principal of leading matrices:
- https://www.reddit.com/r/askmath/comments/3o5a66/what_exactly_are_principal_minors/
- https://math.stackexchange.com/questions/551422/deducing-that-a-matrix-is-indefinite-using-only-its-leading-principal-minors
well the determinant means product of eigen values and step by step we check each eigen value https://math.stackexchange.com/questions/507641/show-that-the-determinant-of-a-is-equal-to-the-product-of-its-eigenvalues
the trace means sum of eigen values
- https://www.reddit.com/r/math/comments/bpxhem/intuition_of_the_trace_of_a_matrix/
- https://mathoverflow.net/questions/13526/geometric-interpretation-of-trace
- https://math.stackexchange.com/questions/4711970/whats-the-intuitive-meaning-of-matrix-trace-if-any
- https://www.quora.com/What-is-a-mathematically-intuitive-way-to-understand-the-trace-of-a-matrix
3Blue1Brwon visualization of linear transformation of matrix
- https://www.reddit.com/r/3Blue1Brown/comments/18ojyhc/guys_i_made_interactive_graph_for_linear_algebra/
- https://www.reddit.com/r/3Blue1Brown/comments/aemh1t/3b1binspired_interactive_matrix_visualization/
linear algebra (3blue1brown lessons)