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Sometimes I do read in a sentence:

some statement iff another statement

At first I thought it is a mispelling, but then I realized, that I do encounter this at many different posts. Does this mean equivalence?

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    @PaulSlevin actually it is $\phi \Leftrightarrow \varphi$ and $\theta \Leftrightarrow \vartheta$.2012-07-12

3 Answers 3

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http://en.wikipedia.org/wiki/If_and_only_if using google helps a lot sometimes.

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    @the responder: please do not discourage users to ask questions, while refering to google!2017-10-29
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As Zhen Lin already mentioned, "iff" is shorthand for "if and only if". Also in french literature one can find "ssi", which means "si et seulement si".

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    Well yes, we have that abbreviation, but it does not have the form of a misspelling. :D2011-07-08
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Using your example:

some statement iff another statement.

This is a short-hand way of combining the "if-then" form of a statement, and its converse or the vice versa statement when they are both true:

  • if another statement, then some statement;

or

some statement, if another statement (1)


  • if some statement, then another statement;

or

another statement, if some statement (2)


Combining (1) and (2) gives:

some statement iff another statement.