In text (Spivak) it says that a function is a collection of pairs of numbers with the following property: if (a,b) & (a,c) are both in the collection, then b=c; in other words, the collection must not contain two different pairs with the same first element. Now in an other text (Kolmogorov) I found that if b is a range of the function then it could have several preimages or no preimages at all (preimage means the domain) now these two definitions are opposite to each other & I don't really know that which one is true :|(adsbygoogle = window.adsbygoogle || []).push({});

P.S. Just figured out that both definitions are same, please ignore my post

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# What is the exact definition of a function?

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