This Venn diagram is an attempt to visually classify densely-defined linear operators between Banach spaces (self-adjoint operators are an exception, defined between Hilbert spaces). Operators are first divided between the bounded (B) in the left box and the unbounded (uB) in the right box. Red curve contains closable operators, green ellipse closed operators, grey circle compact operators and blue ellipse self-adjoint operators.
Preliminary remarks:
- bounded operators are closable (Bounded Linear Transformation theorem)
- compact operators are bounded (Show that a compact operator is bounded)
- self-adjoint operators are closed (Why is every selfadjoint operator closed?)
Comments for specific subdomains:
e.g. derivative d/dx defined on $C^\infty([a,b]) \subseteq C^0([a,b])$
e.g. Examples of self adjoint compact operators on Hilbert spaces
e.g. multiplication with $i$ on finite dimensional complex normed space?
e.g. any linear bounded functional with proper dense domain?
e.g. multiplication with $i$ on any infinite dimensional complex Banach space?
e.g. identity restricted to proper, dense domain of an infinite dimensional Banach space?
Is this classification correct? Are there any better, more instructive examples?
