This paper considers the valuation of a spread call when asset prices are log-normal. The implicit strategy of the Kirk formula is to exercise if the price of the long asset exceeds a given power function of the price of the short asset. We derive a formula for the spread call value, conditional...

This paper presents a closed-form approximation of the value of the finite-lived American option where the underlying asset provides a constant pay-out rate. It is derived by imposing a restriction on the set of feasible exercise strategies, and thus represents a lower bound to the option value....