Theory of Cauchy filters in uniform spaces. Complete uniform spaces. Totally bounded subsets.
A filter f
is Cauchy if for every entourage r
, there exists an
s ∈ f
such that s × s ⊆ r
. This is a generalization of Cauchy
sequences, because if a : ℕ → α
then the filter of sets containing
cofinitely many of the a n
is Cauchy iff a
is a Cauchy sequence.
A set s
is called complete, if any Cauchy filter f
such that s ∈ f
has a limit in s
(formally, it satisfies f ≤ 𝓝 x
for some x ∈ s
).
Equations
- is_complete s = ∀ (f : filter α), cauchy f → f ≤ filter.principal s → (∃ (x : α) (H : x ∈ s), f ≤ nhds x)
The common part of the proofs of le_nhds_of_cauchy_adhp
and
sequentially_complete.le_nhds_of_seq_tendsto_nhds
: if for any entourage s
one can choose a set t ∈ f
of diameter s
such that it contains a point y
with (x, y) ∈ s
, then f
converges to x
.
If x
is an adherent (cluster) point for a Cauchy filter f
, then it is a limit point
for f
.
Cauchy sequences. Usually defined on ℕ, but often it is also useful to say that a function defined on ℝ is Cauchy at +∞ to deduce convergence. Therefore, we define it in a type class that is general enough to cover both ℕ and ℝ, which are the main motivating examples.
Equations
- cauchy_seq u = cauchy (filter.map u filter.at_top)
If a Cauchy sequence has a convergent subsequence, then it converges.
A complete space is defined here using uniformities. A uniform space is complete if every Cauchy filter converges.
Instances
- complete_of_compact
- complete_of_proper
- complete_space_of_cau_seq_complete
- complete_space.prod
- Pi.complete
- real.complete_space
- continuous_linear_map.complete_space_ker
- bounded_continuous_function.complete_space
- continuous_linear_map.complete_space
- continuous_multilinear_map.complete_space
- Cauchy.complete_space
- uniform_space.complete_space_separation
- uniform_space.completion.complete_space
- CpltSepUniformSpace.is_complete_space
- CpltSepUniformSpace.complete_space
- emetric.closeds.complete_space
- emetric.nonempty_compacts.complete_space
- Gromov_Hausdorff.complete_space
Equations
- complete_space.prod = _
- _ = _
- _ = _
If univ
is complete, the space is a complete space
A Cauchy sequence in a complete space converges
If K
is a complete subset, then any cauchy sequence in K
converges to a point in K
A set s
is totally bounded if for every entourage d
there is a finite
set of points t
such that every element of s
is d
-near to some element of t
.
The closure of a totally bounded set is totally bounded.
The image of a totally bounded set under a unifromly continuous map is totally bounded.
Equations
Sequentially complete space
In this section we prove that a uniform space is complete provided that it is sequentially complete
(i.e., any Cauchy sequence converges) and its uniformity filter admits a countable generating set.
In particular, this applies to (e)metric spaces, see the files topology/metric_space/emetric_space
and topology/metric_space/basic
.
More precisely, we assume that there is a sequence of entourages U_n
such that any other
entourage includes one of U_n
. Then any Cauchy filter f
generates a decreasing sequence of
sets s_n ∈ f
such that s_n × s_n ⊆ U_n
. Choose a sequence x_n∈s_n
. It is easy to show
that this is a Cauchy sequence. If this sequence converges to some a
, then f ≤ 𝓝 a
.
An auxiliary sequence of sets approximating a Cauchy filter.
Equations
- sequentially_complete.set_seq_aux hf U_mem n = classical.indefinite_description (λ (s : set α), ∃ (_x : s ∈ f), s.prod s ⊆ U n) _
Given a Cauchy filter f
and a sequence U
of entourages, set_seq
provides
a sequence of monotonically decreasing sets s n ∈ f
such that (s n).prod (s n) ⊆ U
.
Equations
- sequentially_complete.set_seq hf U_mem n = ⋂ (m : ℕ) (H : m ∈ set.Iic n), (sequentially_complete.set_seq_aux hf U_mem m).val
A sequence of points such that seq n ∈ set_seq n
. Here set_seq
is a monotonically
decreasing sequence of sets set_seq n ∈ f
with diameters controlled by a given sequence
of entourages.
Equations
- sequentially_complete.seq hf U_mem n = classical.some _
If the sequence sequentially_complete.seq
converges to a
, then f ≤ 𝓝 a
.
A uniform space is complete provided that (a) its uniformity filter has a countable basis; (b) any sequence satisfying a "controlled" version of the Cauchy condition converges.
A sequentially complete uniform space with a countable basis of the uniformity filter is complete.