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Static hedge for one over a clamped stock price

Question

A claim pays 1/min⁡(max⁡(ST,K1),K2)1/\min(\max(S_T,K_1),K_2) at expiry, where K1<K2K_1<K_2: that is 1/K11/K_1 below K1K_1, 1/ST1/S_T between the strikes and 1/K21/K_2 above K2K_2. Construct a hedge from vanilla options that works whatever model the stock follows.

Short answer

A bond plus calls weighted by the payoff's curvature. Using f(S)=f(K1)+f′(K1+)(S−K1)++∫K1K2f′′(K)(S−K)+dK+…f(S)=f(K_1)+f'(K_1^+)(S-K_1)^++\int_{K_1}^{K_2}f''(K)(S-K)^+dK+\ldots, hold bonds paying 1/K11/K_1, short 1/K121/K_1^2 calls at K1K_1, a strip of calls with density 2/K32/K^3 for K∈(K1,K2)K\in(K_1,K_2), and long 1/K221/K_2^2 calls at K2K_2 to flatten the payoff. In practice the strip is discretised.

Adapted from Joshi, Denson & Downes, Quant Job Interview Questions and Answers, Q2.42, p.20; question and answer rewritten by SelfMadeQuant.com.

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