Lim Cohen–Macaulay sequences of modules

Abstract We introduce the notion of a lim Cohen–Macaulay sequence of modules. We prove the existence of such sequences in positive characteristic, and show that their existence in mixed characteristic implies the long open conjecture about positivity of Serre intersection multiplicities for all regular local rings, as well as a new proof of the existence of big Cohen–Macaulay modules. We describe how such a sequence leads to a notion of closure for submodules of finitely generated modules: this family of closure operations includes the usual notion of tight closure in characteristic <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:mrow> <m:mi>p</m:mi> <m:mo>&gt;</m:mo> <m:mn>0</m:mn> </m:mrow> </m:math> p&gt;0 , and all of them have the property of capturing colon ideals. In fact, they satisfy axioms formulated by Dietz from which it follows that if a local ring 𝑅 has a lim Cohen–Macaulay sequence then it has a big Cohen–Macaulay module. We also prove the existence of lim Cohen–Macaulay sequences for certain rings of mixed characteristic.