𝐴 is 𝑚×𝑛 matrix Definition 𝐴=𝐴𝑇 and (∀𝑥≠0)𝑥𝑇𝐴𝑥>0. For all 0<𝑖<𝑛 Diagonal entries 𝑎𝑖𝑖>0 Eigenvalues 𝜆𝑖>0 Condition number 𝑐𝑜𝑛𝑑(𝐴)=max𝜆min𝜆