9

## Fundamental Theorems in W*-Algebras

### and the Kaplansky Theorem

### Akio Ikunish i

### i

### School of Commerce, Senshu University, 214-8580 Japan

### Abstract

By using the projection of the second dual of a W*-algebra LM onto./材of norm one,

### we shall prove elemcntarily that any W*-algebra has an identity･ Without, usIIlg the

### Kaplansky theorem, we shall show that the second dual of a algebra is a

### C*-algebra. Conversely, this implies the Kaplansky theorem. Furthermore we shall give

### other proofs of the Kaplansky therorem,

### In a W*-algebra, that the existence of an identity, theinvolution is JIWeakly continuous

### alld the lllllltiplication are separately (7-Weaky contintlOtlS are nOn-trivial. By uslng a

### tlle-orem on extreme points, Sakai showed that any W*-algebra has an identity. The extreme

points of the tlnit ball is relevant to an identity. However the proof of the tlleOrCm Or上

### extreme points is difEcult. So, by uslng the projection of the second dual of a W*-algebra

.ノ材onto Eノ冴of norm one, We shall elemelltarily show that a W*-algebra has an identity.

Let LM be a W*-algebra; then.AW is isometrically isomorphic to ･M**/(.ノ銑)o as normed

spaces･ Regarding./身as a subspace of.AW**, the canonic/almapping E ∴AW** - LM**/(.Ag.)○

-./身is a projection of norm one. The quotient topology of the (丁(Lノ材**7.ノ材*)-topology

by (一m*)o is holneOInOrphic to the 0-(eノ材**/(tノ銑)○,･/銑)-topology and so E is continuous

with respeC/t to the c,(Eノ冴**フE/冴*)-topology and o-(tM,.ノ銑)-topology--M** is a Bana(･/h

*-algebra and, by definition, the involution is (7(.ノ材**,.ノ材*)-continuous and the multiplicati()n is separately J(LM**, ･/材*)-Continuous･ Hence a cluster point of arュ approximate identity (cL) of.ノ材with respect t() the J(Lノ材**, ･ノ材*)-topology is an idelltity of ,ノ材**. Since a unique cluster point in a compact space is a limit, (C,ノ) converges to an identity 1 of.AW**. We shall seel∈.ノ座′.

### Ifwc use the fact that the see/ond dual ofa C*-algebra is a C*-algebra, then E is obviously

positive. The unitality and the continuity of the involution in.ノ材are immediate results of

the positivity of E (←∴f･ [?])･ Without using this factフWe Can See that pM has an identity

### alld E is self-adjoillt and positive･ Furthermore, we shall sh()w that E((i,.,IJ･) - ae(I) f()r any

.7; ∈./材** and a, ∈.ノ材, a special case of the T()miyalna theorem. This ilnplies immediately### 10

### Bulletin of the lnstitute ofNatural Sciences, Senshu Universlty No.37

### we can see that the second dual of a C*-algebra is a C*-algebra. In order to see this, we

### use the Kaplansky's idea oil matrix･ CoIIVerSely, from this we shall show the Kaplansky

### theorem･ Also, we shall glVe Other proofs of the KaplaIISky theorem. There we use the

### polar decornp()sition instead of the Kaplansky's idea on matrix. There are several quite

### different proofs of the Kaplansky theorcln. What is essential points to prove the KaplarlSky

### theorem? At last, we shall see within the lilnits of the C*-algebra and without uslng Of

### representations on Hilbert spaces that a second dllal of a C*-algebra is a C*-algebra.

### 1. W*-ALGEBRAS

### Theorem l･ Any W*-algebra has an identity.

Proof･ Let tノ材be a W*-algebra and (eL) an approxiInate identity of.ノ材. At first, we shall

### show that the Banach *-subalgebra LAW + Cl of LM** is a C*-algebra. Since the norm of

･-M** is lower semi-continuous with respect tO the c,(.ノ材**, LAW*)-topology, we have, for any

### x∈.ノ材and入∈Cフ

### IIx+^1日2 ≦ 1iminf糎+入C,JH2

I,

≦ liminf(lL.7: - I:eI,ll + LIxeL +入cLH)2

(/

- 1iminf=xeI, +入eLH2 - 1iminfH(EEL +入elJ)*(.7:C,. +入eI/)H

i L

- 1iminfHeL(･1,･+Al)*(I+Al)eLH ≦ ll(.T+Al)*(I+^1)ll. L

eノ材+ Cl is therefore a C*-algebra.

Let E be the canonical projection ｡f.ノ冴** olltOと/材of llOrm One. It holds that

llE(1)+i入elJII2 - HE(1+iAeL)ll2 ≦ Ell+i入eLll2

-日(1+,i入eL)*(1+i入eL)ll - lll+入2e2日

### ≦1+入2.

0n the other hand, it follows that, for any state p of tノ材,

LIE(1)+i入eLll ≧ llp(E(1)+i入eI,)E ≧ lImp｡E(1)十人や(eL)l.

Since limLP(eL) - 1, we have (lm甲｡E(1) +A)2 ≦ 1 +入2 for an arbitrary real numbcr入

andsolmp｡E(1) -0, i･e･, p(E(1)) ∈R･ Since ll2C,ノ-1日≦ 1, wehave

29(elJ)-P(E(1)) -Poe(2eL-1) ≦ 1.

### Fundamental Theorems in W*-Algebras and the Kaplansky Theorem

### ll

### Remark. (1) From the first part of the above proof, it follows that any C*-algebra is a

### C*-subalgebra of soITle C*-algebra with an identity･

(2) Without using the fac/t that.ノ材+ Cl is a C*一algebraフWe Can Prove Theorem l･ For,

### we have

Hl +i入eLII2 ≦ 1iminfHeK+i入eLH2 - 1iminfll(eK+i入eI/)*(eK+i入eL)IL

hJ rll

- 1iminf lLe2K + il(eKel/ - eJ,.eK,) +入2eZII

Il,

≦ 1+LAlliminfHeKeL-eL/eKII+LIL2 ≦ 1+1^12･

氏

Moreover, for rt ≧ i, Wehave -eK ≦ 2eL-eK ≦ eL andso ll2el/-EK‥ ≦ 1･ Hence wehave

H2eL-1日≦liminfKll2eL-EK‥ ≦ 1･

Lemma 2. Let tノ財be a W*-algebra and E thJe Ca,run,icalprojection ofL/財** onto.ノ材ofnorm

### one. Then E is self-adjoint.

Proof. Since the involution in LM** is J(.ノ材**,tM*)-Continuous7 the unit ball of tノ好一S is o-(.ノ冴**,亡ノ材*)-dense in the unit ball of the self-adjoint portioll Of tAW**. Let t7; be an element

of the unit ball of the self-adjoint portion of.ノ材** and首afilter on i/財s n jP converging t()

I; then we have, for any real number入,

llE(I)+i^111(2 - lIE(I+ill)ll2 ≦瞳+iA1112

### ≦ li霊nf=y'叫12 - 1iminfll(y'iAl)*(y'iAl)ll

### y,.7

### -li霊nfHyl2.^'21日≦ 1+入2･

Hence we have, for any state p of i/♂,

### (Im(E(I),P)+A)(2 ≦ I(e(I)+ill,9)l2 ≦ 1+入2,

### so that Im(E(.77),P) - 0･ Therefore we obtain (E(I) - E(I)*,P) - 0 alld so E(1:) - E(I)*･

Hence e is self-adjoint. □

Theorem 3. The involution in a W*-al9e加.a LAW is o1-Weakly c()ntinuous･

Proof.Let E be the canollical projection of.ノ材** onto -4 0f norm olle; then there is a

### colnmutative diagram as follows:

tAW** lnVOlution〉 LAW**

### El le

### h闘iiiiiiiiii L闘

1IIVOlutioII

Since the involution in tノ材** is c,(LM**, tノ材*)-Continuous, the involution in tM is J-Weakly

### 12

### Bulletin of the Institute of Natural Sciences, Senshu University No･37

Lemma 4･ The self-adjoint portion.ノ冴s and positive poγ･tion.ノ教トOf a W*-algebra Eノ冴are

### J-Weak･ly closed.

Proof･ By Theorem 3っit is trivial that./材s is J-Weakly closed. Since.JM+ nLSP -.ノ材s nLjPn

(1 - L99), tノ教トn -デis J-Weakly closed and also is tノ教ト. □

Proposition 5･ Let Eノ材be a W*-algebra and E the canonical projection ofLM** onto tノ材

### of norm one. Then E is positive.

Proof･ By Theorem 3, the self-adjoint portion of tノ材is cT-Weakly closed and also is the positive portion of ･/材･ It holds that, for p ∈.ノ材* and elementsこr and y of the unit ball of

### LM**,

lp(y*y) - p(X*X)l ≦ lp(y*(y-I))I + lp((y-.7:)*X)F

### - lp(y*(y-I))E+lp(X*(y-I))l

≦2 sup rpa(y-I)l.

‖可I≦1

Since the set (pa L a ∈ tM**, lla= ≦ 1) is a J(LM*7LM**)-colnPaCt balanced convex set, the furlCtion I L- X*x is continuous on the unit ball of tノ材** with respect to the

7-(LM**,.M*)-topology and J(-M**フtノ材*)-7-(LM**,.M*)-topology･ The unit ball of LAW is J(.M**,.M*)-dense in the unit

ball of ･AW**and so 7-(-AW**,-4g*)-dense in the unit ball of.ノ財**･ Since ll1 - X*xH ≦ 1 for every I ∈ Esp, we hve ll1 -X*xII ≦ 1 for every element I of the unit ball ofLノ材**. For any element I of the unit ball ofLM** and state p oftAW, we have p(E(11X*X)) ≦ lll-I,r*xII ≦ 1 and s() p(E(17:*X)) ≧ 0･ Hence we have E(X*.7:) ≧ 0. E is therefore positive. □

### Theorcm 1 and Theorem 3 imply the spectral decomposition of a self-adjoint eleIIlellt Of

### a W*-algebra ･M and so the set of all I)rOjections of ･AW is uniforlnly total in LAW (C.f. [?]).

### The followlng PrOPOSitJion is a special case of the Tomlyama theorem, however we do not

### need the fact that the se(nnd dual of a C*-algebra is a C*-algebra.

### Proposition 6･ Let LM be a W*-algebra and E a Canonical projectior"f.AW** onto LM of

I/,JrII/ ･JI/I. r/I･I/ /1 //(,/,/･J/川/. I,･I ･lIII/.). ･_.〝‥ ,I//I/'I二.〟.

### E(ax) -aE(X･) and E(-) -E(.7:)a.

Proof･ Letこr be an element of -M** with l回l ≦ 1 and g afi1ter on the unit ball of tノ財

### Fundamental Theorems in W*-Algebras and the Kaplansky Theorem

### 13

projection e of tノ材. It holds that, for an arbitrary positive real number入,

### (1+A)2Ilye=2-日(y+Åye)e=2 ≦ Ily+入ye‖2

- I匡(I(1-e)+Åye)Il2 ≦ llx(1-e)+入ye‖2### ≦ 1iminfHz(1-｡)+入yeH2

I,育### -1iminfH(I(1-e)+Åye)(I(1-e)+Åye)*H

I,5 - 1iminfHz(1 - e)Z*十人2(ye)(ye)*FI I,育 ≦ 1+入2日yell2.### Therefore we have ye - 0. Replacing e by 1 -C, we have E(xe)(1 - e) - 0 and so

E(LTe) - E(xe)e - E(X)e･ Since the set of all projections of I/身is uniformly total in tM, we obtainE(xa) - E(I)a for any a ∈.ノ冴･ Since E is self-adjoint, we have E(a*X*) - a♯e(X*)･ □### Theorem 7. The multiplication in a W*-algebra LM is separately contirmous with respect

### to the (7-Weak iopology.

Proof. Let a be an element of i/財and E the canonical projection of t4g** onto LM of norm

### one; then there is a commutative diagram as follows:

L/材**∋ X ) ax Etノ材**

### El lE

tノ財∋X .一･一一 aX ∈.ノ材Since the mapping LM** ∋ I ‥ ax ∈ i/材** is J(i/材**,.ノ材*)-Continuous, the mapping

I/材∋ I r- ax ∈ LAW is JIWeakly continuous. Similarly, the mapping.ノ冴∋ I ‥ xa ∈レ冴is

J-Weakly continuous. □

### 2. SECOND DUALS OF C*-ALGEBRAS AND THE KAPLANSKY THEOREM

### In this section, Without uslng the Kaplansky theorem, we show that the second dual of

### a C*-algebra is *-isomorphic and homeomorphic to some nondegenerate JIWeakly closed

*-subalgebra of cg(負) and that the second dual of a C*-algebra is a C*-algebra･ For the former we need the fact that ｡g(負) is a W*-algebra. However, for the latter we do not

### need that.

### Notice that a positive linear form p on a C*-algebra A is positive on A**. For, the

### function I L- P(X*X) is T(A**, A*)-continuous on the unit ball of A**･

### Lemma 8･ Let A be a C*-algebra ar7Jd S(A) the state space ofA･ Then it holds that

### 14

### Bulletin of the Institute of Natural Sciences, Senshu University No.37

Proof.Let二r be a self-adjoint elernent of A** and 6 an arbitrary positive real number;

then there exists an element p ofA* such that p(I) ≧ l回l -6 and =pII ≦ 1･ We have

### p(I) - 211(p+p*)(I). put中- 2~1(p+p*) an°let 4, - ,4,+一軒beaJordandecomposition

### of 4); then we have

### p(･T) - 4)(I) ≦ I,中十(I)a + 1虹(I)I

≦(lld,+lI+lLl付F) sup lp(I)[-ll,中日 sup lp(I)l

p∈S(A) p∈S(A)

<_ sup tp(i:)l <_ ttx日. p∈S(A)

Therefore we obtain HxH - supp∈S(A) Ip(I)r･

Lemma 9･ LetA be a C*-algebra andy a state ofA･ Let (7Tp,丸p) and (㌔,丸p) be the GNS-repr･esentations ofA and A** associated with p, respectively･ Thenカp is isomorphic to

カp oJt9 Hilbert spaces･ Therefore we may regard the representation (方97九戸) as the e･,IJ･tenSion

### of (7Tp, jjp) by contirl/uity with respect to the J(A**, A*)-topology and c,-weak topology･

Proof For any I,y ∈ A**, we have目方p(I)Ep -斤p(y)Ep=2 - p((I-y)*(I-y))･ since the mapping A** ∋ I Lj P(X*X) is 7-(A**,A*)-continuous on boullded sets and theunit ball

of A is T(A**,A*)-dense in the unit ball of A**,斉p(A)Ep is dense in斤p(A**)Ep･ Hence

舟p is isomCtrically isomorphic toカp･ Therefore we may regard毎as a repsentation oil

舟p extending 7Tp･ For I ∈ A** and y,I ∈ 4 we have p(Z*xy) - (斤p(I)斤p(y)E諦p(I)Ep)･

HeII｡e, SiIICe yPZ* ∈ A*,斤p is contilluOuS On the unit ball of A** and so continuous on A** With respec/t to the o-(A**, A*)-topology and c,-weak topology. ロ

Theorem lO･ Let A be a C*-algebra and S(A) the state space ofA･ Let (7T,負) derwte the

direct sum ∑pO∈S(A)(打9,恥) of GNS-representations and (斤, 35) the direct sum ∑冨∈S(A)(斤9,

{1,1l I,守)I(.I ･T,･ ,Ir･ //,I ,･I･l･"Il/,I//I),/･､日.仁l‥ ,Ih /// I･･,I,,//,I !J･ T/l･/I訂卜I

,､り′I･･,/-degenerate J-Weakly closed 求-subalgebra of cY(負). FurtherγTWre,斤is an isometry and a

### homeomorphism with respect to the o-(A**, A*)-topology and JIWeak topology. Therefore

### A** is a C*-algebra.

Proof. It is easy that斤is continuous with respect to the J(A**, A*)-topology and the

J-weak topology. If斤(I) - 0, then p(I) - 0 for all states 早 of A. Since S(A) is linearly

total in A*, we have a1 - 0, so that斤is faithful. By LemIna 8, we have, for any self-adjoint elemellt二r Of A**,

llxH - sup Jp(I)l - sup ILJEや(斤p(X･))I ≦ sup ll斤p(I)= - Llk(･T)‖･

p∈S(A) p∈S(A) p∈S(A)

Hence we have植‖ -悼(X)=･ Therefore the intersection of the self-adjoint portion of

斉(A**) alld the ullit ball of亡g(負) is (7-Weakly compact and hence the self-adjoint portioll Of

### Fundamental Theorems in W*-Algebras and the Kaplansky Theorem

### L5

J-Weak closure of斉(A**) and首afilter on斤(A**) converging 0--weakly toこr･ Then we have

2~1(I +X*) - limy,172~1(y + y*) ∈斤(A**)･ Similarly7 We have (2i)Ll(･7: - X*) ∈斤(A**) and

so I ∈斉(A**)･ Hence元･(A**) is J-Weakly closed and so is a W*-algebra･ Since a closed ball

of A** is J(A**, A*)-compact, the restriction of斤to a closed ball is a hoIIleOmOrphism with

respect to the (丁(A**, A*)-topology and the (7-Weak topology･ For an element I of斉(A**), wc

have牌~1(2-1(I+.7;*))Ll ≦ Hx‖ and目方~1((2i)-i(I-X*))lt ≦ LIx= andso目元~1(I)‖ ≦ 2日緋 Hence斤~1 is continuous on the unit ball and so contilluOuS On斤(A**), because that斤(A**)

is a W*-algebra･斤is therefore a homeomorphisln.

Consider 7T(A) ㊨ M2(C) as a C*-subalgebra of亡g(カ㊤ C2) and define the norm ()II

### A㊤ M2(C) by LIx‖ - ll7T⑭id(I)日. Since the C*-algebra A㊤ M2(C) is homeomorphic to

### the topological direct sum AoAOAo4 (A⑭M2(C))** is *-isomorphic to A** ㊤M2(C)･

Let p～ be the representation of (A A M2(C))** associated with tile direct sum of all

### GNS-representations of A㊤M2(C)･ ij((A㊤M2(C))**) is *-isomorphic to斉(A**) ㊤M2(C)I SiIICe

### lLi5(I)‖ ≦目刺≦ 2lli)(I)= for every x･ ∈ (A㊤M2(C))**, the normed space i)((A㊤M2(C))**)

is complete and so is a C*-algebra. Hence the *-isomorphism between i5((A ㊨ M2(C))**)

and斤(A**) ㊨ M2(C) is an isometry.

### A⑭M2(C) - (A⑭M2(C))**一･一- i5((A㊧M2(C))**)

1 1 1

### 7T(A)⑭M2(C) A**㊤M2(C) - 斤(A**)㊤M2(C)

If I is an element of the unit ball of斤(A**), then (望xo*) is a self-adjoillt element of the

### unit ball of斤(A**) ⑭M2(C)･ Ify is an element of (A㊤ M2(C))** corresponding to (望零) ,

then we have Hy‖ - lL(告.～)Llフbecause that the self-adjoint portion of斉(A**) ㊨ M2(C) is

### isometric to the self-adjoint portion of (A@ M2(C))*'･ Since the unit ball of A@ M2(C) is

### weakly* dense in the unit ball of (A@M2(C))**, y belongs to the weak* closure of the unit

ball of A ㊨ M2(C)･ Since (A ㊨ M2(C))* is algebraically isomorphic to the product space

### A* × A* × A* × A*, the J((A㊤M2(C))**, (A⑭M2(C))*)-topology is homeomorphic to the

c,(A**㊤M2(C), A* ×A* ×A* ×A*)-topology and so is homeomorphic to the product topology of the J(A**,A*)-topologies･ Since ce(負) ㊨ M2(C) acts on jう0 35, the J-Weak topology

on cY(負) ㊨ M2(C) is homeomorphic to the product topology of the o･-Weak topologies on

cZ(jVI Hence the *-isomorphism of (A ㊨ M2(C))** onto斤(A**) ㊨ M2(C) is continuous with respect to the weak* topology and the 0--weak topology･ Hence (望xo') belongs to the

c,-weak closure of the unit ball of 7T(A) ㊨ M2(C). Definillg Pr21 by prL21 ((芸…壬芸li三)) - X21, al belongs to the J-Weak closure of the ilnage Of the unit ball of 7T(A) ㊦ M2(C) under prl21･

Hence二r belongs to the J-Weak closure of the unit ball of 7T(A)･ The image of the unit ball

### of A** under i is o･-Weakly compact and so coincides with the unit ball of i(A**)･ i is

### 16

### Bulletin of the institute of Natural Sciences, Senshu University No.37

Proposition ll. Let LAW andレ〟 be two W*-algebras and ◎ a J-Weakly contirmous

*-homo-II/･･/I/,///､JJH･./'.〟 /I/I,I ･ I. 'r//,IJ ･1･t･/Y) /､汀-II…(./I/ ,･/I)､′･/ ･l//,/ //), ,II/// I"lI/ (,I･1･(./r)

･･,,/I/, /,/･･､ II.//// //), /Il川†/-,I //I-I///I IJ,lil ,,[･/y Ill/,I･ r小.

Proof･ Let i be the canonical mapping of LAW onto LAW/ ker ◎; then there exists a

### *-isomOr-phismせofLAW/ker◎ into ｡〟 such that ◎ -せ｡j. Since LM/ker◎ is a C*-algebra,せis

### an isometry. The image of the open unit ball of LAW under i coincides with the open unit

ball of LM/ker◎. Hence the image of the open unit ball of tM under ◎ Coincides with the

open unit ball of ◎(LM)･ Since the closed unit ball of tノ材is J-Weakly compact, the image

of the c/losed unit ball of LAW under ◎ is cT-Weakly compact alld so coiIICides with the closed unit ball of ◎(I,iW)･ Therefore ◎(tAW) is 0--weakly closed･ □

Proposition 12･ LetA be a C*-algebra and (7T,瑚a representation of AI Then there exists

a representation (斤,3H ofA** extending (7T7 jう) by continuity with respect to the J(A**,

A*)-topology and J-Weak A*)-topology･ Furthermore斉(A**) is JIWeakly closed.

proof There exists a family (pL) of states of A such that (7T湧) - ∑㌘(7TpL,カpL). Putting

(斤7jう) - ∑㌘(斤pL,カpL),斤is a continuous represelltation of A** with respect to the

### J(A**, A*)-topology and o1-Weak topology. By Theorem 10, A** is a W*-algebra. Therefore,

by Pr()positi()Il ll,斤(A**) is a (7-Weakly closed *-subalgebra of cg(卵. □

### Theorern 10 is an immediate result of the KaplaIISky theorem. However, Without usIIlg

### the Kaplansky theorem, We have seen that the second dual of a C*-algebra is a C*-algebra.

Conversely, this implies tlle Kaplansky theorem. The KaplanskyフS original proof consists### of the calculation of ftlnCtions and the idea uslng matrices, but at present it is known that

### the disctlSSion on matrices is ullneCeSSary. However the Kaplansky theorem follows from

### the idea uslng matrices, too.

### Let LAW be a W*-algebra and V a ulliformly dense linear subspace of LAW. such that, for

### anyp∈ Vanda∈JW, p*,apandpabelongto V.

Lemma 13･ Let JW and V be as above and A a o-(LM,V)-dense 求-subalgebra oftAW. Then

_1 /ヾ汀-II.''I/./I/ I/Ill.､′ /II./Y. I/III･/'rIIIl'/･'../r l./ /､ /I/'･IIll/''/ /I) //)I '丁-Il･'ll/.I '･I'JヾII/･'

### 'J/-A+∩2L99.

Proof･ (Ⅰ) A is deIISe ill tM with respect to the 7-(LM,V)-topology. Let e be a projection

### in LAW and g afi1ter on A converging to e with respect to the 7-(LM,V)-topology. The

balanced convex set PLSP is J(V,LM)-compact for every p ∈ V･ Sinceフfor any I ∈亡AW, 2(1+xx*)LIE ∈ Lip and (1+xx*)-1 ∈ L99, we have### Fundamental Theorems in W*-Algebras and the Kaplansky Theorem

### 17

where the topology is the c,(LAW, V)-topology･ Hence we have limx,17(1+xx*)-1(xx*一e) - 0･

### Therefore we obtain

### 聖(2(1+xx*)11xx*-e) -盟(1+a)ー1(xx*-e)(2-e) -0･

Since (1 + xx*)~1xx* ∈ LjP, we have e - 1imx,52(1 +xx*)~1xx* with respect to the J-Weak topology. By spectral decompostion, LAW+nL99 is incluede in the J-Strong Closure of A+∩2LSP.

### Hence A is JIStrOngly dense in LM.

(ⅠⅠ) Let g be an ultrafilter on A converging to I ∈ LM with respect to the 7-(LAW,V)-topology. In (Ⅰ), replacing e by I ∈ LAW, We have limy,17(1 + yy*十1(yy* - xx*) - o･ where the topology is the cT(i/材, V)-topology. Since the image of an ultrafi1ter is an ultrafilter base

### and an ultrafilter is convergent in a compact space, there exists a limit a - limy,.7(1+yy*)Ll

### in the JWeak closure ofA+CI with respect to the JWeak topology. Since (1+yy*)11yy*

### -ll (1+yy*)~1, We have limy,17(1+yy*)11yy* - 11a･ Therefore we have 1 -a-axx* - 0

### and so a(1 +xx*) - 1. Hence a is invertible and a-1 belongs to the J-Weak closure of

### A+ Cl. Since 1 - a belongs to the o1-Weak closure of4 xx* - aIl(1 - a) belongs to the

J-Weak closure of A. Consequently, A is o1-Weakly dense in i/材. □ Let LM be a J-Weakly closed *-subalgebra of ,y(負) alld A a strongly dense *-subalgebraof LAW. For an element I of LAg, there is afilter g on A converging strongly toこr. Since

### (y-I)*(y-I)-y*y-X*X-X*(y-I)-(y*-X*)I

### and

### liT17n(y~X)*(y~X) =O alld liTlil(y~L') =liT.il(y* ~a'*) =0,

### with respect to the weak topology, we have x*X - limy,.7y*y with respect to the weak

### topology and hence AnLM+ is weakly dense in LM+, or equivalently, strongly dense in LM+.

Therefore, for a projection e in LAWフWe have

### x→e盟｡.須(2(1 +I)-1X-e) - X→e浩1LAW.(1 +I)-1(I- e)(2 I e) - 0,

With respect to the strong topology. Since ll(1 + I)JIxLl ≦ 1 for every I ∈ tM+, e belongs

### to the (7-Strong Closure of A, So that A is JIStrOngly dense in tM, in virtue of spectral

decomposition. From this fact, it follows that J-Weakly closed *-subalgebra of `g(負) is strongly closed7 0r equlValentlyフWeakly closed.

Theorem 14 (Kaplansky)･ Let tAW and V be as above and A a 求-subalgebra oftAW which

/ヾ〔(./7㌧l■卜′I･I/､･ //I.〟. nI,I/ //I･ IIII// I"I/I･,I.I /･､ Tt./7.1/Y.Ill/･Il･､HI/ //I, Ill/// I,,lil,,]./Y.

### Proof. We may assume, without loss of generality, that A is a C*-algebra. Let id denote

### 18

### Bulletin of the Institute of Natural Sciences, Senshu Universlty No.37

thell ◎ is a continuous *-homomorphism of A** equipped with the (7(A**, A*)-t()pology into

eノ冴equipped with the (7-Weak topology. We regard ◎ as all eXtenSioll Of id. By Theorem 10, A** is a W*-algebra･ Hence, ◎(A**) is c,-Weakly closed and the image of the ullit ball of A** under ◎ coincides with the ullit ball of ◎(A**)つin virtue of Proposition ll. SiIICe ◎(A**) is (丁(tAW, V)-dense ill tM, ◎(A**) coincides with LAW, ill Virtue of Lemma 13. SiIICe

### the unit ball ofA is rT(A**,A*)-dense in the unit ball ofA**, the unit ball ofA is J-Weakly

deIISe ill the llIlit ball of tノ材alld so 7-(i/材, eAW.)-dense ill the unit ball of LAW. □

Proposition 15 (Polar Decompositioll)･ Let eノ材be a W*-algebra･ For arlJy elemer7Jt.,r Of

.〟. I//,I･ ･//､/､ =′-lI/I/,,II/,/ ,,/I･ I"IrII･II /､‖/Ill/Ill/ I･ /II./7 ､m/∫ /i/'I/.'･ 〝.I･ 'III'/ I.l'･

### S(回)･

Proof･ Put?,rn, - I(n~1 1+回)~1 for each positive natural Ilumber n; then we have lI7,"J= ≦ 1･

Since卜由- (n-11十回)~lrxr, ((,unf)n is increasing alld so J-Strongly convergellt. Since

s(La7L)lvnl - I,unl, we have s(回)limnーCX, lvnL - 1iIIlnー∞ L7,nL. Since同一剛vnl - n,~1l,unI, we

### have回回lim,(/ー∞卜項･ Hence we have s(回)(1 11imnーJvnJ) 0and so limn→cJu,n,I

-S(回)･ Since vニV,m -ド)nll,i,ml, we have (vn - vm)*(vn - vm) - (Lvnトlて)mL)2. Hence (vn)n### is a Cauchy sequence with respect to the J-Strong tOpOlogy. Since the unit ball of LAW is

Complete with respect t｡ the J-Strong tOpOlogy, (7)γ～ノ)γ～ノCOnVergeS J-Strongly to some eleIIlellt 7, ∈ eAW､ Since ･1㌧Vnlxl - nALIvn, we obtain I -申ト

Let w be a partial isometry in LAW such that二r - Wr可and w*W - S(回); then we have

1,- 1im I(nー11+lxL)-1 - 1im wlxl(r7rll+回)~1 -ws(回) -,u'.

n→∝) nーDO

[コ

### Now, we shall describe the pr()ofs of the Kaplansky theoreln based OII continuity of

functions. The function f: tAW+ ∋ I L., I(1 + Ll)一l ∈ LAW+ n L99 is most sirnple aIIIOIlg

### continuous fullCtions such that f(LM+) is cT-StrOrlgly denseinLAW+ n L99. We sllall show

A+ ∩ ･9 -.,4W+ n Lプ. For a noII Self-adjoint element二r ∈ LjPフC()IISidering the polar

decom-position I -申1, We call See V ∈才F｢タand so I ∈オ斤タ. of course, we can apply tllis

### manner to the proof of Theorem lO･ We can sllOW A+ nL90 - LAW+ n L99 by an arbitrary

### non-negative valued continuous function.(j such that g is defined on R+ and such that

g(0) - 0 and suptER. 9(i) - 1･託軒- eJ4g. n L99 is an interlnediate-value theorem･

### OthJer Proofs of the Kaplansky lheoremJ･ (I) We may assume that A is uniformly closed. Let

L5P, A+ and As dellOte the unit ball of i/材, the positive portion of A and the self-adjoillt

### portion of 4 respectively･ By the proof of Lemma 13 and spectral dec/ompositioll, A+ is

### JIWeakly dense in eM+, or equivalelltly, JIStrOngly dense in LAW+. Define the functiorl f by

f(.,r) - I(1+∫)~l for I ∈亡AW+. It is obvious that the function eM+ ∋ I - (1+I)ll ∈ tM+

### Fundamental Theorems in W*-Algebras and the Kaplansky Theorem

### 19

hence f(LAW+) is iIICluded ill the JIStrOng Closure f(A+) of f(A+)･ For I ∈ LAW+ n L99 and

any real number α ∈ (0,1), we have α∬ ∈ f(LM.) and so I - 1imαT1αX ∈アて云つ. Hence

### we have eAW+nL99 ⊂ f(A+)･ Since f(A+) ⊂ A+nL99, we have LM+nt90 ⊂ A+∩｡99. If

0 ≦ a≦ 1 andO ≦ b≦ 1, thenwehave lla-bH ≦ 1･ For anyself-adjointelementxinL99,

the positive part and negative part ofx belong to A+ n L99. Therefore we haveこr ∈ As n L99

### and so L90 is included in the J-StrOng* closure A n 2L99 of A n 2L90.

### There exists a sequence (pn)n of polynomials with real coefhcients such that

### lim sup lpn(i)-tl/21 -0.

n-Cno<t<4

### It llOlds that

### sup Hpn(I)-xl/211≦ sup lpn(i)-tl/2トO as n-+∞,

JJ･∈.M+ ∩4～9P O<t<4

in virtue of Gelfand representation･ The furICtion eAW+ ∩ 4L90 ∋ I - pn(I) ∈ LAW is

### J-strollgly colltinuous. SiIICe the limit of a sequence ｡f colltinuous fuIICtioIIS With respect

to the topology of ulliform convergence is continuousっthe func/tion i/畝ト∩ 4亡ブヨx

L-EI/2 ∈ LAW. is J-StrOIlgly coIltinuous･ Since the function 2LjP ∋ I L- X*X ∈ LM+ ∩ 4L99 is c,-strollgly* Continuous, the function 2L5P ∋.r L- (1 + n(X*X)l/2)~1 ∈ LM is J-StrOngly* Continuous. Therefore the function 2L99 ∋ I i- I(n111 + (X*X)1/2)~1 ∈ LAW is continuous with respect to the J-StrOIlg* topology and the J-Strong tOpOlogy. For any I ∈ 4 we have .77(n-ll+(X*X)1/2)-1 ∈ AnLSP. HeIICe, for ally.,r ∈ L99, JJ･(n~11+(X*X)1/2)~1 ∈オ斤夕. since

lxL ∈ A+∩｡99, we havex(nJll+

### (X*X)1/2)~1回∈オ斤夕. since I-I(n111+ lxI)-1回

### n-1X(n111+回)~1, we obtain I - limn→∞(I(n-ll+ lxL)｣/2)回∈オ斤夕. AnL99 is

### therefore JIStrOngly dense in ｡99, 0r equivalently, 7-(LM, LAW.)-dense ill L99.

(ⅠⅠ) Next, We assume that LAW is a JIWeakly closed *-subalgebra of ｡g(35) and A is a strongly dense *-subalgebra o仁/材. Then, as seen above, A+ is strollgly dense in tM+. siIICe f is strollgly contilluOuS7 We have f(LAW.) ⊂許す⊃. By the above discussion, we obtain tAW+ nLSP ⊂ A+ n L99 and t99 ⊂ A n 2L99っwhere the closures are takell by the strong*

### topology, or equlValently, by the J-StrOIlg* topology･ Repeating the above discussion, We

### conclude that the unit ball of A is strongly dense in the unit ball of LAW.

### (III) We c/an take a gelleral function iIIStead of the above function f. Let g be a continuous

### non-negative valued function such that g is defined on R+ or a bounded interval l0, r] alld

### such that g(0) - O and supfg(i) - 1･ In case that g is defined on a bounded interval,

### puttillg g(i) - 9(r) for i > r, we may assume, without loss of geIICrality, that g is defined

### onR十

We shall show that g(LM+) is uniformly dense in LM+ n L99. Let二r be in tAW+ n L90; then,

### for an arbitrary positive rlumber 6, there exist a mutually orthogonalfinite sequence (et)i

of projections and afinite sequence (li)i in (0, 1) such that H31. - ∑%入甘eiH < 6, iII Virtue of### 20

### Bulletin of the Institute of Natural Sciences, Senshu Universlty No.37

0 itscharacterspace. Therearepi ∈ R+ with入i - g(FLi)･ Put I - ∑iPtei･ If∂i(LJ) - 0 for

all i, then a(LJ) - 0 for all a ∈ B, which is a contradiction･ Hence, for any LJ ∈ 0, there is

### an index i with 6%(LJ) - 1. When 8%(LJ) - 1, we have g(2(LJ)) - g(pi) - Ai - a(LJ), because

that ∂j(LJ) - 0 ford ≠ i･ Therefore we have 9 - g｡2 or y - g(I)･ 9(tAW+) is therefore

### uniformly dense in LM+ n L99. 9 may be approximated uniformly on each bounded interval

### lo,28] by polynomials pn such that the coemcients ofpn are real and pn(0) - 0･ Hence it

### holds that

sup llg(I)-pn(I)=≦ sup lg(i)-pn(i)I-0 as n-+∞,

X己AW+ ∩25･y O<t<25

ill Virtue of Gelfalld repsresentation. Therefore the function LAW+∩2sL99 ∋ I L- g(I) ∈ LAW+ n L99 is J-Strongly continuous. By Lemma 13, LM+ n s｡プis iIICluded in the cTIStrOng Clusure

A+ ∩2sL99 and so g(LM+ nsL5P) ⊂ g(A+ ∩2sL99)･ Since pn(0) - 0, for any I ∈ A+ ∩2sLSP,

we havepn(I) ∈ A and so g(I) ∈ A･ Hence we have g(A+∩2sL57) ⊂ A+nL99 and s(,

g(LAW+ n st99) ⊂ A+ n LSP･ Therefore we have

### g(亡弟) - Ug(LM. nsL99) ⊂

S>0

### A+ n LSP.

Since A+ nL99 is uniformly closed, it holds that LAW+ ∩ ｡99 ⊂ A+ nL5P･ Considering polar decomposition as in (Ⅰ), it follows that the unit ball of A is J-Strongly dense in the unit

### ballofLM.

If g is a continuous real-valued function on [0, +∞) such that g is strictly increasing,

g(o) - o and limt→+∞9(i) - 1, then it is obvious that g(tAW+) is uniformly dense in

### LM+ n LjP, because that the inverse function gll is continuousI

Puttingg(i) - t∧1 for i ∈ R+, we haveg(I) - I for every I ∈ LAW+nL99 and so

g(LAW+ n L99) - tAW+ n -ア･

(IV) The function亡M ∋ I L- (1 + xx*)-1 ∈ tAW+ n L99 is J-StrOngly* Continuous･ For, it

### holds that

### (1+yy*)-1 - (1+xx')~1 - (1+yy*)~1(xx* -yy*)(1+xx*)~1

### - (1 +yy*)~1(I-y)X*(1 +xx*)-1

### + (1+yy*)-1y(X* -y*)(1+X*X)~1,

and ll2(1 + yy*)llyH ≦ 1日. The function LM. nL99 ∋ I - xl/2 is c,-strongly continuous, and hence the function LM ∋ I L- (1 + xx*)ll/2 ∈ LM. n L99 is J-StrOngly* Continuous･ Therefore the function h‥ LAW ∋ I r- (1 + xx*)~1/2X ∈ tAW n亡jP is continuous with respect to the J-StrOng* topology and c'-strong topology. The function R ∋ i i- (1 + t2)~1/2t is strictly increasing and inft(1 + t2)ll/2tニー1 and supt(1 + t2)ll/2t - 1･ It is obvious that

### Fundamental Theorems in W*-Algebras and the Kaplansky Theorem

### y - h(I), and hence h(LAW) is the open unit ball of LAW. Since

### h(JW) ⊂可否⊂万石二戸,

we have LSP ⊂ A n L90, that is, A n L99 is cT-Strongly dense in LSP.

### 21

### Now, We can prove, Without using Of representations on Hilbert spaces, that the second

### dual of a C*-algebra is a C*-algebra.

### Let A be a C*-algebra. Since a positive linear form p on A is positive on A**, we can

define the seminorms pp and p; on A**‥### pp(I)-p(X*X)1/2 and p;(I)-p(xx*)1/2

We call the topology defined by all pp (resp･, all pp and pを) the J-Strong tOpOlogy (resp･,

### the J-StrOrlg* topology)･ By Jordan decompostion, the u-strong topology is finer than the

a-(A**,A*)-topology･ Since (Px I x ∈ A**, lLxIL ≦ 1) is J(A*,A'*)-compact, pp and pをare

### continuous on the unit ball with respect to the T(A**, A*)-topology･ Hence, for a J-StrOngly*

colltinuous linear form ¢ on A**, the intersection of ker4, and the unit ball is 7-(A**,

### A*)-closed and so J(A**, A*)-A*)-closed･ By the Banach theorem, ker4, is c,(A**, A*)-A*)-closed, and

hence ¢ is J(A**,A*)-Continuous, that is, 4, ∈ A*･ Therefore the J-StrOIlg tOPOlogy and

### o1-StrOng* topology arc compatible with the duality (A**, A*).

### Notice that the second dualof a C*-algebra has an identity.

### Lemma 16. Let A be a C*-algebra. The/n, for any self-adjoint element I ofA**, we have

IJx2日- ‖J/･JZ2 and so ‖LTH - r(I)′ where r(I) denotes the spectral radius ofx.

Proof･ By Lemma 8 and the Cauchy-Schwarz inequality, for any self-adjoint elementこr ∈

### A**, we have

### l酬≦ sup p(X2)1/2≦植211]/2,

p∈S(A)

and so Hx2日- llx=2. Therefore we have

r(I) - 1im lLx2nlI'2~"メ- l回L.

nー()○

[コ

### Let B be a commutative Banach *-subalgebra of A** containing 1 and the mapplng

B ∋ I Lj金∈ C(0) the Gelfand representation of B. If I and y are self-adjoint elements

of B and金- a, then we have I - y･ For, since the spectral radius in a Banach subalgebra

### coincides with the spectral radius in A**, we have

IJx-yJI -r(.1/･-y) -Sup匝(LJ)一g)(LJ)I -0

LJ∈fl

### and so.7:-y.

### 22

### Bulletin of the Institute ofNatural Sciences, Senshu Universlty No.37

Lemma 17. LetA be a C*-algebra. Then we have Sp(I) ⊂ Rfor ever･y self-adjoint element

I ∈ A** and Sp(X*X) ⊂ R+ for every element L･ ∈ A**･ Therefore th,e spectrum, of a

selF-'I･!/I,/Il/ I/I lI/I/I/.I･二.l‥ /IHI /''･/I川･// ､′lIJ･lI･/I/JI･,I /,'L･,JI//,I//,,I/I/ ･I･ ･lll,/ 1 L･･,///･･/I/- II･,//I /i/I

### spectrum ofx in A**.

Proof. (I) Let I be a self-adjoint element of A** with Hx‖ ≦ 1; then there is afilter g

on the unit ball of the self-adjoint portion of A which converges toこr With respect to the

### J(A**,A*)-topology. Since, by the proof of Theorem 1, A + Cl is a C*-algebra, it holds

that, for any入∈ Sp(.7;) and integer γ7Jっ### llm入+rl/l≦1人+inl≦順+inlH

≦ 1irrylinf lly 'i,n/llI - limillf =(y'inl)*(y +inl)lll/(2

### y,t7

- 1inylinflly*y+n21日1/2 ≦ (1 +n2)1/2

Hencewe have IIn入-0, i.e.,入∈ R, so that Sp(I) ⊂ R･

### Forx∈A** with L回L ≦ 17 WehaveO≦ p(X*X) ≦ 1 foreverystatepofA, andso

IIl-X*L･ll - sup Lp(1-.,r*L7:)t ≦ 1･

p∈S(A)

Hence, forany入∈ Sp(X*X),wehave 1-人≦ lll-X*xll ≦ 1 andso入≧ 0･ Thereforcwc

obtaiII Sp(.7:*t7:) ⊂ R+.

### Since the boundary of the spectrum of I in a Banach subalgebra is included in the

### boundary of the spectrum of I in A** alld there is a llOIl real boundary p()illt if there is a

### llOIl real lltlIllber in tlle Spectrum, it follows the remainder.

(ⅠⅠ) For any入¢ Rand anyself-adjoint clcrllent I ∈ 4 wellaVe =(^1-I)ー1日≦ lIII1人｢1 <

+∞. HcIICe, if 首is a ultrafi1ter oll the self-adjoint portioll Of A whicll COIIVergeS tO a Selfadjoint elelnent a･ Of A** with the 7(A**, A*)topology, then we have limy,17(入1y)~1(yt7;) -o with respect t-o the J(A**,A*)-t-op-ol-ogy. There exists a lilnit a - limy,17(入1 l y)~1 with respect to the J(A**,A*)-topology･ Hence we have -1 +入a - aLT - O or a(入1 - I,r) - 1･

Therefore we obtaill入官Sp(LT), SO that Sp(.,r) ⊂ RI

### Forany入gR十and二r∈4wehave

### ll(入1 1X*X)~]ll ≦ min(lRe入｢1,LIm入｢1) < +∞

### and

### FundamentalTheorems in W*-Algebras and the KaplanLqky Theorem

### 23

Hence, if音is a ultrafilter on A which converges to an elemellt I,r ∈ A** Witll tlle T(A**,

### A*)-t()I)()logy, tlleII We llaVe

liTgl(入1 - y*y)~1(y*y Jal) - I,iT51(入1 - y*y)Jly*(y 1,I)

### '恕(Å1 - y*y)一1(y - tT)*L7:

### =O

with respect to the J(A**, A*)-topology･ There exists a lilnit a - limy,17(入1 - y*y)~1 with respect to the (丁(A**, A*)-topology. Hence we have -1 +入aJ - (1,ll/･*lj1 - 0 or oJ(入1 - X*X) - ll Therefore we obtain入¢ Sp(X･*X), so that SI)(.7:*.7:) ⊂ R+. □

Let -アaIld Al* denote the unit ball of the second dual A** of a C*-algebra A and the

set ofal1.,IJ･*.7: Withニr ∈ A**, rcspcctively.

Lemma 18･ Let A be a C*-algebra arZJd I a self-adjoir7/i element ofA** with Sp(I) ⊂ R+･

I/J･/I //,… I//､/､ I( ,Ill/,III･ ･uu-I,I,/J･･/I// ,I,Il/･Il/ I/ ,I/.l‥. ,/,//,I/,I/ I"/.Ill -'. ､′′･/∫ //I,/(.I･ - /了 ･lII,/叫-(I/) ･ Il . 'r//･I･,/･Jr･ .1" '･,I/I/I/I/- III'//I //I･ ､･/ ･JJ I/I/ ･､･〃一･l,//･･/I// ,/･Il/I/II､.I･ ､′l,･//

///'l/ト申.,･l二R . /:'l////'17II',I･'///'IIl//I//'･//.1‥ l･/ .'･-.l･l -I/.ll/l ･/ ′､汀-.､//･･･I/.I//I/

Continuous.

Proof･ There exists a sequencJe (pγL)n of polyn()mials with real (ミ｡efBcieIltS Su(て11 that

### lim sup lpn(i)-tl/2l-0.

nー∞o≦t5;llal=

Let B bc the comnlutative BaIlaCh *-sl1balgebra of A** generated byこr alld 1, alld tllC

mappillg B ∋ y i- ,a ∈ C(f2) the Gelfand rcpresentatioll. By LemIna 16っit holds that

llpn(al) -Pm(I)‖ - SuPLpn(金(LJ)) -Pm(金(LJ))I ≦ sup lpn(i) -pm(i)卜

LJ∈O o≦t_<‖J/･JI

### HeIICe the sequence (pn(I))n is a Cauchy sequence alld so coIIVergeS in^norm to some

sclf-adjoint element y. Since a(LJ) - 1iIIlnーCWr.(金(LJ)) -.,i(LJ)1/2, we have y2 - ∂2 -金alld so yL2 -.1,･ alld Sp(y) - ,a(fl) ⊂ R+. If I is a self-adjoint clement, I,･ - 22 alld Sp(I) ⊂ R+7 theII I COmIIluteS Withニr. Hence there is a commutative BallaCll *-Hllbalgebra C containlllg i:,I and 1. Since y ∈ C, CoIISidering the Gelfand representatioll OfC, we have ,a -金1/2 -乏

### alld so y- 2:.

Sincc Al* ∩ ･y ∋ L･ r--i Pn(･7:) ∈ A** is J-Strongly contiIluOuS and

sup Hpn(.,r)-xl/2日- sup suplpn(金(LJ))-金(LJ)1/2l x∈Al*∩,y x∈Al*∩･-プW∈0

≦ sup 7pn(i)-tl/2l7

0<t<1

### 24

### Bulletin of the Institute of Natural Sciences, Senshu University No.37

### Theorem 19. The second dual of a C*-algebra is a C+-algebra.

Proof･ Let A be a C*-algebra･ For any lT ∈ Al*, we have Sp(1 +I) ⊂ [1,+∞) and so

Sp((1+I)~1) ⊂ (0,1]. Hence, byLemma 16, wehave H(1+I)-1日-r((1+I)-1) ≦ 1. By Lemma 18, wehave (1+I)-1 ∈ Al*nL5PI Hencethefunction Al* ∋ I r-- (1+I)~1 ∈ Al*nL99

is 0--strongly continuous･ Since the function L99 ∋ I L- X*X ∈ Al'n L99 is o1-StrOngly* Continuous, for a positive natural number n, the function L99 ∋ I L- (1 + n(X*X)1/2)-] ∈ Al* n L99 is J-StrOngly* Continuous, in virtue of Lemma 18･ Therefore the function LSP ∋ I - I(n-ll + (X*X)i/2)~1 ∈ A** is continuous with respect to the o1-StrOng* topology and

J-Strong tOPOlogy. For any I ∈ A, we have Hx(n~11 + (X*X)1/2)-1日≦ 1. Since the J-Strong

### topology and J-StrOng* topology are compatible with the duality (A**, A*), L5P is J-Strongly

closed and A nL99 is c,-strongly* dense in L99. Hence we obtain llx(n-ll + (X*X)1/2)~111 ≦ 1 for everyこr ∈ LSP. Since

### x -I(n-ll + (X*X)1/2)~1(X*X)1/2 - n-Ix(n-ll + (X*X)1/2)~1,

### we have

Hx-I(n111 + (X*X)1/2)ll(X*X)1/211 ≦ n~1.

### Therefore it follows that

llxH - lim =X(n-ll + (X*X)1/2)~1(X*X)1/2日

rLー00

≦ H(X*X)1/2日- llx*xlll/2,

### so that lLx*xH - lLx=2. consequently, A** is a C*-algebra.

In a W*-algebra LM and its second dual亡M**, the involutions are continuous and the

multiplications are separately continuous. Since the canonical projection E Of i/材** onto

### LAW of norm one is an extension of the identity mapping of eAW by continuity, E is a

### *-homomorphism. In the following, We do not use the fact that the second dual of a

C*-algebra is a C*-C*-algebra and we need only the fact that the *-isomorphism斉in Theorem

### 10 is a homeomorphism.

### Theorem 20 (Sakai). Any W*-algebra LM is isometrically *-isomorphic and c,-weakly

homeomorphic to somJe nOndegenerate c,-weakly closed 求-subalgebra of,y(負) fort a Hilbert

spacejう.

Proof･ kerE - (LAW.)o is a u(LAW**,tAW*)-closed two sided ideal･Let斤be the representation

of JiW** as in Theorern lO･ Since斉is a homeomorphism,斤(kerE) is a o1-Weakly closed two

sided ideaL Hence LAW is *-isomorphic and homeomorphic to寿(LM**)/斤(kerE). Therefore ･ノ冴is *-isomorphic and homeomorphic to some reduced Yon Neumann algebra斤(モノ材**)e