### The obstruction to excision in K-theory and in cyclic homology, by Guillermo Cortiñas

Let f : A --> B be a ring homomorphism of not necessarily unital rings and I
an ideal of A 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 K_{*}(A:I) --> K_{*}(B:f(I)) to be an isomorphism; it is
measured by the birelative groups K_{*}(A,B:I). 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.

Guillermo Cortiñas <gcorti@dm.uba.ar>