# mathlib3documentation

order.well_founded

# Well-founded relations #

THIS FILE IS SYNCHRONIZED WITH MATHLIB4. Any changes to this file require a corresponding PR to mathlib4.

A relation is well-founded if it can be used for induction: for each x, (∀ y, r y x → P y) → P x implies P x. Well-founded relations can be used for induction and recursion, including construction of fixed points in the space of dependent functions Π x : α , β x.

The predicate well_founded is defined in the core library. In this file we prove some extra lemmas and provide a few new definitions: well_founded.min, well_founded.sup, and well_founded.succ, and an induction principle well_founded.induction_bot.

@[protected]
theorem well_founded.is_asymm {α : Type u_1} {r : α α Prop} (h : well_founded r) :
r
@[protected]
theorem well_founded.is_irrefl {α : Type u_1} {r : α α Prop} (h : well_founded r) :
r
@[protected, instance]
@[protected, instance]
theorem well_founded.mono {α : Type u_1} {r r' : α α Prop} (hr : well_founded r) (h : (a b : α), r' a b r a b) :
theorem well_founded.on_fun {α : Sort u_1} {β : Sort u_2} {r : β β Prop} {f : α β} :
theorem well_founded.has_min {α : Type u_1} {r : α α Prop} (H : well_founded r) (s : set α) :
s.nonempty ( (a : α) (H : a s), (x : α), x s ¬r x a)

If r is a well-founded relation, then any nonempty set has a minimal element with respect to r.

noncomputable def well_founded.min {α : Type u_1} {r : α α Prop} (H : well_founded r) (s : set α) (h : s.nonempty) :
α

A minimal element of a nonempty set in a well-founded order.

If you're working with a nonempty linear order, consider defining a conditionally_complete_linear_order_bot instance via well_founded.conditionally_complete_linear_order_with_bot and using Inf instead.

Equations
theorem well_founded.min_mem {α : Type u_1} {r : α α Prop} (H : well_founded r) (s : set α) (h : s.nonempty) :
H.min s h s
theorem well_founded.not_lt_min {α : Type u_1} {r : α α Prop} (H : well_founded r) (s : set α) (h : s.nonempty) {x : α} (hx : x s) :
¬r x (H.min s h)
theorem well_founded.well_founded_iff_has_min {α : Type u_1} {r : α α Prop} :
(s : set α), s.nonempty ( (m : α) (H : m s), (x : α), x s ¬r x m)
@[protected]
noncomputable def well_founded.sup {α : Type u_1} {r : α α Prop} (wf : well_founded r) (s : set α) (h : s) :
α

The supremum of a bounded, well-founded order

Equations
@[protected]
theorem well_founded.lt_sup {α : Type u_1} {r : α α Prop} (wf : well_founded r) {s : set α} (h : s) {x : α} (hx : x s) :
r x (wf.sup s h)
@[protected]
noncomputable def well_founded.succ {α : Type u_1} {r : α α Prop} (wf : well_founded r) (x : α) :
α

A successor of an element x in a well-founded order is a minimal element y such that x < y if one exists. Otherwise it is x itself.

Equations
• wf.succ x = dite ( (y : α), r x y) (λ (h : (y : α), r x y), wf.min {y : α | r x y} h) (λ (h : ¬ (y : α), r x y), x)
@[protected]
theorem well_founded.lt_succ {α : Type u_1} {r : α α Prop} (wf : well_founded r) {x : α} (h : (y : α), r x y) :
r x (wf.succ x)
@[protected]
theorem well_founded.lt_succ_iff {α : Type u_1} {r : α α Prop} [wo : r] {x : α} (h : (y : α), r x y) (y : α) :
r y r y x y = x
theorem well_founded.min_le {β : Type u_2} [linear_order β] {x : β} {s : set β} (hx : x s) (hne : s.nonempty := _) :
h.min s hne x
theorem well_founded.eq_strict_mono_iff_eq_range {β : Type u_2} {γ : Type u_3} [linear_order β] {f g : β γ} (hf : strict_mono f) (hg : strict_mono g) :
f = g
theorem well_founded.self_le_of_strict_mono {β : Type u_2} [linear_order β] {f : β β} (hf : strict_mono f) (n : β) :
n f n
noncomputable def function.argmin {α : Type u_1} {β : Type u_2} (f : α β) [has_lt β] [nonempty α] :
α

Given a function f : α → β where β carries a well-founded <, this is an element of α whose image under f is minimal in the sense of function.not_lt_argmin.

Equations
theorem function.not_lt_argmin {α : Type u_1} {β : Type u_2} (f : α β) [has_lt β] [nonempty α] (a : α) :
¬f a < f h)
noncomputable def function.argmin_on {α : Type u_1} {β : Type u_2} (f : α β) [has_lt β] (s : set α) (hs : s.nonempty) :
α

Given a function f : α → β where β carries a well-founded <, and a non-empty subset s of α, this is an element of s whose image under f is minimal in the sense of function.not_lt_argmin_on.

Equations
@[simp]
theorem function.argmin_on_mem {α : Type u_1} {β : Type u_2} (f : α β) [has_lt β] (s : set α) (hs : s.nonempty) :
s hs s
@[simp]
theorem function.not_lt_argmin_on {α : Type u_1} {β : Type u_2} (f : α β) [has_lt β] (s : set α) {a : α} (ha : a s) (hs : s.nonempty := _) :
¬f a < f s hs)
@[simp]
theorem function.argmin_le {α : Type u_1} {β : Type u_2} (f : α β) [linear_order β] (a : α) [nonempty α] :
f h) f a
@[simp]
theorem function.argmin_on_le {α : Type u_1} {β : Type u_2} (f : α β) [linear_order β] (s : set α) {a : α} (ha : a s) (hs : s.nonempty := _) :
f s hs) f a
theorem acc.induction_bot' {α : Sort u_1} {β : Sort u_2} {r : α α Prop} {a bot : α} (ha : acc r a) {C : β Prop} {f : α β} (ih : (b : α), f b f bot C (f b) ( (c : α), r c b C (f c))) :
C (f a) C (f bot)

Let r be a relation on α, let f : α → β be a function, let C : β → Prop, and let bot : α. This induction principle shows that C (f bot) holds, given that

• some a that is accessible by r satisfies C (f a), and
• for each b such that f b ≠ f bot and C (f b) holds, there is c satisfying r c b and C (f c).
theorem acc.induction_bot {α : Sort u_1} {r : α α Prop} {a bot : α} (ha : acc r a) {C : α Prop} (ih : (b : α), b bot C b ( (c : α), r c b C c)) :
C a C bot

Let r be a relation on α, let C : α → Prop and let bot : α. This induction principle shows that C bot holds, given that

• some a that is accessible by r satisfies C a, and
• for each b ≠ bot such that C b holds, there is c satisfying r c b and C c.
theorem well_founded.induction_bot' {α : Sort u_1} {β : Sort u_2} {r : α α Prop} (hwf : well_founded r) {a bot : α} {C : β Prop} {f : α β} (ih : (b : α), f b f bot C (f b) ( (c : α), r c b C (f c))) :
C (f a) C (f bot)

Let r be a well-founded relation on α, let f : α → β be a function, let C : β → Prop, and let bot : α. This induction principle shows that C (f bot) holds, given that

• some a satisfies C (f a), and
• for each b such that f b ≠ f bot and C (f b) holds, there is c satisfying r c b and C (f c).
theorem well_founded.induction_bot {α : Sort u_1} {r : α α Prop} (hwf : well_founded r) {a bot : α} {C : α Prop} (ih : (b : α), b bot C b ( (c : α), r c b C c)) :
C a C bot

Let r be a well-founded relation on α, let C : α → Prop, and let bot : α. This induction principle shows that C bot holds, given that

• some a satisfies C a, and
• for each b that satisfies C b, there is c satisfying r c b and C c.

The naming is inspired by the fact that when r is transitive, it follows that bot is the smallest element w.r.t. r that satisfies C.