Let x 1,...,x n be vectors of a unitary space X. Then $$G\,\left( {{x_1}\,,\,...\,,{x_n}} \right)\, = \,\left[{\begin{array}{*{20}{c}}{\left( {{x_1},\,{x_n}} \right)\,...\,\left( {{x_1}\,,\,{x_n}} \right)} \\ \vdots \\ {\left( {{x_1}\,,\,{x_n}} \right)\,...\,\left( {{x_n}\,,{x_n}} \right)} \end{array}} \right]$$ is called the Gram matrix of the vectors x 1,...,x n .