C2 space refers to all functions with second-order continuously differentiable and continuous second-order partial derivatives in a specified region. In mathematics, C2 space can be used to study some problems, such as: in calculus, C2 space is the basis of the existence and uniqueness theorem of solutions of differential equations; In functional analysis, C2 space is a subset of H 1 space, and H 1 space is a basic concept in real variable analysis. In topology, C2 space is a topological space.