Extension Theory

Download or read book Extension Theory of Formally Normal and Symmetric Subspaces written by Earl A. Coddington and published by American Mathematical Soc.. This book was released on 1973 with total page 87 pages. Available in PDF, EPUB and Kindle. Book excerpt: Let [italic]H be a Hilbert space. Formally normal, normal, symmetric, selfadjoint, and semibounded subspaces of [italic]H2=[italic]H2[circled plus][italic]H are defined by means of the corresponding properties of the graphs of operators in H which are formally normal, normal, symmetric, selfadjoint, or semibounded, respectively. The author gives a complete description of all formally normal and normal subspace extensions in [italic]H2 of a given formally normal subspace [italic]N of [italic]H2. Those extensions which are graphs of operators are explicitly characterized. The symmetric and selfadjoint extensions of a given symmetric subspace are also classified; this result extends the well-known result of von Neumann characterizing the selfadjoint extensions of a (densely defined) symmetric operator. The construction of the "Friedrichs extension'' of a semibounded symmetric subspace is outlined. The existence of formally normal and symmetric extensions in a larger Hilbert space is also studied. A formally normal subspace need not have any normal subspace extension in a bigger subspace. But (as is known for operators), every symmetric subspace has selfadjoint extensions in suitable larger spaces; these extensions are completely characterized.