On the Characters and the Plancherel Formula of Nilpotent Groups ...

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374 BUSBY COROLLARY 3.5. (to Theorem 3.3.) If B is a C*-akebra with identity and A is a closed, two-sided ideal in B, then Prim (A) consists entirely of closed, Hausdogpoints in Prim (B) if and only if A is central. Proof. If A is central, then the correspondence P-+ P n C(A) is a homeomorphism of Prim (A) onto the maximal ideal space of C(A) ([9], Theorem 9.1). The corollary then follows. COROLLARY 3.6. In any C*-algebra B with identity, the unique closed, two-sided ideal of B corresponding to $(Prim (B)) (which exists by [3], Proposition 3.3.2) is the largest central ideal in B. Following the notation of [6], Section 4, we denote this ideal by L(B). COROLLARY 3.7. With notation as above, ;f J ~Prim(B) then J E S(Prim (B)) if and only if th ere is a central ideal I in B, with I $ J. COROLLARY 3.8. With B as above, the following are equivalent: (1) Prim (B) is a HausdotJg space. (2) All primitive ideals in B are central. (3) There are two primitive ideals, J1 and Jz , which are central. (4) B is central. Proof. (1) S- (2). Follows from Corollary 3.5. (2) =+ (3) is obvious (3) 3 (1) The only difficulty here is that, a priori, J1 (say) could be in the closure of Jz . But since J1 is central, and JS is a point in the complement of (x}, Jz is closed. Now Prim (B) is the union of two sets, both in &Prim (B)), and so (1) follows. (1) * (4) also follows from Corollary 3.5. (4) 3 (1) is just Theorem 9.1. of [9]. We now refer the reader to [3] Chapter 10 and [A, Chapter 1 for the definition and properties of a continuous field, (9, 0), of C*-algebras over a Hausdorff space X. We will use Definition 10.1.2 of [3]. If, for each x E X, F(x) (the fiber over x) is a full matrix algebra of some dimension, (depending on x) we will refer to (9, e), or simply to 8, as a continuous field of matrix algebras. If condition (iv) of [3], Definition 10.1.2 does not hold we will call (9, 8) a continuous prefield of C*-algebras. THEOREM 3.9. Let A be a separable C.C.R. algebra with identity (see [3], Chapter 4, where they are called h’minal (?-algebras). Then there is a compact Hausdorff space X, and a continuous prefield 8 of matrix algebras over X such that A g 8.
STRUCTURE SPACES AND EXTENSIONS OF C*-ALGEBRAS 375 Proof. Since A has identity, Prim (A) is compact (see [3], Proposition 3.1.8, we do not assume HausdorfI when we say compact). It was shown ([S], L emma 2) that if A is separable, with compact structure space, then S(Prim (A)) is dense in Prim (A). Again let L(A) be the ideal corresponding to $Prim (A)). ThenL(A) is a central C.C.R. algebra. Since L(A) is C.C.R. with HausdorfI structure space, we know that there is a continuous field of C*-algebras, (9, fQ, over S(Prim (A)) such that for each I E Prim (A), F(I) is the algebra of all compact operators on some Hilbert space HI , and I,(A) is isomorphic with the algebra of all sections 5 E 8 which vanish at infinity (see [3], Theorem 10.54). Since I,(A) is central each g(I) has center and so must be a full matrix algebra. Now M&(A)) s 13. In fact the proof of [I], Theorem 3.15, can easily be modified to include the case under discussion. Also by [I] Theorem 3.15, since S(Prim (A)) is dense in Prim (A), A can be considered to be a subalgebra of M(L(A)) = 19. It is then easy to see that A satisfies (i)-(iii) of [3], Definition This completes the proof. 10.1.2. We refer to [3], Section 4.3 for the definition of G.C.R. (postliminal) C*-algebra, and to [3], 4.7.12 for the definition of generalized trace class (G.T.C.) we prove: algebras. As a final application of Theorem 3.3, THEOREM 3.10. If A is a G.C.R. algebra with identity, the following statements are equivalent: (i) A is G.T.C. (ii) If I is a proper, closed, two-sided ideal in A, there is a proper closed, two-sided ideal J 3 I with J/I central. (iii) If I is a proper, closed, two-sided ideal in A, there is a proper, closed, two-sided ideal J 3 I, such that C(J/L) # {O}. Proof. Dixmier has shown in [6-J Prop. 4.2. that if A is G.C.R., then A is G.T.C. if and only if for all closed, two-sided ideal I of A, W/I) # {O} (using p revious notation). This together with Corollary 3.6. shows that (i) o (ii), and obviously (ii) 3 (iii). Finally, (iii) + (ii) follows easily from Theorem 3.3, and the proof is complete. 4. APPLICATIONS TO EXTENSION THEORY If A is a C*-algebra, and M(A) is its double centralizer algebra, we will denote the quotient M(A)/A by O(A). An extension of A by C, where A and C are C*-algebras, is the equivalence class of some short
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PLANCHEREL FORMULA OF NILPOTENT GRO
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and PLANCHEREL FORMULA OF NILPOTENT
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VECTEURS SePARATEURs DES ALGhBRES D
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VECTEURS SfiPARATEURS DES ALGkBRES
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VECTEURS SltPARATEURS DES ALGhBRBS
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VECTEURS SiPARATEURS DES ALGBBRES D
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SIZES OF COMPACT SUBSETS OF HILBERT
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Thus where SIZES OF COMPACT SUBSETS
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On the Characters and the Plancherel Formula of Nilpotent Groups ...

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