Positive Definite Matrix -- from Wolfram MathWorld

An n×n complex matrix A is called positive definite if R[x^*Ax]>0 (1) for all nonzero complex vectors x in C^n, where x^* denotes the conjugate transpose of the vector x. In the case of a real matrix A, equation (1) reduces to x^(T)Ax>0, (2) where x^(T) d

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