n Mathematical measure of length given by "the square root of the squares." Denoted by ||\cdot||_2, the two-norm of a vector \vec v= is ||\vec v||_2=\sqrt{a_1^2+a_2^2+\cdots+a_n^2}. The two norm of an m\times m matrix A is defined by \max_{\vec v\neq\vec 0}\frac{||A\vec v||_2}{||\vec v||_2} where \vec v is an m-dimensional vector that is not the zero vector.