Extensions of Symmetric Operators - Extensions of Symmetric Operators

Extensions of Symmetric Operators

Problem Given a densely defined closed symmetric operator A, find its self-adjoint extensions.

This question can be translated to an operator-theoretic one. As a heuristic motivation, notice that the Cayley transform on the complex plane, defined by

maps the real line to the unit circle. This suggests one define, for a symmetric operator A,

on Ran(A + i), the range of A + i. The operator U_A is in fact an isometry between closed subspaces that takes (A + i)x to (A - i)x for x in Dom(A). The map

is also called the Cayley transform of the symmetric operator A. Given U_A, A can be recovered by

defined on Dom(A) = Ran(U - 1). Now if

is an isometric extension of U_A, the operator

acting on

is a symmetric extension of A.

Theorem The symmetric extensions of a closed symmetric operator A is in one-to-one correspondence with the isometric extensions of its Cayley transform U_A.

Of more interest is the existence of self-adjoint extensions. The following is true.

Theorem A closed symmetric operator A is self-adjoint if and only if Ran (A ± i) = H, i.e. when its Cayley transform U_A is a unitary operator on H.

Corollary The self-adjoint extensions of a closed symmetric operator A is in one-to-one correspondence with the unitary extensions of its Cayley transform U_A.

Define the deficiency subspaces of A by

and

In this language, the description of the self-adjoint extension problem given by the corollary can be restated as follows: a symmetric operator A has self-adjoint extensions if and only if its Cayley transform U_A has unitary extensions to H, i.e. the deficiency subspaces K₊ and K_- have the same dimension.