$$A=PDP^{-1}$$$$A = U \\Sigma V^{\\top}$$The SVD exists for any matrix $\\bold{A}\\in\\R^{m\\times n}$

Derivate -$f:\reals^ \to \reals$ -derivative of $f$ with respect to $x$ > $$ >\phi(x)=\lim_{t\to x}\frac{f(t)-f(x)}{t-x} >$$ provided this limit exi