Assume that there are two risky securities and two states of the world. Show that it is always possible to construct an asset which has a certain payoff, i.e. one which has the same yield in all...

Assume that there are two risky securities and two states of the world. Show that it is always possible to construct an asset which has a certain payoff, i.e. one which has the same yield in all states of the world if the spanning requirement is satisfied and short sales are permitted. When is it possible to construct a certain asset if short sales are not permitted?