It\^{o}'s formula for noncommutative $C^2$ functions of free It\^{o} processes

Abstract

In a recent paper, the author introduced a rich class $NC^k(\mathbb{R})$ of "noncommutative $C^k$" functions $\mathbb{R} \to \mathbb{C}$ whose operator functional calculus is $k$-times differentiable and has derivatives expressible in terms of multiple operator integrals (MOIs). In the present paper, we explore a connection between free stochastic calculus and the theory of MOIs by proving an It\^{o} formula for noncommutative $C^2$ functions of self-adjoint free It\^{o} processes. To do this, we first extend P. Biane and R. Speicher's theory of free stochastic calculus -- including their free It\^{o} formula for polynomials -- to allow free It\^{o} processes driven by multidimensional semicircular Brownian motions. Then, in the self-adjoint case, we reinterpret the objects appearing in the free It\^{o} formula for polynomials in terms of MOIs. This allows us to enlarge the class of functions for which one can formulate and prove a free It\^{o} formula from the space originally considered by Biane and Speicher (Fourier transforms of complex measures with two finite moments) to the strictly larger space $NC^2(\mathbb{R})$. Along the way, we also obtain a useful "traced" It\^{o} formula for arbitrary $C^2$ scalar functions of self-adjoint free It\^{o} processes. Finally, as motivation, we study an It\^{o} formula for $C^2$ scalar functions of $N \times N$ Hermitian matrix It\^{o} processes.