Filtration (mathematics)

In mathematics, a filtration $\mathcal$ is an indexed family $(S_i)_$ of subobjects of a given algebraic structure $S$, with the index $i$ running over some totally ordered index set $I$, subject to the condition that

::if $i\leq j$ in $I$, then $S_i\subseteq S_j$.

If the index $i$ is the time parameter of some stochastic process, then the filtration can be interpreted as representing all historical but not future information available about the stochastic process, with the algebraic structure $S_i$ gaining in complexity with time. Hence, a process that is adapted to a filtration $\mathcal$ is also called non-anticipating, because it cannot "see into the future".

Sometimes, as in a filtered algebra, there is instead the requirement that the $S_i$ be subalgebras with respect to some operations (say, vector addition), but not with respect to other operations (say, multiplication) that satisfy only $S_i \cdot S_j \subseteq S_$, where the index set is the natural numbers; this is by analogy with a graded algebra.

Sometimes, filtrations are supposed to satisfy the additional requirement that the union of the $S_i$ be the whole $S$, or (in more general cases, when the notion of union does not make sense) that the canonical homomorphism from the direct limit of the $S_i$ to $S$ is an isomorphism. Whether this requirement is assumed or not usually depends on the author of the text and is often explicitly stated. This article does ''not'' impose this requirement.

There is also the notion of a descending filtration, which is required to satisfy $S_i \supseteq S_j$ in lieu of $S_i \subseteq S_j$ (and, occasionally, $\bigcap_ S_i=0$ instead of $\bigcup_ S_i=S$). Again, it depends on the context how exactly the word "filtration" is to be understood. Descending filtrations are not to be confused with the dual notion of cofiltrations (which consist of quotient objects rather than subobjects).

Filtrations are widely used in abstract algebra, homological algebra (where they are related in an important way to spectral sequences), and in measure theory and probability theory for nested sequences of σ-algebras. In functional analysis and numerical analysis, other terminology is usually used, such as scale of spaces or nested spaces.

Examples

Algebra

Algebras

See: Filtered algebra

Groups

In algebra, filtrations are ordinarily indexed by $\mathbb$, the set of natural numbers. A ''filtration'' of a group $G$, is then a nested sequence $G_n$ of normal subgroups of $G$ (that is, for any $n$ we have $G_\subseteq G_n$). Note that this use of the word "filtration" corresponds to our "descending filtration".

Given a group $G$ and a filtration $G_n$, there is a natural way to define a topology on $G$, said to be ''associated'' to the filtration. A basis for this topology is the set of all cosets of subgroups appearing in the filtration, that is, a subset of $G$ is defined to be open if it is a union of sets of the form $aG_n$, where $a\in G$ and $n$ is a natural number.

The topology associated to a filtration on a group $G$ makes $G$ into a topological group.

The topology associated to a filtration $G_n$ on a group $G$ is Hausdorff if and only if $\bigcap G_n=\$.

If two filtrations $G_n$ and $G'_n$ are defined on a group $G$, then the identity map from $G$ to $G$, where the first copy of $G$ is given the $G_n$-topology and the second the $G'_n$-topology, is continuous if and only if for any $n$ there is an $m$ such that $G_m\subseteq G'_n$, that is, if and only if the identity map is continuous at 1. In particular, the two filtrations define the same topology if and only if for any subgroup appearing in one there is a smaller or equal one appearing in the other.

Rings and modules: descending filtrations

Given a ring $R$ and an $R$-module $M$, a ''descending filtration'' of $M$ is a decreasing sequence of submodules $M_n$. This is therefore a special case of the notion for groups, with the additional condition that the subgroups be submodules. The associated topology is defined as for groups.

An important special case is known as the $I$-adic topology (or $J$-adic, etc.): Let $R$ be a commutative ring, and $I$ an ideal of $R$. Given an $R$-module $M$, the sequence $I^n M$ of submodules of $M$ forms a filtration of $M$. The ''$I$-adic topology'' on $M$ is then the topology associated to this filtration. If $M$ is just the ring $R$ itself, we have defined the ''$I$-adic topology'' on $R$.

When $R$ is given the $I$-adic topology, $R$ becomes a topological ring. If an $R$-module $M$ is then given the $I$-adic topology, it becomes a topological $R$-module, relative to the topology given on $R$.

Rings and modules: ascending filtrations

Given a ring $R$ and an $R$-module $M$, an ''ascending filtration'' of $M$ is an increasing sequence of submodules $M_n$. In particular, if $R$ is a field, then an ascending filtration of the $R$-vector space $M$ is an increasing sequence of vector subspaces of $M$. Flags are one important class of such filtrations.

Sets

A maximal filtration of a set is equivalent to an ordering (a permutation) of the set. For instance, the filtration $\ \subseteq \ \subseteq \$ corresponds to the ordering $(0,1,2)$. From the point of view of the field with one element, an ordering on a set corresponds to a maximal flag (a filtration on a vector space), considering a set to be a vector space over the field with one element.

Measure theory

In measure theory, in particular in martingale theory and the theory of stochastic processes, a filtration is an increasing sequence of $\sigma$-algebras on a measurable space. That is, given a measurable space $(\Omega, \mathcal)$, a filtration is a sequence of $\sigma$-algebras $\_$ with $\mathcal_ \subseteq \mathcal$ where each $t$ is a non-negative real number and

:$t_ \leq t_ \implies \mathcal_ \subseteq \mathcal_.$

The exact range of the "times" ''$t$'' will usually depend on context: the set of values for $t$ might be discrete or continuous, bounded or unbounded. For example,

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