Approximate Homotopy of Homomorphisms from $C(X)$ into a Simple $C^*$-algebra

Abstract

Let $X$ be a finite CW complex and let $h_1, h_2: C(X)\to A$ be two unital \hm s, where $A$ is a unital C*-algebra. We study the problem when $h_1$ and $h_2$ are approximately homotopic. We present a $K$-theoretical necessary and sufficient condition for them to be approximately homotopic under the assumption that $A$ is a unital separable simple C*-algebra of tracial rank zero, or $A$ is a unital purely infinite simple C*-algebra. When they are approximately homotopic, we also give a bound for the length of the homotopy. Suppose that $h: C(X)\to A$ is a monomorphism and $u\in A$ is a unitary (with $[u]=\{0\}$ in $K_1(A)$). We prove that, for any $\ep>0,$ and any compact subset ${\cal F}\subset C(X),$ there exists $\dt>0$ and a finite subset ${\cal G}\subset C(X)$ satisfying the following: if $\|[h(f), u]\|<\dt$ and $\text{Bott}(h,u)=\{0\},$ then there exists a continuous rectifiable path $\{u_t: t\in [0,1]\}$ such that $$ \|[h(g),u_t]\|<\ep, \rforal g\in {\cal F},u_0=u\andeqn u_1=1_A. $$ Moreover, $$ \text{Length}(\{u_t\})\le 2\pi+\ep. $$ We show that if $\text{dim}X\le 1,$ or $A$ is purely infinite simple, then $\dt$ and ${\cal G}$ are universal (independent of $A$ or $h$). In the case that ${\rm dim} X=1,$ this provides an improvement of the so-called the Basic Homotopy Lemma of Bratteli, Elliott, Evans and Kishimoto for the case that $A$ is mentioned above. Moreover, we show that $\dt$ and ${\cal G}$ can not be universal whenever $\text{dim} X\ge 2.$ Nevertheless, we also found that $\dt$ can be chosen to be dependent on a measure distribution but independent of $A$ and $h.$