Differential equations in metric spaces
We give a meaning to derivative of a function , where is a complete metric space. This enables us to investigate differential equations in a metric space. One can prove in particular Gronwall’s Lemma, Peano and Picard Existence Theorems, Lyapunov Theorem or Nagumo Theorem in metric spaces. The main idea is to define the tangent space of . Let , be continuous at zero. Then by the definition and are in the same equivalence class if they are tangent at zero, that is if By we denote...