feat(combinatorics/set_family/shadow): Shadow of a set family (#10223)

This defines `shadow 𝒜` where `𝒜 : finset (finset α))`.

src/combinatorics/set_family/shadow.lean

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+/-
+Copyright (c) 2021 Bhavik Mehta. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Bhavik Mehta, Alena Gusakov, Yaël Dillies
+-/
+import data.finset.lattice
+
+/-!
+# Shadows
+
+This file defines shadows of a set family. The shadow of a set family is the set family of sets we
+get by removing any element from any set of the original family. If one pictures `finset α` as a big
+hypercube (each dimension being membership of a given element), then taking the shadow corresponds
+to projecting each finset down once in all available directions.
+
+## Main definitions
+
+The `shadow` of a set family is everything we can get by removing an element from each set.
+
+## Notation
+
+`∂ 𝒜` is notation for `shadow 𝒜`. It is situated in locale `finset_family`.
+
+We also maintain the convention that `a, b : α` are elements of the ground type, `s, t : finset α`
+are finsets, and `𝒜, ℬ : finset (finset α)` are finset families.
+
+## References
+
+* https://github.com/b-mehta/maths-notes/blob/master/iii/mich/combinatorics.pdf
+* http://discretemath.imp.fu-berlin.de/DMII-2015-16/kruskal.pdf
+
+## Tags
+
+shadow, set family
+-/
+
+open finset nat
+
+variables {α : Type*}
+
+namespace finset
+variables [decidable_eq α] {𝒜 : finset (finset α)} {s t : finset α} {a : α} {k : ℕ}
+
+/-- The shadow of a set family `𝒜` is all sets we can get by removing one element from any set in
+`𝒜`, and the (`k` times) iterated shadow (`shadow^[k]`) is all sets we can get by removing `k`
+elements from any set in `𝒜`. -/
+def shadow (𝒜 : finset (finset α)) : finset (finset α) := 𝒜.sup (λ s, s.image (erase s))
+
+localized "notation `∂ `:90 := finset.shadow" in finset_family
+
+/-- The shadow of the empty set is empty. -/
+@[simp] lemma shadow_empty : ∂ (∅ : finset (finset α)) = ∅ := rfl
+
+/-- The shadow is monotone. -/
+@[mono] lemma shadow_monotone : monotone (shadow : finset (finset α) → finset (finset α)) :=
+λ 𝒜 ℬ, sup_mono
+
+/-- `s` is in the shadow of `𝒜` iff there is an `t ∈ 𝒜` from which we can remove one element to
+get `s`. -/
+lemma mem_shadow_iff : s ∈ ∂ 𝒜 ↔ ∃ t ∈ 𝒜, ∃ a ∈ t, erase t a = s :=
+by simp only [shadow, mem_sup, mem_image]
+
+lemma erase_mem_shadow (hs : s ∈ 𝒜) (ha : a ∈ s) : erase s a ∈ ∂ 𝒜 :=
+mem_shadow_iff.2 ⟨s, hs, a, ha, rfl⟩
+
+/-- `t` is in the shadow of `𝒜` iff we can add an element to it so that the resulting finset is in
+`𝒜`. -/
+lemma mem_shadow_iff_insert_mem : s ∈ ∂ 𝒜 ↔ ∃ a ∉ s, insert a s ∈ 𝒜 :=
+begin
+  refine mem_shadow_iff.trans ⟨_, _⟩,
+  { rintro ⟨s, hs, a, ha, rfl⟩,
+    refine ⟨a, not_mem_erase a s, _⟩,
+    rwa insert_erase ha },
+  { rintro ⟨a, ha, hs⟩,
+    exact ⟨insert a s, hs, a, mem_insert_self _ _, erase_insert ha⟩ }
+end
+
+/-- `s ∈ ∂ 𝒜` iff `s` is exactly one element less than something from `𝒜` -/
+lemma mem_shadow_iff_exists_mem_card_add_one :
+  s ∈ ∂ 𝒜 ↔ ∃ t ∈ 𝒜, s ⊆ t ∧ t.card = s.card + 1 :=
+begin
+  refine mem_shadow_iff_insert_mem.trans ⟨_, _⟩,
+  { rintro ⟨a, ha, hs⟩,
+    exact ⟨insert a s, hs, subset_insert _ _, card_insert_of_not_mem ha⟩ },
+  { rintro ⟨t, ht, hst, h⟩,
+    obtain ⟨a, ha⟩ : ∃ a, t \ s = {a} :=
+      card_eq_one.1 (by rw [card_sdiff hst, h, add_tsub_cancel_left]),
+    exact ⟨a, λ hat,
+      not_mem_sdiff_of_mem_right hat ((ha.ge : _ ⊆ _) $ mem_singleton_self a),
+      by rwa [insert_eq a s, ←ha, sdiff_union_of_subset hst]⟩ }
+end
+
+/-- Being in the shadow of `𝒜` means we have a superset in `𝒜`. -/
+lemma exists_subset_of_mem_shadow (hs : s ∈ ∂ 𝒜) : ∃ t ∈ 𝒜, s ⊆ t :=
+let ⟨t, ht, hst⟩ := mem_shadow_iff_exists_mem_card_add_one.1 hs in ⟨t, ht, hst.1⟩
+
+/-- `t ∈ ∂^k 𝒜` iff `t` is exactly `k` elements less than something in `𝒜`. -/
+lemma mem_shadow_iff_exists_mem_card_add :
+  s ∈ (∂^[k]) 𝒜 ↔ ∃ t ∈ 𝒜, s ⊆ t ∧ t.card = s.card + k :=
+begin
+  induction k with k ih generalizing 𝒜 s,
+  { refine ⟨λ hs, ⟨s, hs, subset.refl _, rfl⟩, _⟩,
+    rintro ⟨t, ht, hst, hcard⟩,
+    rwa eq_of_subset_of_card_le hst hcard.le },
+  simp only [exists_prop, function.comp_app, function.iterate_succ],
+  refine ih.trans _,
+  clear ih,
+  split,
+  { rintro ⟨t, ht, hst, hcardst⟩,
+    obtain ⟨u, hu, htu, hcardtu⟩ := mem_shadow_iff_exists_mem_card_add_one.1 ht,
+    refine ⟨u, hu, hst.trans htu, _⟩,
+    rw [hcardtu, hcardst],
+    refl },
+  { rintro ⟨t, ht, hst, hcard⟩,
+    obtain ⟨u, hsu, hut, hu⟩ := finset.exists_intermediate_set k
+      (by { rw [add_comm, hcard], exact le_succ _ }) hst,
+    rw add_comm at hu,
+    refine ⟨u, mem_shadow_iff_exists_mem_card_add_one.2 ⟨t, ht, hut, _⟩, hsu, hu⟩,
+    rw [hcard, hu],
+    refl }
+end
+
+end finset