# F=ℝ: Normal matrix with real eigenvalues but not diagonalizable

1. Dec 12, 2013

### Bipolarity

I am going through Friedberg and came up with a rather difficult problem I can't seem to resolve.
If $F = ℝ$ and A is a normal matrix with real eigenvalues, then does it follow that A is diagonalizable? If not, can I find a counterexample?

I'm trying to find a counterexample, by constructing a matrix A that is normal and has real eigenvalues, but is not diagonalizable. It is giving me some problems! Any ideas?

If I did not have the assumption that A has real eigenvalues, the rotation matrix would suffice as a counterexample.

If I had the complex field instead of the real field, then easily A is diagonalizable since normality implies orthogonal diagonalizibility in a complex inner product space.

BiP

2. Dec 13, 2013

### Hawkeye18

Just a hint: there is no counterexample, look for proof.