We prove that every unital stably finite simple amenable $C^*$-algebra $A$ with finite nuclear dimension and with UCT such that every trace is quasi-diagonal has the property that $Aotimes Q$ has generalized tracial rank at most one, where $Q$ is the
universal UHF-algebra. Consequently, $A$ is classifiable in the sense of Elliott.
Let $X$ be an infinite compact metric space with finite covering dimension and let $alpha, beta : Xto X$ be two minimal homeomorphisms. We prove that the crossed product $C^*$-algebras $C(X)rtimes_alphaZ$ and $C(X)rtimes_beltaZ$ are isomorphic if and
only if they have isomorphic Elliott invariant. In a more general setting, we show that if $X$ is an infinite compact metric space and if $alpha: Xto X$ is a minimal homeomorphism such that $(X, alpha)$ has mean dimension zero, then the tensor product of the crossed product with a UHF-algebra of infinite type has generalized tracial rank at most one. This implies that the crossed product is in a classifiable class of amenable simple $C^*$-algebras.
We present a classification theorem for a class of unital simple separable amenable ${cal Z}$-stable $C^*$-algebras by the Elliott invariant. This class of simple $C^*$-algebras exhausts all possible Elliott invariant for unital stably finite simple
separable amenable ${cal Z}$-stable $C^*$-algebras. Moreover, it contains all unital simple separable amenable $C^*$-alegbras which satisfy the UCT and have finite rational tracial rank.
Let $beta: S^{2n+1}to S^{2n+1}$ be a minimal homeomorphism ($nge 1$). We show that the crossed product $C(S^{2n+1})rtimes_{beta} Z$ has rational tracial rank at most one. More generally, let $Omega$ be a connected compact metric space with finite cov
ering dimension and with $H^1(Omega, Z)={0}.$ Suppose that $K_i(C(Omega))=Zoplus G_i$ for some finite abelian group $G_i,$ $i=0,1.$ Let $beta: OmegatoOmega$ be a minimal homeomorphism. We also show that $A=C(Omega)rtimes_{beta}Z$ has rational tracial rank at most one and is classifiable. In particular, this applies to the minimal dynamical systems on odd dimensional real projective spaces. This was done by studying the minimal homeomorphisms on $Xtimes Omega,$ where $X$ is the Cantor set.
Let $x, y$ be two normal elements in a unital simple C*-algebra $A.$ We introduce a function $D_c(x, y)$ and show that in a unital simple AF-algebra there is a constant $1>C>0$ such that $$ Ccdot D_c(x, y)le {rm dist}({cal U}(x),{cal U}(y))le D_c(x,y
), $$ where ${cal U}(x)$ and ${cal U}(y)$ are the closures of the unitary orbits of $x$ and of $y,$ respectively. We also generalize this to unital simple C*-algebras with real rank zero, stable rank one and weakly unperforated $K_0$-group. More complicated estimates are given in the presence of non-trivial $K_1$-information.
Let $A$ be a unital $C^*$-algebra and let $U_0(A)$ be the group of unitaries of $A$ which are path connected to the identity. Denote by $CU(A)$ the closure of the commutator subgroup of $U_0(A).$ Let $i_A^{(1, n)}colon U_0(A)/CU(A)rightarrow U_0(math
rm M_n(A))/CU(mathrm M_n(A))$ be the hm, defined by sending $u$ to ${rm diag}(u,1_n).$ We study the problem when the map $i_A^{(1,n)}$ is an isomorphism for all $n.$ We show that it is always surjective and is injective when $A$ has stable rank one. It is also injective when $A$ is a unital $C^*$-algebra of real rank zero, or $A$ has no tracial state. We prove that the map is an isomorphism when $A$ is the Villadsens simple AH--algebra of stable rank $k>1.$ We also prove that the map is an isomorphism for all Blackadars unital projectionless separable simple $C^*$-algebras. Let $A=mathrm M_n(C(X)),$ where $X$ is any compact metric space. It is noted that the map $i_A^{(1, n)}$ is an isomorphism for all $n.$ As a consequence, the map $i_A^{(1, n)}$ is always an isomorphism for any unital $C^*$-algebra $A$ that is an inductive limit of finite direct sum of $C^*$-algebras of the form $mathrm M_n(C(X))$ as above. Nevertheless we show that there are unital $C^*$-algebras $A$ such that $i_A^{(1,2)}$ is not an isomorphism.
We found that if $u$ and $v$ are any two unitaries in a unital $C^*$-algebra with $|uv-vu|<2$ such that $uvu^*v^*$ commutes with $u$ and $v,$ then the SCA, $A_{u,v}$ generated by $u$ and $v$ is isomorphic to a quotient of the rotation algebra $A_thet
a$ provided that $A_{u,v}$ has a unique tracial state. We also found that the Exel trace formula holds in any unital $C^*$-algebra. Let $thetain (-1/2, 1/2)$ be a rational number. We prove the following: For any $ep>0,$ there exists $dt>0$ satisfying the following: if $u$ and $v$ are two unitary matrices such that $$ |uv-e^{2pi itheta}vu|<dtandeqn {1over{2pi i}}tau(log(uvu^*v^*))=theta, $$ then there exists a pair of unitary matrices $tilde{u}$ and $tilde{v}$ such that $$ tilde{u}tilde{v}=e^{2pi itheta} tilde{v}tilde{u},,, |u-tilde{u}|<epandeqn |v-tilde{v}|<ep. $$ Furthermore, a generalization of this for all real $theta$ is obtained for unitaries in unital infinite dimensional simple $C^*$-algebras of tracial rank zero.
Let $A$ be a unital separable simple amenable $C^*$-algebra with finite tracial rank which satisfies the Universal Coefficient Theorem (UCT). Suppose $af$ and $bt$ are two automorphisms with the Rokhlin property that {induce the same action on the $K
$-theoretical data of $A$.} We show that $af$ and $bt$ are strongly cocycle conjugate and uniformly approximately conjugate, that is, there exists a sequence of unitaries ${u_n}subset A$ and a sequence of strongly asymptotically inner automorphisms $sigma_n$ such that $$ af={rm Ad}, u_ncirc sigma_ncirc btcirc sigma_n^{-1}andeqn lim_{ntoinfty}|u_n-1|=0, $$ and that the converse holds. {We then give a $K$-theoretic description as to exactly when $af$ and $bt$ are cocycle conjugate, at least under a mild restriction. Moreover, we show that given any $K$-theoretical data, there exists an automorphism $af$ with the Rokhlin property which has the same $K$-theoretical data.
Let ${cal Z}$ be the Jiang-Su algebra and ${cal K}$ the C*-algebra of compact operators on an infinite dimensional separable Hilbert space. We prove that the corona algebra $M({cal Z}otimes {cal K})/{cal Z}otimes {cal K}$ has real rank zero. We actually prove a more general result.
Let $A$ be a unital separable simple ${cal Z}$-stable C*-algebra which has rational tracial rank at most one and let $uin U_0(A),$ the connected component of the unitary group of $A.$ We show that, for any $epsilon>0,$ there exists a self-adjoint ele
ment $hin A$ such that $$ |u-exp(ih)|<epsilon. $$ The lower bound of $|h|$ could be as large as one wants. If $uin CU(A),$ the closure of the commutator subgroup of the unitary group, we prove that there exists a self-adjoint element $hin A$ such that $$ |u-exp(ih)| <epsilon and |h|le 2pi. $$ Examples are given that the bound $2pi$ for $|h|$ is the optimal in general. For the Jiang-Su algebra ${cal Z},$ we show that, if $uin U_0({cal Z})$ and $epsilon>0,$ there exists a real number $-pi<tle pi$ and a self-adjoint element $hin {cal Z}$ with $|h|le 2pi$ such that $$ |e^{it}u-exp(ih)|<epsilon. $$
