85 research outputs found
The obstruction to excision in K-theory and in cyclic homology
Let be a ring homomorphism of not necessarily unital rings and
an ideal which is mapped by f isomorphically to an ideal of
B. The obstruction to excision in K-theory is the failure of the map between
relative K-groups to be an isomorphism; it is
measured by the birelative groups . We show that these are
rationally isomorphic to the corresponding birelative groups for cyclic
homology up to a dimension shift. In the particular case when A and B are
\Q-algebras we obtain an integral isomorphism.Comment: Final version to appear in Inventione
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