie law. Such a statement is logically true.

A compound statement that is always false, regardless ofwhat

truth values are assigned to its statement variables, is called a

contradiction. Such a statement is logically false.

List of the most important tautologies (logie laws):

(1)    p <=>p, p =>p (the identity law for equivalence and implication)

(2)    (p =>q) <=>( ~q =>~p) (the contrapositive law)

(3)    p 1/(t?) (law of ”the excluded center”)

(4)    ~\p /1(7?)] (law of contradiction)