In this paper we study short time asymptotics of a density function of the solution of a stochastic differential equation driven by fractional Brownian motion with Hurst parameter H (1/2 < H < 1) when the coefficient vector fields satisfy an ellipticity condition at the starting point. We prove both on-diagonal and off-diagonal asymptotics under mild additional assumptions. Our main tool is Malliavin calculus, in particular, Watanabe's theory of generalized Wiener functionals.
IntroductionLet (w t ) t≥0 be the standard d-dimensional Brownian motion and let V i (0 ≤ i ≤ d) be smooth vector fields on R n with sufficient regularity. Consider the following stochastic differential equation (SDE) of Stratonovich-type;If the set of vector fields satisfies a hypoellipticity condition, the solution y t = y t (a) has a smooth density p t (a, a ′ ) with respect to Lebesgue measure on R n . From an analytic point of view, p t (a, a ′ ) is a fundamental solution of the parabolic equation ∂u/∂t = Lu, where, and is also called a heat kernel of L. In many fields of mathematics such as probability, analysis, mathematical physics, and differential geometry, short time asymptotic of p t (a, a ′ ) is a very important problem and has been studied extensively. Although analytic methods are also well-known, we * Revised on 7 Nov. 2013. Mathematics Subject Classification: 60H07, 60F99, 60H10, 60G15.