An Invitation to von Neumann Algebras by V.S. Sunder

By V.S. Sunder

Why This booklet: the speculation of von Neumann algebras has been starting to be in leaps and boundaries within the final two decades. It has continuously had powerful connections with ergodic conception and mathematical physics. it's now starting to make touch with different parts akin to differential geometry and K-Theory. There looks a powerful case for placing jointly a e-book which (a) introduces a reader to a few of the fundamental idea had to relish the new advances, with no getting slowed down via an excessive amount of technical aspect; (b) makes minimum assumptions at the reader's heritage; and (c) is sufficiently small in dimension not to attempt the stamina and endurance of the reader. This booklet attempts to satisfy those specifications. as a minimum, it's only what its name publicizes it to be -- a call for participation to the interesting global of von Neumann algebras. it really is was hoping that when perusing this e-book, the reader may be tempted to fill within the a variety of (and technically, capacious) gaps during this exposition, and to delve extra into the depths of the idea. For the specialist, it suffices to say the following that once a few preliminaries, the publication commences with the Murray - von Neumann class of things, proceeds in the course of the simple modular conception to the III). type of Connes, and concludes with a dialogue of crossed-products, Krieger's ratio set, examples of things, and Takesaki's duality theorem.

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U {oo} where E = D( N). It is easy to see that (i) if le is finite and [lei N ] = n, then I:!.. = {k E : k = 0,1, ... , n} and (ii) if le is infinite, then I:!.. = {n E : n = 0, 1, ... , oo}. Case (ii): M is of type II. Since M has a fundamental sequence, it follows that I:!.. does not contain a smallest positive number. Let a = D(le). l, then ka E I:!.. for any integer k such that ka Taken together, the preceding two sentences guarantee that I:!.. is dense in [O,a]. If now, < a < ex, pick a sequence {an} f I:!..

We get: = Mo M= [m~o (Mn Q Nn») $ [n~1(MnQNn») $ [n~o (N n Q Mn+l)] $:R L~o(NnQ Mn+l»)$:R No ..... N. where we have used the fact proved above that (Mo Q No) ..... (M1 Q N1) 0 ..... (M2 9 N2 ) ..... 9. Suppose M is a factor. If M and subspaces affiliated to M. then either MIN or N 1 11 N are closed Proof. Assume. with no loss of generality. that both M and N are non-zero. Let f denote the set, whose typical member is a family 22 1. ,1 1 N. 1 1. 1 1. J for i 11 "I- j, Ni for all i, and f 11, Nj f N for all 1.

To prove 11 is isometric it suffices thanks to the C*-identi ty to verify that 111(x) II = IIx I when x = x* E M. Fix such an x; since the norm of a self-adjoint operator is equal to its spectral radius, it is more than sufficient to show that sp l1(x) = sp x. For this, if >. r l E M, by the double commutant theorem; hence l1(x) - ). r l ). Thus, sp l1(x) f sp x. Suppose this inclusion is strict; then there exists a continuous real function I on sp x, which vanishes on sp l1(x) but not everywhere in sp x.

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