Analytic Approximation to Constant Maturity Swap Convexity Correction in a Multi-factor Sabr Model

We consider the convexity correction in a multi-factor SABR type stochastic volatility model, in which the volatility and the short-term forward rate are modeled as independent factors. In general, the convexity correction is not analytically tractable in a multi-factor model, but based on the assumption of linear swap rates an analytic solution is available. Linear swap rate models are popular among practitioners for their efficiency and their ability to capture the swaption volatility smile. For an efficient approximation of the solution, we adopt the small disturbance asymptotics technique and construct a stochastic Taylor series of the underlying process. Several numerical experiments compare the accuracy of the approximation with a Monte Carlo benchmark solution.