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.
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:
denotes AND, 
denotes "is equivalent
to," 
denotes NOT, 
denotes OR, and 
denotes implies.
