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In current practice, the single 'word' "iff" is almost always read as the four words "if and only if". However, in the preface of General Topology , Kelley suggests that it should be read differently: "In some cases where mathematical content requires 'if and only if' and euphony demands something less I use Halmos' 'iff ' ".
Venn diagram of (true part in red) In logic and mathematics, the logical biconditional, also known as material biconditional or equivalence or bidirectional implication or biimplication or bientailment, is the logical connective used to conjoin two statements and to form the statement "if and only if" (often abbreviated as "iff " [1]), where is known as the antecedent, and the consequent.
In mathematics, theorems are often stated in the form "P is true if and only if Q is true". Because, as explained in previous section, necessity of one for the other is equivalent to sufficiency of the other for the first one, e.g. P ⇐ Q {\displaystyle P\Leftarrow Q} is equivalent to Q ⇒ P {\displaystyle Q\Rightarrow P} , if P is necessary ...
This statement expresses the idea "' if and only if '". In particular, the truth value of p ↔ q {\displaystyle p\leftrightarrow q} can change from one model to another. On the other hand, the claim that two formulas are logically equivalent is a statement in metalanguage , which expresses a relationship between two statements p {\displaystyle ...
Since the statement and the converse are both true, it is called a biconditional, and can be expressed as "A polygon is a quadrilateral if, and only if, it has four sides." (The phrase if and only if is sometimes abbreviated as iff.) That is, having four sides is both necessary to be a quadrilateral, and alone sufficient to deem it a quadrilateral.
An informal fallacy that presents two options as the only possibilities when in fact more possibilities exist. false dilemma A form of false dichotomy where two choices are presented as the only options when other alternatives are available, often used to force a decision. falsemaker An entity or set of conditions that makes a proposition false.
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Equivalence (if and only if): , , , , (prefix) in which is the most modern and widely used, and is commonly used where is also used. For example, the meaning of the statements it is raining (denoted by p {\displaystyle p} ) and I am indoors (denoted by q {\displaystyle q} ) is transformed, when the two are combined with logical connectives: