Is it possible that $L+L^T$ is a positive definite matrix?

86 Views Asked by At

If $L$ is a nonsymmetric Laplacian matrix, is it possible that $L+L^T$ is a positive definite matrix?

1

There are 1 best solutions below

0
On BEST ANSWER

The answer is no. It is impossible for $L+L^T$ to be a positive definite matrix. This is because of \begin{align} 1^T(L+L^T)1 &= 1^TL1 + 1^T L^T 1 \\ &= 1^T(L1) + (L1)^T 1 \\ &= 0 \end{align} which has used the property $L1=0$. Therefore, this contradicts the definition of a positive definite matrix. This means $L+L^T$ has at least one non-positive eigenvalue.