Description: Define the adjoint of an operator (if it exists). The domain of
U adj W is the set of all operators from U to W that have an
adjoint. Definition 3.9-1 of Kreyszig p. 196, although we don't
require that U and W be Hilbert spaces nor that the operators be
linear. Although we define it for any normed vector space for
convenience, the definition is meaningful only for inner product spaces.
(Contributed by NM, 25-Jan-2008)(New usage is discouraged.)