if and only if

A term used by mathematicians and logicians to indicate that something happens under very specific conditions. Often seen in proofs and theories.

1) The sum a geometric sequence is given by a/(1-r) if and only if |r| < 1.

2) In a bar:
dumped guy: "My girlfriend left me because I told her I loved her!"
other guy: "What a bitch!"
dumped guy: "No, it's all my fault, she's a mathematician, and I should have said 'I love you and only you'".

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