Chaichian, M. and Demichev, A. (2001) Path Integrals in Physics. Vol. 1, Stochastic Processes and Quantum Mechanics, IOP Publishing, Bristol.
 Lay, D.C. (2012) Linear Algebra and its Applications. Addison-Wesley, Boston.
 Gilbert, G.T. (1991) Positive Definite Matrices and Sylvester’s Criterion. American Mathematical Monthly, 98, 44-46.
 Yu, B., Rumer, M. and Rivkin, Sh. (1980) Thermodynamics, Statistical Physics and Kinetics. Mir Publishers, Moscow.