# How are R^2 and R^4 isomorphic as VS's over Q?

1. Mar 24, 2011

### joeblow

I came across a problem asserting that the C-tensor product of C and C and the R-tensor product of C and C are isomorphic as Q-modules. How does this begin to make sense as they do not have the same dimension over R? The first is isomorphic to R2 and the second is isomorphic to R4 over R.

Also, I'm struggling with tensors. Does anyone have a good source I should check out? (I've tried many)

Last edited: Mar 24, 2011
2. Mar 25, 2011

### lavinia

Over Q these vector spaces are infinite dimensional. They will be isomorphic if the cardinality of their bases is the same - which I think is the cardinality of the Continuum.

The tensor product is a way to extend the field of scalars of a vector space. A real vector space of dimension n (n can be infinite) becomes a complex vector space of dimension n when tensored with C. It becomes a real vector space of twice the dimension over R.

If one views R as a 1 dimensional vector space over itself with single basis vector v, the all elements are of the form rV for real numbers,r. Over C the basis is also v and the resulting vector space is 1 dimensional over C. But over R the basis is V and iV and so is 2 dimensional over R.

Last edited: Mar 25, 2011
3. Mar 25, 2011

### Landau

Indeed; we discussed this here.