  If A is a homotopy cartesian square of ring spectra satisfying connectivity
hypotheses, then the cube induced by Goodwillie's integral cyclotomic trace
from K(A) to TC(A) is homotopy cartesian. In other words, the homotopy fiber of
the cyclotomic trace satisfies excision. The method of proof gives as a
spin-off new proofs of some old results, as well as some new results, about
periodic cyclic homology, and - more relevantly for our current application -
the T-Tate spectrum of topological Hochschild homology, where T is the circle
group
