Sylvester equation.
sylv(A, B, C, flag)
three real matrices of appropriate dimensions.
character string ('c' or 'd')
X= sylv(A, B, C, 'c') computes X, solution
of the "continuous time" Sylvester equation
X=sylv(A, B, C, 'd') computes X, solution
of the modified "discrete time" Sylvester equation
X=-sylv(-A, B, C, 'd') computes X, solution
of the real "discrete time" Sylvester equation