Nonlinear Stochastic Filtering with Volterra Gaussian noises

We consider a nonlinear filtering problem for a signal-observation system driven by a Volterra-type Gaussian rough path, whose sample paths may exhibit greater roughness than those of Brownian motion. The observation process includes a Volterra-type drift, introducing both memory effects and low regularity in the dynamics. We prove well-posedness of the associated rough differential equations and the Kallianpur-Striebel. We then establish robustenss properties of the filter and study the existence, smoothness, and time regularity of its density using partial Malliavin calculus. Finally, we show that, in the one-dimensional case, the density of the unnormalized filter solves a rough partial differential equation, providing a rough-path analogue of the Zakai equation.