(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 image under inv of Ioo 0 a is Ioi a⁻¹.