Tautology
A tautology is a logical statement in which the conclusion is equivalent to the premise. More colloquially, it is formula in propositional calculus which is always true (Simpson 1992, p. 2015; D'Angelo and West 2000, p. 33; Bronshtein and Semendyayev 2004, p. 288).
If 
is a tautology, it is written 
. A sentence
whose truth table contains only 'T' is called a tautology.
The following sentences are examples of tautologies:
 |
(1)
|
 |
(2)
|
 |
(3)
|
(Mendelson 1997, p. 26), where 
denotes AND,
denotes "is equivalent
to," 
denotes NOT,
denotes OR, and 
denotes implies.
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