I've found two examples in Friedberg's Linear Algebra book that confused me.
He shows a Diagonal Matrix with Diagonal Entries $3, 1, -3$ (after reducing). And he refers to the eigenvalues as -1 and 3. But isn't there also -3?
For another Matrix with diagonals -1 1 and 3 he has Eigenvalues 1 and 3. But isn't there also an Eigenvalue -1?
From the Characteristic equation i get $(3 - t)(1-t)(-3 - t) = 0$. So there are three distinct roots in my opinion? Could somebody explain this to me? I'm super confused. Thank you!