On validity of the asymptotic expansion approach in contingent claim analysis

Kunitomo and Takahashi (1995, 2001) have proposed a new methodology, called small disturbance asymptotics, for the valuation problem of financial contingent claims when the underlying asset prices follow a general class of continuous Itô processes. It can be applicable to a wide range of valuation problems, including complicated contingent claims associated with the Black--Scholes model and the term structure model of interest rates in the Heath--Jarrow--Morton framework. Our approach can be rigorously justified by an infinite-dimensional analysis called the Watanabe--Yoshida theory on the Malliavin calculus recently developed in stochastic analysis.