DefinitionSo we have positive definite matrix A if it is symmetric, i.e. if (A)^T = A for all whose eigenvalues are greater than zero. That is, the quadratic form xTAx>0 for all non-zero x. Optimization and numerical analysis Positive definite StringIO matrices are a key ingredient in optimization and numerical analysis. For example, well-known ar positive definite where B=⟨2112⟩.