By Graham Ellis

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We have that c−1 (c(T )) = T and c(c−1 (U )) = U ; c(T ) is permutable with T , and commutes with any S ∈ L(H) which is permutable with T . 2 Definition. The operator c(T ) is the Cayley transform of T , and c−1 (U ) is the inverse Cayley transform of U . If E is a spectral projection of c(T ) corresponding to a Borel subset A of T, then E is permutable with T ; in fact, EU is a unitary on EH, and it is easily veriﬁed that c−1 (EU ), which is a self-adjoint operator on EH, agrees with T |EH . Furthermore, E commutes with any bounded operator which is permutable with T .

Let S, T ∈ L(H) with S ∗ S ≤ T ∗ T . Then there is a unique W ∈ L(H) with W ∗ W ≤ QT (hence W ≤ 1), and S = W T . If R ∈ L(H) commutes with S, T , and T ∗ , then RW = W R. Proof: W is deﬁned on R(T ) by W (T ξ) = Sξ (W is well deﬁned since Sξ ≤ T ξ for all ξ). W extends to an operator on QT H by continuity; set ∗ ∗ ∗ W = 0 on Q⊥ T H. Then W W ≤ QT and S = W T . 6 The Spectral Theorem 23 and thus RW η = W Rη for η ∈ QT H. Since R(T T ∗ ) = (T T ∗ )R, RQT = QT R, and thus R leaves (I −QT )H invariant; thus if η ∈ Q⊥ T H, then W Rη = RW η = 0.

Let T be a densely deﬁned operator on H. Then T is symmetric if T ⊆ T ∗ , and T is self-adjoint if T = T ∗ . A symmetric operator is closable (its closure is also symmetric) and a selfadjoint operator is closed. 2 Examples. 7 Unbounded Operators 31 D0 = {f ∈ D2 : f (0) = f (1) = 0}. Then D0 is dense in L2 [0, 1], and Dk has codimension one in Dk+1 (k = 0, 1). Let Tk be deﬁned by Tk f = f with domain Dk (k = 0, 1, 2). Then each Tk is closed, T1 is self-adjoint, and T0∗ = T2 , so T0 is closed and symmetric, but not self-adjoint.