I am a newbie. Please help me understand how to resolve the exercise 2.2 from the book "The concept and practice of Mathematical Finance". The solution from the book says that our super-replicating portfolio will be $\alpha$ shares and $\beta$ bonds. It must dominate at zero. This implies that $\beta$ >= 0. First of all, what does it mean "it must dominate at zero". Secondly, why if it dominates at zero, then $\beta$ >= 0? Thanks so much for your help!
"It must dominate at zero" means that when the final spot level is zero, the value of the super-replicating portfolio must be greater than or equal to the value of the payoff, which is zero. Since the super-replicating portfolio consists of some stock (which has zero value when the spot price is zero) and some bonds (which have value one), there must be a non-negative number of bonds.