model which ensure a
solution of the condition of a positive–definite conditional variance matrix in the process of optimisation, but induce
the problems of interpretation of parameters. Derivate from the VECH –GARCH model, the GARCH model can be
used unrestricted for a small number of variables, or restricted either as a diagonal or a scalar. In the case of
diagonal restricted model, the covariance of the GARCH (1, 1) model