[Math] A concrete example of a Hermitian positive definite matrix with complex numbers

linear algebra

I hope someone could provide an example of a Hermitian positive definite matrix with complex numbers in it. I searched the internet and all I can find are symmetric ones with only real values. Many thanks!

Best Answer

Example: $$\left( \begin{array}{cc} 2 & i \\ -i & 2\\ \end{array} \right)$$

Because the sum of positive definite matrices is also positive definite we have also, say:

$$\left( \begin{array}{cc} 2 & i \\ -i & 2\\ \end{array} \right) +\left( \begin{array}{cc} 3 & 1 \\ 1 & 2\\ \end{array} \right)= \left( \begin{array}{cc} 5 & 1+i \\ 1-i & 4\\ \end{array} \right) $$

As pointed out in a comment, you cannot have all entries complex, as the elements in the diagonal must be real.

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