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Swaption Vol Cube Arbitrage

How can I exploit an arbitrage by violating the following no-arbitrage condition (taken from the paper "Arbitrage-Free Construction of the Swaption Cube" by Simon Johnson and Bereshad Nonas):

Swptn(K,T1,T2)+Swptn(K,T2,T3) >= Swptn(K,T1,T3) with Swptn(A,B,C) being the price of a swaption of strike A, time to option maturity B, time to underlying swap maturity C.

Thanks for any hints. L.

2 responses

Well, if two swaptions were less than third, you'd buy them, sell the third, and the credit is free money - there's no outcome where you do not get to keep at least the credit and the interest on it. If at T2 your first swaption is in the money, you keep that as well, otherwise it's a scratch, and you're still hedged for T2-T3.

Thank you Simon. In T3 I still have Swptn(K,T2,T3)-Swptn(K,T1,T3). Why should this payoff be always non-negative?Thanks.