Exact volatility calibration based on a dupiretype call-put duality for perpetual American options

This paper investigates the calibration of a model with a time-homogeneous local volatility function to the market prices of the perpetual American Call and Put options. The main step is the derivation of a Call-Put duality equality for perpetual American options similar to the equality which is equivalent to Dupire's formula (Dupire in Risk 7(1):18-20, 1994) in the European case. It turns out that in addition to the simultaneous exchanges between the spot price and the strike and between the interest and dividend rates which already appear in the European case, one has to modify the local volatility function in the American case. To show this duality equality, we exhibit non-autonomous nonlinear ODEs satisfied by the perpetual Call and Put exercise boundaries as functions of the strike variable. We obtain uniqueness for these ODEs and deduce that the mapping associating the exercise boundary with the local volatility function is one-to-one onto. Thanks to this Dupire-type duality result, we design a theoretical calibration procedure of the local volatility function from the perpetual Call and Put prices for a fixed spot price x₀. The knowledge of the Put (resp. Call) prices for all strikes enables to recover the local volatility function on the interval (0, x₀) (resp. (x₀, +∞)). We last prove that equality of the dual volatility functions only holds in the standard Black-Scholes model with constant volatility.