Umbral theory and the algebra of formal power series

Abstract

Umbral theory, formulated in its modern version by S. Roman and G.~C. Rota, has been reconsidered in more recent times by G. Dattoli and collaborators with the aim of devising a working computational tool in the framework of special function theory. Concepts like umbral image and umbral vacuum have been introduced as pivotal elements of the discussion, which, albeit effective, lacks of generality. This article is directed towards endowing the formalism with a rigorous formulation within the context of the formal power series with complex coefficients $(\mathbb{C}[[ t ]], \partial)$. The new formulation is founded on the definition of the umbral operator $\operatorname{\mathfrak{u}}$ as a functional in the "umbral ground state" subalgebra of analytically convergent formal series $\varphi \in \mathbb{C}\{t\}$. We consider in detail some specific classes of umbral ground states $\varphi$ and analyse the conditions for analytic convergence of the corresponding umbral identities, defined as formal series resulting from the action on $\varphi$ of operators of the form $f(\zeta \operatorname{\mathfrak{u}}^\mu)$ with $f \in \mathbb{C}\{t\}$ and $\mu, \zeta \in \mathbb{C}$. For these umbral states, we exploit the Gevrey classification of formal power series to establish a connection with the theory of Borel-Laplace resummation, enabling to make rigorous sense of a large class of -- even divergent -- umbral identities. As an application of the proposed theoretical framework, we introduce and investigate the properties of new umbral images for the Gaussian trigonometric functions, which emphasise the trigonometric-like nature of these functions and enable to define the concept of "Gaussian Fourier transform", a potentially powerful tool for applications.