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OPERATOR ALGEBRAS and THEIR REPRESENTATIONS Contents 1 OPERATOR ALGEBRAS AND THEIR REPRESENTATIONS YAMAGAMI SHIGERU Contents 1. Algebras and Representations 2 2. Gelfand Theory 7 3. Positivity in C*-algebras 16 4. Representations and W*-algebras 21 5. Linear Functionals on W*-algebras 31 6. Tomita-Takesaki Theory 36 7. Standard Hilbert Spaces 42 8. Universal Representations 53 9. Reduction Theory 57 9.1. Commutative Ampliations 58 9.2. Measurable Fields 61 Appendix A. Analytic Elements 69 Appendix B. Haar Measure 74 Appendix C. Pontryagin Duality 74 Appendix D. Group Representations 75 Appendix E. Projective Representations 77 Appendix F. Tensor Products 80 Appendix G. Infinite Tensor Products 80 Appendix H. Polarity in Banach Spaces 83 Appendix I. Radon Measures 84 Appendix J. Sesquilinear Forms 85 Appendix K. Transition Probabilities 91 Appendix L. Random Operators 98 L.1. Polar Decomposition 98 L.2. Sesquilinear Forms 100 L.3. Normal Homomorphisms 101 Appendix M. Geometric Approach 101 Appendix N. Stone-Cechˇ Compactification 103 Dixmier, Von Neumann Algebras. North-Holland, 1981. 1 2 YAMAGAMI SHIGERU Bratteli-Robinson, Operator Algebras and Quantum Statistical Me- chanics I, II, Springer, 1979. Takesaki, Theory of Operator Algebras I, Springer, 1979. Pedersen, C*-Algebras and their Automorphism Groups, Academic Press, 1979. Reed-Simon, Functional Analysis, Academic Press, 1980. Sakai, C*-Algebras and W*-Algebras, Springer, 1971. 1. Algebras and Representations By a *-algebra, we shall mean an algebra A over the complex num- ber field C which is furnished with a distinguished conjugate linear transformation ∗ : A ! A (called a *-operation) satisfying (a∗)∗ = a (ab)∗ = b∗a∗; a; b 2 A: ∗ Exercise 1. If A has a unit 1A (i.e., A is unital), then 1 = 1. An element a in a *-algebra A is said to be hermitian if a∗ = a.A hermitian element p is called a projection if p2 = p. When A has a unit 1, a is said to be unitary if aa∗ = a∗a = 1. A *-algebra is said to be unitary1 if it is generated by unitaries. Example 1.1. Given a *-algebra A, the n × n matrix algebra Mn(A) with entries in A is a *-algebra. Example 1.2. Let C[X] be the polynomialP algebraP of indeterminate n ∗ n X and make it into a *-algebra by ( n≥0 anX ) = n≥0 anX . Then 0 and 1 are all the projections and constant polynomials of modulus 1 are all the unitaries. P C C Example 1.3. Given a group G, the free vector space G = g2G g generated by elements in G is a *-algebra (called a group algebra) by extending the group product to the algebra multipliction and defining the *-operation so that elements in G are unitary. The group algebra CG is unitary. Exercise 2. Let A be the vector space of functions of finite support on a group G and make it into a *-algebra (convolution algebra) by X (ab)(g) = a(g0)b(g00); a∗(g) = a(g−1): g0g00=g The convolution algebra A of G is naturally isomorphic to the group algebra CG. 1Warning: This is not a common usage of terminology. OPERATOR ALGEBRAS AND THEIR REPRESENTATIONS 3 Given *-algebras A and B, their direct sum A⊕B and tensor product A ⊗ B are again *-algebras in an obvious manner. Exercise 3. The matrix algebra Mn(A) is naturally identified with the tensor product Mn(C) ⊗ A. Let H be a pre-Hilbert space; H is a complex vector space with a positive definite inner product ( j ). A linear operator T : H ! H is called the adjoint of a linear operator S : H ! H (and denoted by S∗) if it satisfies (ξjSη) = (T ξjη) (for ξ; η 2 H). A linear operator S on H is sadi to be bounded (on the unit ball) if kSk = supfkSξk; ξ 2 H; kξk ≤ 1g is finite. Let L(H) be the set of linear operators on H having adjoints, which is a unital *-algebra in an obvious way. The subset B(H) of L(H) consisting of bounded operators is a *-subalgebra. When H is complete, a linear operator on H has an adjoint if and only if it is bounded thanks to the closed graph theorem and the Riesz lemma, whence L(H) = B(H). By a *-representation of a *-algebra A on a pre-Hilbert space H, we shall mean an algebra-homomorphism π : A ! L(H) satisfying π(a)∗ = π(a∗) for a 2 A. When kπ(a)k < 1 for every a 2 A, π is said to be bounded. If A is unitary, any *-representation is automatically bounded, i.e., π(A) ⊂ B(H). Two *-representations πi : A ! L(Hi) (i = 1; 2) are said to be unitarily equivalent if we can find a unitary map T : H1 ! H2 satisfying T π1(a) = π2(a)T (a 2 A). It is often convenient to regard the representation space H as a left A-module by aξ = π(a)ξ. A right A-module structure then corre- sponds to a *-antirepresentation, i.e., an algebra-antihomomorphism π : A ! L(H) satisfying π(a)∗ = π(a∗), by the relation ξa = π(a)ξ. A pre-Hilbert space H is called an A-B bimodule (B being another *-algebra) if we are given a *-representation λ : A ! L(H) and a *-antirepresentation ρ : B ! L(H) satisfying λ(a)ρ(b) = ρ(b)λ(a) for a 2 A and b 2 B, i.e., (aξ)b = a(ξb) in the module notation. An A-A bimodule H is called a *-bimodule if we are given an antiunitary 2 involution ξ∗ on H satisfying (aξb)∗ = b∗ξ∗a∗ for a; b 2 A and ξ 2 H. A linear functional ' on a *-algebra A is defined to be positive if '(a∗a) ≥ 0 for a 2 A. A positive linear functional ' on a unital *- algebra A is called a state if '(1A) = 1 (1A being the unit element of A). A linear functional τ on an algebra A is called a trace or said to be tracial if τ(ab) = τ(ba) for a; b 2 A. 2A conjugate-linear operator J on a pre-Hilbert space H is called an antiunitary if it satisfies (JξjJη) = (ηjξ) and JH = H. 4 YAMAGAMI SHIGERU Example 1.4. Given a *-representation π : A ! L(H) and a vector ξ 2 H, '(a) = (ξjπ(a)ξ) gives a positive linear functional. When ξ is a unit vector, ' is a state. This kind of state is called a vector state. Exercise 4. Given a bounded *-representationP π : A ! B(H) and a f g H j sequence ξn n≥1 of vectors in satisfying n≥1(ξn ξn) = 1, observe that X1 '(a) = (ξnjπ(a)ξn) n=1 defines a state on A. By specializing to A = M2(C), recognize the difference between states of this form and vector states. Example 1.5. Let C0(H) be the set of finite rank operators on a Hilbert space H. Then C0(H) is a *-ideal of B(H) and the ordinary trace defines a positive tracial functional tr on C0(H). Example 1.6. Every probability measure µ on the real line of finite moments defines a state on the polynomial algebra C[X] by ! X X Z n n ' anX = an t µ(dt): R n n Conversely, any state arises in this way (the existence part of the Ham- burger moment problem). See xX.1 in Reed-Simon for more informa- tion. Example 1.7. In the group algebra CG, positive linear functionals ' are one-to-one correspondence with positive definite functions on G by restriction and linear extension. The state associated to the positive definition function ( 1 if g = e, δ(g) = 0 otherwise is called the standard trace. Exercise 5. The standard trace δ has the trace property: δ(ab) = δ(ba) for a; b 2 CG. Given a positive linear functional ' on a *-algebra A, we define a *-representation as follows: The inner product (ajb) = '(a∗b) on A is positive semidefinite and the representation space is given by the associated pre-Hilbert space H, i.e., H is the quotient vector space relative to the kernel of ( j ). The non-degenerate inner product on the quotient space is also denoted by ( j ), whereas the quotient vector of x 2 A in H is denoted by x'1=2. The inner product then looks like (x'1=2jy'1=2) = '(x∗y) and we introduce a representation π by OPERATOR ALGEBRAS AND THEIR REPRESENTATIONS 5 π(a)(x'1=2) = (ax)'1=2, which is well-defined in view of the Schwarz inequality j'(a∗b)j2 ≤ '(a∗a)'(b∗b); a; b 2 A: In fact, if x is in the kernel of the inner product, '(x∗a∗ax) ≤ '(x∗a∗aa∗ax)1=2'(x∗x)1=2 = 0 shows that ax is in the kernel as well. Moreover, the relation (x'1=2jπ(a)y'1=2) = '(x∗ay) = (π(a∗)x'1=2jy'1=2) shows that π(a)∗ = π(a∗), whence π is a *-representation. The representation obtained in this way is referred to as the GNS- representation or its process as the GNS-construction. When A is 1=2 1=2 unital, we have a distinguished vector ' = 1A' in the repre- sentation space, which is cyclic with respect to π in the sense that H = π(A)'1=2. Conversely, if we are given a *-representation (π; H) of a *-algebra A and a cyclic vector ξ 2 H for π, the formula '(a) = (ξjπ(a)ξ) defines a positive linear functional and the associated GNS-representation is unitarily equivalent to the initial one by the unitary map a'1=2 7! π(a)ξ (a 2 A).
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