# Re: [isabelle] Can I define direct sum of sets in R^n and R^m

Once again, this formalisation of finite Cartesian products is showing itself to be unsuitable for abstract mathematics. It is only useful if you intend to work in one fixed dimension. I hope it isn't used extensively.
Larry
On 28 Jun 2010, at 17:56, grechukbogdan wrote:
> If A and B are sets, direct sum C of A and B is a set of pairs (a, b) where a \in A and b \in B.
> My first question – is this direct sum defined somewhere in Isabelle?
>
> Now, in my case A is a subset of R^n and B is a subset of R^m. In this case, clearly, C is a subset of R^{n+m}. Currently, finite Cartesian product R^n formalized as type real^'n where 'n is some finite type. Now, is it possible to say that C is a subset of real^('n+'m) ? It seems that I can add types (theory Sum_Type), so I would imagine definition of direct sum as a function from (real^'n, real^'m) to real^('n+'m) such that first n coordinates of sum(x, y) coincides with x, and the next m coordinates - with y. Any ideas how to write down such a definition, which theories to look at, or may be suggestions how to formalize this in a different, more convenient way, would be appreciated.

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