125 lines
4.0 KiB
Plaintext
125 lines
4.0 KiB
Plaintext
import Mathlib.Data.Finset.Basic
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import Mathlib.Data.List.Sort
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import Mathlib.Data.Set.Intervals.Basic
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import Common.List.Basic
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/-! # Common.Set.Intervals.Partition
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Additional theorems and definitions useful in the context of sets.
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-/
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namespace Set.Intervals
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open List
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/--
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A `Partition` is a finite subset of `[a, b]` containing points `a` and `b`.
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-/
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structure Partition (α : Type _) [Preorder α] [@DecidableRel α LT.lt] where
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/- The left-most endpoint of the partition. -/
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a : α
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/- The right-most endpoint of the partition. -/
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b : α
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/- The subdivision points. -/
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xs : List α
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/- Ensure the subdivision points are in sorted order. -/
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sorted_xs : Sorted LT.lt xs
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/- Ensure each subdivision point is in our defined interval. -/
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within_xs : ∀ x ∈ xs, x ∈ Ioo a b
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namespace Partition
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/--
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An object `x` is a member of a `Partition` `p` if `x` is an endpoint of `p` or a
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subdivision point of `p`.
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Notice that being a member of `p` is different from being a member of some
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(sub)interval determined by `p`.
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-/
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instance [Preorder α] [@DecidableRel α LT.lt] : Membership α (Partition α) where
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mem (x : α) (p : Partition α) := x = p.a ∨ x ∈ p.xs ∨ x = p.b
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/--
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Return the endpoints and subdivision points of a `Partition` as a sorted `List`.
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-/
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def toList [Preorder α] [@DecidableRel α LT.lt] (p : Partition α) : List α :=
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(p.a :: p.xs) ++ [p.b]
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/--
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`x` is a member of `Partition` `p` **iff** `x` is a member of `p.List`.
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-/
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theorem mem_self_iff_mem_toList [Preorder α] [@DecidableRel α LT.lt]
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(p : Partition α) : x ∈ p ↔ x ∈ p.toList := by
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apply Iff.intro
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· sorry
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· sorry
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/--
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Every member of a `Partition` is greater than or equal to its left-most point.
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-/
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theorem left_le_mem_self [Preorder α] [@DecidableRel α LT.lt]
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(p : Partition α) : ∀ x ∈ p, p.a ≤ x := by
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sorry
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/--
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Every member of a `Partition` is less than or equal to its right-most point.
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-/
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theorem right_ge_mem_self [Preorder α] [@DecidableRel α LT.lt]
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(p : Partition α) : ∀ x ∈ p, x ≤ p.b := by
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sorry
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/-
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Return the closed subintervals determined by the `Partition`.
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-/
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def closedSubintervals [Preorder α] [@DecidableRel α LT.lt]
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(p : Partition α) : List (Set α) :=
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p.toList.pairwise (fun x₁ x₂ => Icc x₁ x₂)
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/-
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Return the open subintervals determined by the `Partition`.
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-/
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def openSubintervals [Preorder α] [@DecidableRel α LT.lt]
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(p : Partition α) : List (Set α) :=
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p.toList.pairwise (fun x₁ x₂ => Ioo x₁ x₂)
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/--
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A member of an open subinterval of a `Partition` `p` is a member of the entire
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open interval determined by `p`.
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-/
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theorem mem_open_subinterval_mem_open_interval
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[Preorder α] [@DecidableRel α LT.lt] {p : Partition α}
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(hI : I ∈ p.openSubintervals) (hy : y ∈ I) : y ∈ Ioo p.a p.b := by
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have ⟨i, ⟨x₁, ⟨x₂, ⟨hx₁, ⟨hx₂, hI'⟩⟩⟩⟩⟩ :=
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List.mem_pairwise_imp_exists_adjacent hI
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have hx₁' : p.a ≤ x₁ := by
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refine p.left_le_mem_self x₁ ?_
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rw [p.mem_self_iff_mem_toList]
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have : ↑i < p.toList.length := calc ↑i
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_ < p.toList.length - 1 := i.2
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_ < p.toList.length := by
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unfold List.length Partition.toList
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simp
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exact List.mem_iff_exists_get.mpr ⟨⟨↑i, this⟩, Eq.symm hx₁⟩
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have hx₂' : x₂ ≤ p.b := by
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refine p.right_ge_mem_self x₂ ?_
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rw [p.mem_self_iff_mem_toList]
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have : ↑i + 1 < p.toList.length := add_lt_add_right i.2 1
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exact List.mem_iff_exists_get.mpr ⟨⟨↑i + 1, this⟩, Eq.symm hx₂⟩
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have hx_sub := Set.Ioo_subset_Ioo hx₁' hx₂'
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rw [hI'] at hy
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exact Set.mem_of_subset_of_mem hx_sub hy
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/--
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A member of an open subinterval of a `Partition` `p` is a member of the entire
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closed interval determined by `p`.
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-/
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theorem mem_open_subinterval_mem_closed_interval
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[Preorder α] [@DecidableRel α LT.lt] {p : Partition α}
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(hI : I ∈ p.openSubintervals) (hy : y ∈ I) : y ∈ Icc p.a p.b := by
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have := mem_open_subinterval_mem_open_interval hI hy
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exact Set.mem_of_subset_of_mem Set.Ioo_subset_Icc_self this
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end Partition
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end Set.Intervals |