MODEL SPACES FOR COMPOSITIONS OF Q-FUNCTIONS

Given two Q-functions f and g, a symmetric relation and a selfadjoint extension is constructed whose Q-function is the composition f og. To obtain this result we use the concept of tensor products of Hilbert sp aces. Furthermore for a symmetric relation S and a selfadjoint extensi on A the Q-function of S and A is related to the Q-function of S-1 and A(-1). This connection is used to characterize some properties of S a nd A by means of their Q-function.