The purpose of this paper is to study pro excision in algebraic K-theory and cyclic homology, after Suslin-Wodzicki, Cuntz-Quillen, Cortiñas, and Geisser-Hesselholt, as well as continuity properties of André-Quillen and Hochschild homology. A key tool is first to establish the equivalence of various pro Tor vanishing conditions which appear in the literature.This allows us to prove that all ideals of commutative, Noetherian rings are pro unital in a suitable sense. We show moreover that such pro unital ideals satisfy pro excision in derived Hochschild and cyclic homology. It follows hence, and from the Suslin-Wodzicki criterion, that ideals of commutative, Noetherian rings satisfy pro excision in derived Hochschild and cyclic homology, and in algebraic K-theory.In addition, our techniques yield a strong form of the pro Hochschild-Kostant-Rosenberg theorem; an extension to general base rings of the Cuntz-Quillen excision theorem in periodic cyclic homology; a generalisation of the Feȋgin-Tsygan theorem; a short proof of pro excision in topological Hochschild and cyclic homology; and new Artin-Rees and continuity statements in André-Quillen and Hochschild homology.MSC: 19D55 (primary), 16E40 13D03 (secondary).
Pro unitality and pro excisiontheir topological counterparts, and André-Quillen homology. When k is a field, condition (iii) is equivalent to the existing notion of H-unitality of the pro k-algebra I ∞ (see Eg. 1.6), which is central in Wodzicki's original approach to excision [39], as well as in J. Cuntz and D. Quillen's approach to excision in periodic cyclic homology [10]. The importance of (iv) is thus not only its relevance to pro excision in K-theory, but also that it reveals that conditions (i)-(iii) are intrinsic properties of the non-unital ring I, depending neither on the ring A nor on the algebra structure from the base ring k.The first concrete application of Theorem 0.2 is to commutative rings: if I is an ideal of a commutative, Noetherian ring A, then M. André [2] noted that condition (i) is always true (see Lem. 2.1), and so we obtain: Theorem 0.3 (See Thm. 2.3). Let I be an ideal of a commutative, Noetherian ring. Then I is pro Tor-unital. Applying Geisser-Hesselholt's aforementioned pro version of the Suslin-Wodzicki criterion, we have the following consequence of Theorem 0.3 which completely solves the pro excision problem in K-theory for commutative, Noetherian rings: Corollary 0.4 (See Corol. 2.4). Ideals of commutative, Noetherian rings satisfy pro excision in algebraic K-theory. In particular, if A → B is a homomorphism of commutative, Noetherian rings, and I is an ideal of A mapped isomorphically to an ideal of B, then the map of pro abelian groups {K n (A, I r )} r −→ {K n (B, I r )} r is an isomorphism for all n ∈ Z.Now we turn to Hochschild and cyclic homology. Just as for K-theory in the first paragraph of the Introduction, these homology theories do not in general satisfy excision. To further justify the usefulness of pro Tor-unitality, we prove that it is sufficient to ensure that...