(Pre)images of intervals #
In this file we prove a bunch of trivial lemmas like “if we add a
to all points of [b, c]
,
then we get [a + b, a + c]
”. For the functions x ↦ x ± a
, x ↦ a ± x
, and x ↦ -x
we prove
lemmas about preimages and images of all intervals. We also prove a few lemmas about images under
x ↦ a * x
, x ↦ x * a
and x ↦ x⁻¹
.
The lemmas in this section state that addition maps intervals bijectively. The typeclass
has_exists_add_of_le
is defined specifically to make them work when combined with
ordered_cancel_add_comm_monoid
; the lemmas below therefore apply to all
ordered_add_comm_group
, but also to ℕ
and ℝ≥0
, which are not groups.
TODO : move as much as possible in this file to the setting of this weaker typeclass.
Preimages under x ↦ a + x
#
Preimages under x ↦ x + a
#
Preimages under x ↦ -x
#
Preimages under x ↦ x - a
#
Preimages under x ↦ a - x
#
Images under x ↦ a + x
#
Images under x ↦ x + a
#
Images under x ↦ -x
#
Images under x ↦ a - x
#
Images under x ↦ x - a
#
Bijections #
Multiplication and inverse in a field #
The (pre)image under inv
of Ioo 0 a
is Ioi a⁻¹
.