Intervals #
In any preorder α
, we define intervals (which on each side can be either infinite, open, or
closed) using the following naming conventions:
i
: infiniteo
: openc
: closed
Each interval has the name I
+ letter for left side + letter for right side. For instance,
Ioc a b
denotes the interval (a, b]
.
This file contains these definitions, and basic facts on inclusion, intersection, difference of
intervals (where the precise statements may depend on the properties of the order, in particular
for some statements it should be LinearOrder
or DenselyOrdered
).
TODO: This is just the beginning; a lot of rules are missing
An interval Ici a
is nonempty.
An interval Iic a
is nonempty.
In an order without maximal elements, the intervals Ioi
are nonempty.
In an order without minimal elements, the intervals Iio
are nonempty.
If a ≤ b
, then (b, +∞) ⊆ (a, +∞)
. In preorders, this is just an implication. If you need
the equivalence in linear orders, use Ioi_subset_Ioi_iff
.
If a ≤ b
, then (b, +∞) ⊆ [a, +∞)
. In preorders, this is just an implication. If you need
the equivalence in dense linear orders, use Ioi_subset_Ici_iff
.
If a ≤ b
, then (-∞, a) ⊆ (-∞, b)
. In preorders, this is just an implication. If you need
the equivalence in linear orders, use Iio_subset_Iio_iff
.
If a ≤ b
, then (-∞, a) ⊆ (-∞, b]
. In preorders, this is just an implication. If you need
the equivalence in dense linear orders, use Iio_subset_Iic_iff
.
Unions of adjacent intervals #
Two infinite intervals #
A finite and an infinite interval #
An infinite and a finite interval #
Two finite intervals, I?o
and Ic?
#
Two finite intervals, I?c
and Io?
#
Two finite intervals with a common point #
We cannot replace <
by ≤
in the hypotheses.
Otherwise for b < a = d < c
the l.h.s. is ∅
and the r.h.s. is {a}
.