Filter.atTop
and Filter.atBot
filters on preorders, monoids and groups. #
In this file we define the filters
Filter.atTop
: corresponds ton → +∞
;Filter.atBot
: corresponds ton → -∞
.
Then we prove many lemmas like “if f → +∞
, then f ± c → +∞
”.
atTop
is the filter representing the limit → ∞
on an ordered set.
It is generated by the collection of up-sets {b | a ≤ b}
.
(The preorder need not have a top element for this to be well defined,
and indeed is trivial when a top element exists.)
Equations
- Filter.atTop = ⨅ (a : α), Filter.principal (Set.Ici a)
Instances For
atBot
is the filter representing the limit → -∞
on an ordered set.
It is generated by the collection of down-sets {b | b ≤ a}
.
(The preorder need not have a bottom element for this to be well defined,
and indeed is trivial when a bottom element exists.)
Equations
- Filter.atBot = ⨅ (a : α), Filter.principal (Set.Iic a)
Instances For
Equations
- ⋯ = inst
Equations
- ⋯ = inst
Alias of the forward direction of Filter.eventually_atTop
.
Alias of the forward direction of Filter.eventually_atBot
.
Sequences #
If u
is a sequence which is unbounded above,
then after any point, it reaches a value strictly greater than all previous values.
If u
is a sequence which is unbounded below,
then after any point, it reaches a value strictly smaller than all previous values.
If u
is a sequence which is unbounded above,
then it Frequently
reaches a value strictly greater than all previous values.
If u
is a sequence which is unbounded below,
then it Frequently
reaches a value strictly smaller than all previous values.
A function f
grows to +∞
independent of an order-preserving embedding e
.
Alias of Filter.tendsto_atTop_atTop_of_monotone
.
Alias of Filter.tendsto_atBot_atBot_of_monotone
.
A function f
goes to -∞
independent of an order-preserving embedding e
.
If f
is a monotone sequence of Finset
s and each x
belongs to one of f n
, then
Tendsto f atTop atTop
.
Alias of Filter.tendsto_atTop_finset_of_monotone
.
If f
is a monotone sequence of Finset
s and each x
belongs to one of f n
, then
Tendsto f atTop atTop
.
A function f
maps upwards closed sets (atTop sets) to upwards closed sets when it is a
Galois insertion. The Galois "insertion" and "connection" is weakened to only require it to be an
insertion and a connection above b
.
A function f
maps upwards closed sets (atTop sets) to upwards closed sets when it is a
Galois insertion. The Galois "insertion" and "connection" is weakened to only require it to be an
insertion and a connection above b
.
The image of the filter atTop
on Ici a
under the coercion equals atTop
.
The image of the filter atTop
on Ioi a
under the coercion equals atTop
.
The atTop
filter for an open interval Ioi a
comes from the atTop
filter in the ambient
order.
The atTop
filter for an open interval Ici a
comes from the atTop
filter in the ambient
order.
The atBot
filter for an open interval Iio a
comes from the atBot
filter in the ambient
order.
The atBot
filter for an open interval Iio a
comes from the atBot
filter in the ambient
order.
The atBot
filter for an open interval Iic a
comes from the atBot
filter in the ambient
order.
The atBot
filter for an open interval Iic a
comes from the atBot
filter in the ambient
order.
If u
is a monotone function with linear ordered codomain and the range of u
is not bounded
above, then Tendsto u atTop atTop
.
If u
is a monotone function with linear ordered codomain and the range of u
is not bounded
below, then Tendsto u atBot atBot
.
If a monotone function u : ι → α
tends to atTop
along some non-trivial filter l
, then
it tends to atTop
along atTop
.
If a monotone function u : ι → α
tends to atBot
along some non-trivial filter l
, then
it tends to atBot
along atBot
.
Given an antitone basis s : ℕ → Set α
of a filter, extract an antitone subbasis s ∘ φ
,
φ : ℕ → ℕ
, such that m < n
implies r (φ m) (φ n)
. This lemma can be used to extract an
antitone basis with basis sets decreasing "sufficiently fast".
If f
is a nontrivial countably generated filter, then there exists a sequence that converges
to f
.
An abstract version of continuity of sequentially continuous functions on metric spaces:
if a filter k
is countably generated then Tendsto f k l
iff for every sequence u
converging to k
, f ∘ u
tends to l
.
A sequence converges if every subsequence has a convergent subsequence.