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## Homework Statement

This is for a much larger question I'm working on.

I have four linearly independent vectors in [itex] \mathbb C^9 [/itex], and hence they span a four dimensional space. Now I have a matrix that is composed out of their outerproduct, namely, if we let the vectors be [itex] v_i, i = 0, \ldots 3 [/itex] then the [itex] 9 \times 9 [/itex] matrix is as follows:

[tex] M = \sum_{i=0}^3 v_i^T v_i [/tex]

where the T is transpose.

Now in general, an inverse to this matrix does not exist. However, I have been informed that M has an inverse on the four-dimensional subspace spanned by [itex] \{ v_i \} [/itex]. What I want to know is how do I find the form of this matrix?

## The Attempt at a Solution

I can easily find the four dimesional subspace, the issue is somehow "reducing" M into the space, finding the inverse (which is easy), and then making it 9x9 again. I honestly don't know how to do this at all.