 Schrödinger was the first to raise the question of the relationship between matrix mechanics and wave mechanics in Schrödinger (1926), which was published in Annalen in spring 1926 between the publication of his second and third papers of the famous four. This paper is also reprinted in Schrödinger (1928). It contains the germ of a mathematical equivalence proof, but it does not contain a rigorous proof of equivalency: the mathematical framework that Schrödinger associated with wave mechanics is a space of continuous and normalizable functions, which is too small to establish the appropriate relation with matrix mechanics. Shortly thereafter, Dirac and Jordan independently provided a unification of the two frameworks. But their respective approaches required essential use of \(\delta\) functions, which were suspect from the standpoint of mathematical rigor. In 1927, von Neumann published three papers in Göttinger Nachrichten that placed quantum mechanics on a rigorous mathematical foundation and included a rigorous proof (i.e., without the use of \(\delta\) functions) of the equivalence of matrix and wave mechanics. These papers are reprinted in von Neumann(1961–1963, Volume I, Numbers 8–10). In the preface to his famous 1932 treatise on quantum mechanics (von Neumann 1955), which is an elegant summary of the separable Hilbert space formulation of quantum mechanics that he provided in the earlier papers, he acknowledges the simplicity and utility of Dirac’s formulation of quantum mechanics, but finds it ultimately unacceptable. He indicates that he cannot endure the use of what could then only be regarded as mathematical fictions. Examples of these fictions include Dirac’s assumption that every self-adjoint operator can be put in diagonal form and his use of \(\delta\) functions, which von Neumann characterizes as “improper functions with self-contradictory properties”. His stated purpose is to formulate a framework for quantum mechanics that is mathematically rigorous.