In the empirical analysis of financial time series, multivariate GARCH models have been used in various forms. In most cases it is not well understood how the use of a restricted model has to be paid with loss of valuable information. We investigate the structural implications of two alternative...

We consider the "and" communication mechanism that inputs messages from two players and outputs the public signal "yes" if both messages are "yes", and outputs "no" otherwise. We prove that no correlation can securely be implemented through finite or infinite repetition of this mechanism.

A separation heuristic for mixed integer programs is presented that theoretically allows one to derive several families of " strong" valid inequalities for specific models and compuationally gives results as good as or better than those obtained from several existing separation routines...