[Math] Definition of Hermitian linear operator

linear algebramatrices

1.Given a matrix A, what does $\overline A^t$ mean? I guess it is first taking conjugate for each entry in A and then taking transpose. Or is there anything to do with "Hermitian"?
For example, if $$A=\begin{bmatrix}1+i & 1-i\\2+i & 2-i\\\end{bmatrix} $$ what does $\overline A^t$ look like?

2.Also, I am wondering what is the definition of "a linear transformation is Hermitian"? Does it mean its corresponding matrix is Hermitian?

Best Answer

The concept of Hermitian linear transformations requires your complex vector space to have an additional structure, a Hermitian product, i.e. a conjugated-symmetric inner product: $x\cdot y = (y\cdot x)^*$, with $^*$ denoting complex cojugation. A linear operator $A$ is then called Hermitian if $x\cdot Ay =(y\cdot Ax)^*$. The matrix of a Hermitian operator is a Hermitian matrix (in the sense that the matrix is equal to its transpose complex conjugate) IF the matrix is written in an orthonormal basis (for a general basis the matrix of a Hermitian operator is NOT in general a Hermitian matrix).