Probability densities for conditional statistics in the cubic sensor problem

AbstractThis paper applies the techniques of Malliavin’s stochastic calculus of variations to Zakai’s equation for the one-dimensional cubic sensor problem in order to study the existence of densities of conditional statistics. Let {Xt} be a Brownian motion observed by a cubic sensor corrupted by white noise, and let $$\hat \phi $$ denote the unnormalized conditional estimate of φ(Xi). If φ1,...,φn are linearly independent, and if $$\hat \Phi = (\hat \phi _1 ,...,\hat \phi _n )$$ , it is shown that the probability distribution of $$\hat \Phi $$ admits a density with respect to Lebesgue measure for anyn. This implies that, at any fixed time, the unnormalized conditional density cannot be characterized by a finite set of sufficient statistics.