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What is an example of a birational morphism between $\mathbb{P}^{n} \times \mathbb{P}^{m} \rightarrow \mathbb{P}^{n+m}$?

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The subset $\mathbb A^n\times \mathbb A^m$ is open dense in $\mathbb P^n\times \mathbb P^m$ and the subset $\mathbb A^{n+m}$ is open dense in $\mathbb P^n\times \mathbb P^m$.
Hence the isomorphism $\mathbb A^n\times \mathbb A^m\stackrel {\cong}{\to} \mathbb A^{n+m}$ is the required birational isomorphism.

The astonishing point is that a rational map need only be defined on a dense open subset , which explains the uneasy feeling one may have toward the preceding argument, which may look like cheating.
The consideration of "maps" which are not defined everywhere is typical of algebraic ( or complex analytic) geometry, as opposed to other geometric theories like topology, differential geometry,...

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    ah, missed the finer thing. I've been stuck with this problem, can you have a look please? http://math.stackexchange.com/questions/$1$46377/birational-map-from-a-variety-to-projective-line2012-05-20