Unfolding singular points in linear differential equations is a classical technique for studying the properties of irregular singularities by relating them to regular singularities. In this paper, we propose a general framework for unfolding unramified irregular singularities of meromorphic connections on the trivial principal $G$-bundle over $\mathbb{ P}^{ 1}$. One of our main results is the description of the unfolding of singularities in terms of deformations of their moduli spaces. We show that every moduli space of irreducible meromorphic $G$-connections with unramified irregular singularities on $\mathbb{ P}^{ 1}$ can be deformed into a moduli space of irreducible Fuchsian $G$-connections on $\mathbb{ P}^{ 1}$. Furthermore, we study the unfolding of additive Deligne-Simpson problems, in which the unfolding of irregular singularities naturally generates a family of such problems. As an application of our main result, we prove that a Deligne-Simpson problem for $G$-connections with unramified irregular singularities admits a solution if and only if every unfolded Deligne-Simpson problem in the family admits a simultaneous solution. We also provide a combinatorial and diagrammatic framework of the unfolding process in terms of spectral types and unfolding diagrams. Finally, we address a conjecture proposed by Oshima concerning the existence of irreducible $G$-connections that realize prescribed spectral types and their unfoldings. Our main result gives an affirmative answer to this conjecture.
Categories: math.AG math.CA math.RT