mathlib documentation

order.order_iso_nat

Relation embeddings from the naturals #

This file allows translation from monotone functions ℕ → α to order embeddings ℕ ↪ α and defines the limit value of an eventually-constant sequence.

Main declarations #

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def rel_embedding.nat_lt {α : Type u_1} {r : α → α → Prop} [is_strict_order α r] (f : ℕ → α) (H : ∀ (n : ℕ), r (f n) (f (n + 1))) :
has_lt.lt ↪r r

If f is a strictly r-increasing sequence, then this returns f as an order embedding.

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@[simp]
theorem rel_embedding.nat_lt_apply {α : Type u_1} {r : α → α → Prop} [is_strict_order α r] {f : ℕ → α} {H : ∀ (n : ℕ), r (f n) (f (n + 1))} {n : ℕ} :
⇑(rel_embedding.nat_lt f H) n = f n
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def rel_embedding.nat_gt {α : Type u_1} {r : α → α → Prop} [is_strict_order α r] (f : ℕ → α) (H : ∀ (n : ℕ), r (f (n + 1)) (f n)) :
gt ↪r r

If f is a strictly r-decreasing sequence, then this returns f as an order embedding.

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theorem rel_embedding.well_founded_iff_no_descending_seq {α : Type u_1} {r : α → α → Prop} [is_strict_order α r] :
well_founded r ↔ is_empty (gt ↪r r)
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def nat.order_embedding_of_set (s : set ℕ) [infinite ↥s] [decidable_pred (λ (_x : ℕ), _x ∈ s)] :
ℕ ↪o ℕ

An order embedding from ℕ to itself with a specified range

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noncomputable def nat.subtype.order_iso_of_nat (s : set ℕ) [infinite ↥s] :
ℕ ≃o ↥s

nat.subtype.of_nat as an order isomorphism between ℕ and an infinite subset. See also nat.nth for a version where the subset may be finite.

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@[simp]
theorem nat.coe_order_embedding_of_set {s : set ℕ} [infinite ↥s] :
⇑(nat.order_embedding_of_set s) = coe ∘ nat.subtype.of_nat s
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theorem nat.order_embedding_of_set_apply {s : set ℕ} [infinite ↥s] {n : ℕ} :
⇑(nat.order_embedding_of_set s) n = ↑(nat.subtype.of_nat s n)
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@[simp]
theorem nat.subtype.order_iso_of_nat_apply {s : set ℕ} [infinite ↥s] {n : ℕ} :
⇑(nat.subtype.order_iso_of_nat s) n = nat.subtype.of_nat s n
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theorem nat.order_embedding_of_set_range (s : set ℕ) [infinite ↥s] :
set.range ⇑(nat.order_embedding_of_set s) = s
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theorem nat.exists_subseq_of_forall_mem_union {α : Type u_1} {s t : set α} (e : ℕ → α) (he : ∀ (n : ℕ), e n ∈ s ∪ t) :
∃ (g : ℕ ↪o ℕ), (∀ (n : ℕ), e (⇑g n) ∈ s) ∨ ∀ (n : ℕ), e (⇑g n) ∈ t
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theorem exists_increasing_or_nonincreasing_subseq' {α : Type u_1} (r : α → α → Prop) (f : ℕ → α) :
∃ (g : ℕ ↪o ℕ), (∀ (n : ℕ), r (f (⇑g n)) (f (⇑g (n + 1)))) ∨ ∀ (m n : ℕ), m < n → ¬r (f (⇑g m)) (f (⇑g n))
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theorem exists_increasing_or_nonincreasing_subseq {α : Type u_1} (r : α → α → Prop) [is_trans α r] (f : ℕ → α) :
∃ (g : ℕ ↪o ℕ), (∀ (m n : ℕ), m < n → r (f (⇑g m)) (f (⇑g n))) ∨ ∀ (m n : ℕ), m < n → ¬r (f (⇑g m)) (f (⇑g n))

This is the infinitary Erdős–Szekeres theorem, and an important lemma in the usual proof of Bolzano-Weierstrass for ℝ.

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theorem well_founded.monotone_chain_condition' {α : Type u_1} [preorder α] :
well_founded gt ↔ ∀ (a : ℕ →o α), ∃ (n : ℕ), ∀ (m : ℕ), n ≤ m → ¬⇑a n < ⇑a m
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theorem well_founded.monotone_chain_condition {α : Type u_1} [partial_order α] :
well_founded gt ↔ ∀ (a : ℕ →o α), ∃ (n : ℕ), ∀ (m : ℕ), n ≤ m → ⇑a n = ⇑a m

The "monotone chain condition" below is sometimes a convenient form of well foundedness.

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noncomputable def monotonic_sequence_limit_index {α : Type u_1} [preorder α] (a : ℕ →o α) :
ℕ

Given an eventually-constant monotone sequence a₀ ≤ a₁ ≤ a₂ ≤ ... in a partially-ordered type, monotonic_sequence_limit_index a is the least natural number n for which aₙ reaches the constant value. For sequences that are not eventually constant, monotonic_sequence_limit_index a is defined, but is a junk value.

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noncomputable def monotonic_sequence_limit {α : Type u_1} [preorder α] (a : ℕ →o α) :
α

The constant value of an eventually-constant monotone sequence a₀ ≤ a₁ ≤ a₂ ≤ ... in a partially-ordered type.

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theorem well_founded.supr_eq_monotonic_sequence_limit {α : Type u_1} [complete_lattice α] (h : well_founded gt) (a : ℕ →o α) :
supr ⇑a = monotonic_sequence_limit a