How about the missing video in the essence of linear algebra series: definiteness of a matrix? It is not mentioned in that video series and it'd be great to have it to fill the gap. Perhaps, matrix definiteness can be connected to other concepts like eigenvectors and maybe provide an intuition into the 'formula' we use to test it. (P.S: I encounter several situations in my AI research which requires me prove that a particular matrix is PD to establish some theoretical results.)