2 Garrett: `A Good Spectral Theorem' 1. von Neumann algebras, density theorem The commutant of a subring S of a ring R is S 0 = fr 2 R : rs = sr; 8s 2


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1 1 A Good Spectral Theorem c1996, Paul Garrett, version February 12, Measurable Hilbert bundles Measurable Banach bundles Direct integrals of Hilbert spaces Trivializing Hilbert bundles 1 Typeset with AMSLaTeX
2 2 Garrett: `A Good Spectral Theorem' 1. von Neumann algebras, density theorem The commutant of a subring S of a ring R is S 0 = fr 2 R : rs = sr; 8s 2 Sg A von Neumann algebra is a stable subalgebra A of the space B(V ) of bounded operators on a Hilbert space V so that A is closed in the strong topology on operators. Theorem: (von Neumann Density) Let A be a stable subalgebra of the space B(V ) of bounded operators on a Hilbert space, and suppose that A contains the identity. Then A 00 = closure of A in the strong operator topology That is, if A is closed and contains the scalar operators, then the strongtopology closure condition is equivalent to the purely algebraic condition A 00 = A Very often the von Neumann algebra in question will indeed include scalar operators, but we reserve the right to have it be otherwise. 2. Direct integrals of Hilbert spaces: denitions Let (; ) be a measure space. We assume that is a positive regular Borel measure. Further, we suppose that is nite, i.e., is a countable union S i (i) of sets (i) with nite measure. And we suppose that there is a countable collection M o of measurable sets in which generate the collection of measurable sets, in the sense that for every measurable set X there is Y in the smallest algebra containing M o so that X? X \ Y and Y? X \ Y are of measure zero. This is to avoid irrelevant trouble. To each s 2 assign a Hilbert space V s with norm j j s and inner product h; i s. This assignment is a Hilbert (space) bundle V over base space. The ber of the bundle V at s 2 is the Hilbert space V s. A section x of fv s : s 2 g is a function x :! a s2 V s so that x(s) 2 V s. We require also that there be chosen a collection F of sections so that For all x; y 2 F the C valued function s! hx(s); y(s)i s is measurable. If z is a section of fv s : s 2 g so that for all x 2 F the function s! hx(s); z(s)i is measurable, then z 2 F. There is a countable collection of elements x 1 ; x 2 ; : : : in F so that for all s 2 the elements fx 1 (s); x 2 (s); : : :g are dense in V s.
3 Garrett: `A Good Spectral Theorem' 3 is a choice of measurable sections of V. Choice of F is part of the specication of a measurable Hilbert bundle. (There certainly may be many dierent choices of such collection of measurable sections). Note that separability is quite explicitly required. Sometimes fv s : s 2 g would be called a measurable eld of Hilbert spaces. We dene the norm of a measurable section x 2 F by jxj 2 = jx(s)j 2 s d(s) If this is zero, then x is a null section; if it is nite, then x is a squareintegrable section. Let V s d(s) denote the collection of squareintegrable sections modulo null sections. This is the direct integral of the Hilbert spaces V s, with respect to the measure, and with respect to (the choice) F of measurable sections. Proposition: The space R V s d(s) is a separable Hilbert space. Proof: This is very similar to the analogous proof for the `plain' L 2 (X; ) for an ordinary measure space (X; ). 3. Trivializing Hilbert bundles Here we show that direct integrals of Hilbert spaces are trivializable in a sense made precise just below. This fact is essential at certain technical points in the sequel, where we reduce various questions to the much easier analogues for trivial Hilbert bundles. We rst need to be explicit about morphisms (i.e., maps) of direct integrals of Hilbert spaces, or of Hilbert bundles, over a xed base space. Let fv s : s 2 g and : s 2 g be two such, with the same base space, with measurable sections F; F 0, respectively. Let ft s : s 2 g be a collection of continuous linear maps T s : V s! V 0 s. We evidently must assume that for v 2 F the section dened by fv 0 s (T v)(s) = T s (v(s)) is in F 0. Then we say that T preserves bers. If each T s is an isometry (or at least almosteverywhere is such), then one easily checks that T is an isometry. If T s is almosteverywhere an isomorphism of Hilbert spaces, then it is easy to show that T is such. And so on. Similarly, we can dene direct sums of direct integrals fv s g; fvsg 0 of Hilbert spaces over : the ber at s is just V s V 0 s, and the measurable sections are of the form s! v(s) v 0 (s) for v 2 cf and v 0 2 F 0. The norm on the direct sum is given by jv v 0 j 2 = jvj 2 + jv 0 j 2 where the latter two norms are those in the two original bundles. The norm also has an obvious integral expression.
4 4 Garrett: `A Good Spectral Theorem' The simplest of all direct integrals of Hilbert spaces is just L 2 (; ) itself, where V s = C with the usual inner product. Extending this example somewhat, a more general but still very simple type of direct integral is a direct integral of a constant or trivial Hilbert bundle, which by denition is one where V o = V s does not depend upon s 2. A direct integral of a constant Hilbert bundle may then be viewed as the space L 2 (; V o ; ) of squareintegrable V o valued functions on, which is dened to be the space of V o valued functions f on so that jfj = hf(s); f(s)i Vo d(s) 1 2 < 1 A countable direct sum ^ i V i of Hilbert spaces V i (with inner products h; i i is dened to be the collection of sequences v = fv i g with v i 2 V i with (nite) norm dened by jvj X 2 = hv i ; v i i i F Generally, if is a countable disjoint union = i i of measurable subsets, then there is an obvious isometric isomorphism V s d(s) = ^ i i V s d(s) For simplicity of statement, we need a convention about cardinalities. In the remainder of this section we will refer to ddimensional Hilbert spaces. Now, if d is a positive integer, this has the usual meaning, but if d = 1, we mean that the Hilbert space is separable but not nitedimensional. Similarly, when we refer to a collection e ; : : : ; e d of vectors, if d is a positive integer this has the usual sense, while if d = 1 we mean this notation to refer to the countable collection e 1 ; e 2 ; : : : ; in which there is no actual element e 1. The following proposition eectively asserts that any direct integral of Hilbert spaces is isomorphic to a countable direct sum of direct integrals of trivial Hilbert bundles, where the sum is indexed only by dimension. An isomorphism of a direct integral to a direct integral of a trivial Hilbert bundle is called a trivialization of the bundle. Proposition: Let V = fv s : s 2 g be a Hilbert bundle with measurable sections F. There are measurable subsets d with indices d = 1; 2; : : : ; 1 so that for s 2 d the dimension of V s is d. For each d 1, there is a ddimensional Hilbert space V (d) o so that we have an isometric isomorphism d V s d(s) L 2 ( d ; V (d) o ; ) which preserves bers and preserves inner products. Remarks: The proof contains a little more information than the statement, in that a semiconstructive `procedure' is described by which we obtain the desired isometric isomorphism. There are many possible dierent version of such a procedure, and it may be both more edifying and more comprehensible if the reader simply nds their own version, rather than read the version here.
5 Garrett: `A Good Spectral Theorem' 5 Proof: Let v 1 ; v 2 ; : : : be a countable collection of sections in F so that v 1 (s); v 2 (s); : : : is dense in the ber V s at s, for all s 2. For xed set I of indices, let I (s) be the absolute value of the determinant of the matrix of inner products hv i (s); v j (s)i s for i; j 2 I. (The choice of ordering of I doesn't matter since we take absolute value). This function is measurable. The vectors fv i (s) : i 2 Ig are linearly independent exactly where I (s) 6= 0. Let d be the supremum of functions I as I varies over all sts of indices with cardinality d. This function is also measurable. Then dim V s < d if and only if d (s) = 0. For 1 d < 1, let d be the measurable set of s where d (s) 6= 0 but d+1 (s) = 0. For d = 1, let d be the set of s where no d vanishes. These are the desired subsets as in the assertion of the proposition. It is easy to check that the restriction to each d of F provides a suitable class of measurable sections for d V s d(s) So now suppose that we have a Hilbert bundle all of whose bers are of the same dimension d 1. We will make a collection e 1 ; e 2 ; : : : ; e d of sections in F so that for all s 2 the collection e 1 (s); : : : ; e d (s) is an orthonormal basis for V s. This will give an isomorphism to a trivial bundle, as follows. Let C d be the standard ddimensional Hilbert space, where by convention d = 1 gives the usual space `2 of squareintegrable sequences. For a measurable section v, we have a map to C d valued functions on given by v! (s! fhv(s); e i (s)i s : i = 1; 2; : : : ; dg) It is easy to check that if v is squareintegrable then the associated `dvalued function is square integrable. That is, we have the desired isomorphism to L 2 (; C d ; ). Again, let v 1 ; v 2 ; : : : be the countable collection whose values pointwise everywhere are dense in the bers. We will `improve' v 1 so that jv 1 (s)j s = 1 for all s 2, basically by adding to it sections of the form fv i where f 2 L 1 (; ) and i > 1. (By refering to the denition one easily checks that such fv 2 F if f 2 L 1 (; ) and v 2 F). First, we normalize the v i. Let f i (s) = 1 for jv i (s)j = 0 and f i (s) = 1=jv i j for v i (s) 6= 0. Replacing v i by f i v i makes jv i (s)j s 1 for all s. Since we are only concerned with measurability and not squareintegrability at the moment, we do not care about the nature of these functions f i beyond the fact that they are measurable. Now let i be the characteristic function of the set of s 2 where v 1 (s) = 0; : : : ; v i?1 (s) = 0 and put v 0 1 = v 1 + 2?1 2 v 2 + 2?2 3 v 3 + : : : + 2?n n v n + : : : By construction, the sequence of partial sums is convergent, the closure of the span of v 0 1 ; v 2; : : : is still the whole of V s at every point, and for all s 2 we have v 0 1(s) 6= 0. Indeed, for v 0 1(s) to vanish requires that all the v i (s) vanish, which is impossible. And then we may as well replace v 0 1 by e 1 (s) = v 0 1(s)=jv 0 1(s)j s to obtain a measurable section e 1 which is pointwise everywhere a unit vector.
6 6 Garrett: `A Good Spectral Theorem' Replace the sections v 2 ; v 3 ; ldots by the results obtained by applying Gram Schmidt pointwise: replace v i by v(s)? hv i (s); e 1 (s)i s e 1 (s) Then we can repeat the previous process as applied to v 2, replacing it by a section e 2 pointwise everywhere a unit vector orthogonal to e 1. By induction, we use this procedure to construct a sequence e i of measurable sections so that for every s 2 the vectors e i (s) are an orthonormal basis for the ber V s, and in particular so that none of these sections vanish at any point s 2. Note that the procedure stops if at any point the GramSchmidt process causes all the subsequent sections to become zero. This will be the case if the bers are all of a xed nite dimension. Generally, many of the original sections v i will become identically zero along the way, but this does not harm the outcome. 4. Decomposable operators For each s 2, let T (s) be a bounded operator on the Hilbert space V s. If the function s! ht (s)x(s); y(s)i is a measurable C valued function for all x; y 2 F, then say that ft s : s 2 g is a measurable eld of operators. If the integral converges, then we can form T s d(s) It turns out that we want to require convergence in the strong topology, and not in the uniform topology. Some especially simple measurable elds of operators are the multipliers: for f 2 L 1 (; ) let M f be the operator given simply by multiplication by f(s) on V s. Here of course we have chosen a function representing the equivalence class of f in L 1 (; ), but this does not aect the multiplier operator M f = obtained on the direct integral space. f(s) id Vs d(s) Theorem: Let M R be the collection of all multipliers M f on V = V s where f 2 L 1 (; ). Let D be the collection of decomposable operators on V. We have commutant relations M 0 = D D 0 = M Proof: We can invoke the results above which express a direct integral as a direct sum (indexed by dimension d of bers) of trivial direct integrals L 2 ( d ; V; ) = V d(s) since the characteristic functions of the subsets d where the bers are ddimensional are measurable. d
7 Garrett: `A Good Spectral Theorem' 7 Note that the projectors to the subspaces where the bers have a xed dimension are given by multiplication by the characteristic functions of the subsets. That is, these projectors lie in M, and so commute with T 2 M. Thus, without loss of generality, we can restrict our attention to the situation that all bers have the same dimension. Then, by the previous section, we may suppose that the Hilbert bundle is trivial, consisting of the space L 2 (; V; ) of squareintegrable V valued functions on (modulo those functions almost everywhere zero, of course), where V is a (separable) Hilbert space. For v 2 V and for scalarvalued 2 L 2 (; ), we use the standard notation v for the function s! (s)v. Given an operator T 2 M 0, dene a eld ft s g of (berwise) operators by T s (v) = (T ( v))(s) for 2 L 1 (; ) having compact support and so that (s) = 1. Note that we make essential use of the triviality of the bundle in writing such an expression. We claim that the integral R Ts is none other than T itself. Proposition: The strong operator topology closure of the collection of multiplier operators M f with continuous f is the collection of multiplier operators M f with (measurable) essentially bounded f. Proof: Later commutative von Neumann Algebras Theorem: Let A be a commutative von Neumann algebra in B(V ) for a separable Hilbert space V. Let A be a C algebra containing 1 and generated by countablymany elements and strongoperator topology dense in A. Let? : A! C o () be the Gelfand isomorphism. Then there is a positive regular Borel measure (described below) on so that? extends to an isomorphism ~? : A! L 1 (; ) There is x o 2 V so that A 0 x o is dense in V, where A 0 is the commutant Lemma: of A. That is, there is a cyclic vector x o for the commutant A 0. Corollary: The functional C o () 3 f! h??1 f(x o ); x o i on C o () is positive, so gives rise to an outer regular positive Borel measure on. Since V is separable, the conclusion of BanachAlaoglu can be sharpened to see that is metrizeable, so is necessarily regular. Lemma: Any other choice of cyclic vector gives rise to a measure absolutely continuous with respect to. Thus, the denition of L 1 (; ) is independent of the choice of cyclic vector.
8 8 Garrett: `A Good Spectral Theorem' Theorem: Let A be a commutative von Neumann algebra in B(V ) for a separable Hilbert space V. Then there is a Hilbert bundle fv s : s 2 g over a compact metric space so that there is an isometry of Hilbert spaces and so that the map : V s d(s)! V L 1 (; ) 3 f! M f?1 is an isomorphism. Now we describe the Hilbert bundle which occurs in the theorem. The heuristic is that in the prototypical case where is the spectrum of a normal operator T, the ber V s is the s`eigenspace' of T. Of course, this cannot be quite right, since there may be no eigenvalues whatsoever. For any x; y 2 V invoke the RieszMarkovKakutani representation theorem to obtain a regular Borel measure x;y from the functional C o () 3 f! h??1 f(x); yi A crucial point is that x;y is absolutely continuous with respect to = xo;x o where x o is a cyclic vector for A 0. Thus, h x;y = d x;y d xo;x o 2 L 1 (; ) We have the crucial property that for f 2 C o () and x; y 2 V, letting T =??1 f 2 B(V ), h T f (x);y = f h x;y Let X be a countable dense subset of V, and let o be a subset of (diering from by a set of measure zero) so that for s 2 o and x; y 2 X the function values h x;y (s) make sense. Let V 0 be the C linear span of??1 C o ()X inside V. Then h x;y (s) makes sense for s 2 o and x; y 2 V 0. Dene (x; y) s = h x;y (s) This is positive semidenite on V 0. Let K s be the kernel of this hermitian form. Then (; ) s induces a positivedenite hermitian form denoted h; i s on the quotient V 0 =K s. Let V s be the Hilbert space obtained by completing this space with respect to the induced metric. For s 2 o and x 2 X, let ~x(s) be the image of x in V 0 =K s V s under the quotient map. Then fv s : s 2 o g is a Hilbert bundle over o with sections ~x for x 2 X. Dene F to be the collection of sections y so that is measurable for every x 2 X. Then s! h~x(s); y(s)i V o V s d(s) Since o diers from by a measurezero set, the distinction is negligible.
9 Garrett: `A Good Spectral Theorem' 9 6. Spectral Theorem for a Normal Operator Let T be a normal operator on a separable Hilbert space V. Let A be the C  algebra generated by 1; T; T, i.e., the uniform operator topology closure of the polynomial ring C [T; T ]. Let A be the strong operator topology closure of A: this is a von Neumann algebra and is the double commutant A 00 of A and of C [T; T ], by the von Neumann density theorem. We may identify the maximal ideal space of A with the spectrum (T ) of T. Let? be the Gelfand isomorphism From above, we have an extension? : A! C o () ~? : A! L 1 (; ) where = xo;x o is the positive regular Borel measure attached to a cyclic vector x o for the commutant A 0. For any measurable subset! of we have the associated e = e! = ~??1 ch! where ch! is the characteristic function of!. If! is of positive measure, then e = e! is a nonzero idempotent lying in the strong closure A 00 of C [T; T ]. The function dened by E : f measurable subsets of g! f selfadjoint projections on V g E(!) = ~??1 (ch! ) 2 B(V ) is easily obtained from ~??1. This function E is sometimes called a `resolution of the identity'. Note that the properties of the extended Gelfand transform ~? immediately yield the required properties of a `resolution of the identity': At the extremes, E(;) = 0 and E() = 1 V. Each E(!) is a selfadjoint projection. For mesaureable subsets!;! 0, we have E(! \! 0 ) = E(!) E(! 0 ) If! \! 0 = ; then E(! [! 0 ) = E(!) + E(! 0 ) For all x; y 2 V the function is a complex measure on. Indeed, for the last property, note that in our earlier notation. E x;y (!) := he(!)x; yi he(!)x; yi = x;y (!)
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