Let
Abe a simple unital AT algebra of real rank zero such that it has a unique tracial state
τand
K1(
A) is neither 0 nor
Z. For each
![](/images/glyphs/CD4.GIF)
![](/images/glyphs/BOA.GIF)
Hom(
K1(
A),
R) with dense range in
Rwe construct a closed derivation
δin
Awhich generates a one-parameter automorphism group
αof
Asuch that
τ(
δ(
u)
u*)=2
πi![](/images/glyphs/CD4.GIF)
([
u]) for any unitary
u
D(
δ). Furthermore we construct such an
αwith the Rohlin property, which is defined in Kishimoto (
Comm. Math. Phys.179(1996), 599–622), in this case the crossed product
A×
αRis a simple AT algebra of real rank zero. As an application we obtain that for such a
C*-algebra
Athe kernel of the natural homomorphism of the group(
A) of approximately inner automorphisms into
Ext(K1(A),K0(A))⊕Ext(K0(A),K1(A)),is the group HInn(
A) of automorphisms homotopic to inner automorphisms. Combining with the result of Kishimoto and Kumjian (
Trans. Amer. Math. Soc., to appear),(
A)/HInn(
A) is isomorphic to the above direct sum. As another application of the construction of
derivations, we show that if
Ais a
C*-algebra of the above type and
α![](/images/glyphs/BOA.GIF)
HInn(
A) has the Rohlin property and comes from
![](/images/glyphs/CD4.GIF)
![](/images/glyphs/BOA.GIF)
Hom(
K1(
A),
R) with dense range as in Kishimoto and Kumjian (preprint), then the crossed product
A×
αZis again of the same type; in particular
A×
αZis an AT algebra. (The other properties are known from Kishimoto [
J. Operator Theory40(1998)].)