In mathematics, almost modules and almost rings are certain objects interpolating between rings and their fields of fractions. They were introduced by Faltings (1988) in his study of p-adic Hodge theory.
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Almost modules
Let V be a local integral domain with the maximal ideal m, and K a fraction field of V. The category of K-modules may be obtained as a quotient of V-mod by the Serre subcategory of torsion modules, i.e. those N such that any element n ∈ N is annihilated by some nonzero element in the maximal ideal. If the category of torsion modules is replaced by a smaller subcategory, we obtain an intermediate step between V-modules and K-modules. Faltings proposed to use the subcategory of almost zero modules, i.e. N ∈ V-mod such that any element n ∈ N is annihilated by all elements of the maximal ideal.
For this idea to work, m and V must satisfy certain technical conditions. Let V be a ring (not necessarily local) and m ⊆ V an idempotent ideal, i.e. m2 = m. Assume also that m ⊗ m is a flat V-module. A module N over V is almost zero with respect to such m if for all ε ∈ m and n ∈ N we have εn = 0. Almost zero modules form a Serre subcategory of the category of V-modules. The category of almost V-modules Va-mod is a localization of V-mod along this subcategory.
The quotient functor
Almost rings
The tensor product of V-modules descends to a monoidal structure on Va-mod. An almost module R ∈ Va-mod with a map R ⊗ R → R satisfying natural conditions, similar to a definition of a ring, is called an almost V-algebra or an almost ring if the context is unambiguous. Many standard properties of algebras and morphisms between them carry to the "almost" world.
Example
In the original paper by Faltings, V was the integral closure of a discrete valuation ring in the algebraic closure of its quotient field, and m its maximal ideal. For example, let V be