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I came across this cross sign when reading a book. Does anyone know what does this mean in the context of a function?

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As @zkutch says, this refers to the cartesian product of two sets.

Its definition has to do with sets, not with functions. The formal definition is as follows:

For two sets $A,B$ we define $$A\times B:= \{(a,b)\mid a\in A, b\in B\}$$ Which is the set of all tuples from A and B.

In the context of functions, it might be simpler to think of this as: a function $f:A\times B \rightarrow C$ essentially takes two arguments $a,b$ and spits out a $c$. This will now be written like that:

$$f(a,b)=c$$

Essentially its the same as accepting a tuple $(a,b)$, so you can rest assured this is exactly the formal mathematical definition for set cartesian product here.

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