import Mathlib.Data.Real.Basic import Mathlib.Data.List.Sort import Common.List.Basic /-! # Common.Real.Set.Partition A description of a partition as defined in the context of stepwise functions. Refer to [^1] for more information. [^1]: Apostol, Tom M. Calculus, Vol. 1: One-Variable Calculus, with an Introduction to Linear Algebra. 2nd ed. Vol. 1. 2 vols. Wiley, 1991. -/ namespace Real open List /-- A `Partition` is some finite subset of `[a, b]` containing points `a` and `b`. It is assumed that the points of the `Partition` are distinct and sorted. The use of a `List` ensures finite-ness. -/ structure Partition where xs : List ℝ sorted : Sorted LT.lt xs has_min_length : xs.length ≥ 2 /-- The length of any list associated with a `Partition` is `> 0`. -/ private lemma length_gt_zero (p : Partition) : p.xs.length > 0 := calc p.xs.length _ ≥ 2 := p.has_min_length _ > 0 := by simp /-- The length of any list associated with a `Partition` is `≠ 0`. -/ instance (p : Partition) : NeZero (length p.xs) where out := LT.lt.ne' (length_gt_zero p) namespace Partition /-- The left-most subdivision point of the `Partition`. -/ def left (p : Partition) : ℝ := p.xs.head (neq_nil_iff_length_gt_zero.mpr (length_gt_zero p)) /-- The right-most subdivision point of the `Partition`. -/ def right (p : Partition) : ℝ := p.xs.getLast (neq_nil_iff_length_gt_zero.mpr (length_gt_zero p)) /-- Define `∈` syntax for a `Partition`. We say a real is a member of a partition provided it lies somewhere in closed interval `[a, b]`. -/ instance : Membership ℝ Partition where mem (x : ℝ) (p : Partition) := p.left ≤ x ∧ x ≤ p.right /-- Every subdivision point is `≥` the left-most point of the partition. -/ theorem subdivision_point_geq_left {p : Partition} (h : x ∈ p.xs) : p.left ≤ x := by unfold left rw [head_eq_get_zero (exists_mem_iff_neq_nil.mp ⟨x, h⟩)] have ⟨i, hi⟩ := mem_iff_exists_get.mp h conv => rhs; rw [← hi] by_cases hz : i = (0 : Fin (length p.xs)) · rw [hz] simp · refine le_of_lt (Sorted.rel_get_of_lt p.sorted ?_) rwa [← ne_eq, ← Fin.pos_iff_ne_zero i] at hz /-- Every subdivision point is `≤` the right-most point of the partition. -/ theorem subdivision_point_leq_right {p : Partition} (h : x ∈ p.xs) : x ≤ p.right := by unfold right have hx := exists_mem_iff_neq_nil.mp ⟨x, h⟩ rw [getLast_eq_get_length_sub_one hx] have ⟨i, hi⟩ := mem_iff_exists_get.mp h conv => lhs; rw [← hi] have ⟨_, ⟨_, hs⟩⟩ := self_neq_nil_imp_exists_mem.mp hx by_cases hz : i = ⟨p.xs.length - 1, by rw [hs]; simp⟩ · rw [hz] · refine le_of_lt (Sorted.rel_get_of_lt p.sorted ?_) rw [← ne_eq, Fin.ne_iff_vne] at hz rw [Fin.lt_iff_val_lt_val] exact lt_of_le_of_ne (le_tsub_of_add_le_right i.2) hz /-- Every subdivision point of a `Partition` is itself a member of the `Partition`. -/ theorem subdivision_point_mem_partition {p : Partition} (h : x ∈ p.xs) : x ∈ p := ⟨subdivision_point_geq_left h, subdivision_point_leq_right h⟩ end Partition end Real