Fourier-Laplace Transforms of the Brownian Signature via Riccati Equations on the Tensor Algebra

We establish an infinite-dimensional affine transform theory for the time-augmented Brownian signature.

Our first main result shows that, for a suitable class of linear functions of the signature, the conditional Fourier-Laplace transform admits an entire signature expansion.

We prove that the associated coefficients solve an infinite-dimensional linear differential equation on the extended tensor algebra.

Our second main result shows that the logarithm admits a local signature expansion whose coefficients satisfy a Riccati equation on the extended tensor algebra, revealing a generalized affine structure of the Brownian signature in a genuinely path-dependent setting.

In contrast to conventional affine processes, we show that this representation is intrinsically local: zeros of the Fourier-Laplace transform in the complex plane prevent any global expansion.

To recover global representations, we introduce a new class of randomized Riccati equations with path-dependent terminal conditions through a recentering argument.

Furthermore, we establish uniqueness of solutions to the linear and Riccati equations within a suitable class of solutions.

Our results provide a theoretical framework for transform methods in non-Markovian settings, with applications to the computation of conditional distributions.