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M. Broadie and J. Detemple [2] specified a three-step procedure for valuing American capped call options when the cap is described by the function . Based on this procedure, S.-L. Chung, M.-W. Hung, and J.-Y. Wang [4] approximated an American call payoff by maximizing the value of a call with exponential cap written on the same asset. If the price of the underlying asset is when , the payoff of the capped call option is , where is the strike price and is the cap's constant growth rate. The payoff is the same as a standard American option, except that the cap function limits the maximum possible payoff. Since the policy of exercising when the asset price reaches the exponential cap is an admissible policy for the American option, for any .

Hence a lower bound is still obtained after optimizing over . Denote the optimal solution as

.

The determination of is a bivariate differentiable optimization problem for any given . With the formulas of derivatives and , the optimal solution should satisfy . In this Demonstration, Mathematica's built-in function FindMaximum is applied to maximize the payoff function and determine the optimal solution .

Following the same idea of M. Broadie and J. Detemple [3], the evaluation of the capped call payoff function derivatives with respect to and , while the underlying asset price approaches from below, leads to an exercise boundary , where is the American call optimal early exercise boundary at time . The boundary is the solution to the equation

,

where and .

In this Demonstration, Mathematica's built-in function FindMinimum is applied to minimize . Following the method of M. Broadie and J. Detemple [3], an upper bound for the American call price is obtained after replacing the optimal early exercise boundary function with in Kim's integral equation [1]. The integral in Kim's equation represents the early exercise premium and is approximated by Simpson's rule.

Compared to the method that uses a constant cap over time [3], the method with the exponential cap function [4] leads to tighter bounds for the theoretical American call price.

References

[1] I. J. Kim, "The Analytic Valuation of American Options," The Review of Financial Studies, 3(4), 1990 pp. 547–572.

[2] M. Broadie and J. Detemple, "American Capped Call Options on Dividend-Paying Assets," The Review of Financial Studies, 8(1), 1995 pp. 161–191.

[3] M. Broadie and J. Detemple, "American Option Valuation: New Bounds, Approximations, and a Comparison of Existing Methods," The Review of Financial Studies, 9(4), 1996 pp. 1211–1250.

[4] S.-L. Chung, M.-W. Hung, and J.-Y. Wang, "Tight Bounds on American Option Prices," Journal of Banking and Finance, 34(1), 2010 pp. 77–89.