Normal matrices

A complex square matrix A is a normal matrix if

$A^{\dagger }A=AA^{\dagger },$

where $A^{\dagger }$ is the conjugate transpose of A. That is, a matrix is normal if it commutes with its conjugate transpose: $[A,A^{\dagger }]=0$.

Normal matrices are precisely those to which the spectral theorem applies: a matrix $A$ is normal if and only if it can be represented by a diagonal matrix $\Lambda$ and a unitary matrix $U$ by the formula

$A=U\Lambda U^{\dagger },$

where

$\Lambda =\mathrm {diag} (\lambda _{1},\lambda _{2},\dots )$

$U^{\dagger }U=UU^{\dagger }=I.$

The entries $\lambda _{i}$ of the diagonal matrix $\Lambda$ are the eigenvalues of $A$, and the columns of $U$ are the eigenvectors of $A$. The matching eigenvalues in $\Lambda$ must be ordered as the eigenvectors are ordered as columns of $U$.

References

• Normal matrix entry in Wikipedia