Quadratic Form

It is a easier way to express the quadratic form of multiple variables.

$$q_A(x_1,\ldots,x_n) = \sum_{i=1}^{n}\sum_{j=1}^{n}a_{ij}{x_i}{x_j} = \mathbf x^\mathrm{T} A \mathbf x.$$

$A$ is a symetric matrix. $x$ is a vector of variables.