lean2/library/data/finset/card.lean
2015-05-11 09:14:48 -07:00

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/-
Copyright (c) 2015 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.
Author: Jeremy Avigad
Cardinality calculations for finite sets.
-/
import data.finset.to_set data.set.function
open nat eq.ops
namespace finset
variables {A B : Type}
variables [deceqA : decidable_eq A] [deceqB : decidable_eq B]
include deceqA
theorem card_add_card (s₁ s₂ : finset A) : card s₁ + card s₂ = card (s₁ s₂) + card (s₁ ∩ s₂) :=
finset.induction_on s₂
(show card s₁ + card ∅ = card (s₁ ∅) + card (s₁ ∩ ∅),
by rewrite [union_empty, card_empty, inter_empty])
(take s₂ a,
assume ans2: a ∉ s₂,
assume IH : card s₁ + card s₂ = card (s₁ s₂) + card (s₁ ∩ s₂),
show card s₁ + card (insert a s₂) = card (s₁ (insert a s₂)) + card (s₁ ∩ (insert a s₂)),
from decidable.by_cases
(assume as1 : a ∈ s₁,
assert H : a ∉ s₁ ∩ s₂, from assume H', ans2 (mem_of_mem_inter_right H'),
begin
rewrite [card_insert_of_not_mem ans2, union.comm, -insert_union, union.comm],
rewrite [insert_union, insert_eq_of_mem as1, insert_eq, inter.distrib_left, inter.comm],
rewrite [singleton_inter_of_mem as1, -insert_eq, card_insert_of_not_mem H, -*add.assoc],
rewrite IH
end)
(assume ans1 : a ∉ s₁,
assert H : a ∉ s₁ s₂, from assume H',
or.elim (mem_or_mem_of_mem_union H') (assume as1, ans1 as1) (assume as2, ans2 as2),
begin
rewrite [card_insert_of_not_mem ans2, union.comm, -insert_union, union.comm],
rewrite [card_insert_of_not_mem H, insert_eq, inter.distrib_left, inter.comm],
rewrite [singleton_inter_of_not_mem ans1, empty_union, add.right_comm],
rewrite [-add.assoc, IH]
end))
theorem card_union (s₁ s₂ : finset A) : card (s₁ s₂) = card s₁ + card s₂ - card (s₁ ∩ s₂) :=
calc
card (s₁ s₂) = card (s₁ s₂) + card (s₁ ∩ s₂) - card (s₁ ∩ s₂) : add_sub_cancel
... = card s₁ + card s₂ - card (s₁ ∩ s₂) : card_add_card
theorem card_union_of_disjoint {s₁ s₂ : finset A} (H : disjoint s₁ s₂) :
card (s₁ s₂) = card s₁ + card s₂ :=
by rewrite [card_union, ↑disjoint at H, inter_empty_of_disjoint H]
theorem card_le_card_of_subset {s₁ s₂ : finset A} (H : s₁ ⊆ s₂) : card s₁ ≤ card s₂ :=
have H1 : disjoint s₁ (s₂ \ s₁),
from disjoint.intro (take x, assume H1 H2, not_mem_of_mem_diff H2 H1),
calc
card s₂ = card (s₁ (s₂ \ s₁)) : union_diff_cancel H
... = card s₁ + card (s₂ \ s₁) : card_union_of_disjoint H1
... ≥ card s₁ : le_add_right
section card_image
open set
include deceqB
theorem card_image_eq_of_inj_on {f : A → B} {s : finset A} :
inj_on f (ts s) → card (image f s) = card s :=
finset.induction_on s
(assume H : inj_on f (ts empty), calc
card (image f empty) = 0 : card_empty
... = card empty : card_empty)
(take t a,
assume H : a ∉ t,
assume IH : inj_on f (ts t) → card (image f t) = card t,
assume H1 : inj_on f (ts (insert a t)),
have H2 : ts t ⊆ ts (insert a t), by rewrite [-subset_eq_to_set_subset]; apply subset_insert,
have H3 : card (image f t) = card t, from IH (inj_on_of_inj_on_of_subset H1 H2),
have H4 : f a ∉ image f t,
proof
assume H5 : f a ∈ image f t,
obtain x (H6l : x ∈ t) (H6r : f x = f a), from exists_of_mem_image H5,
have H7 : x = a, from H1 (mem_insert_of_mem _ H6l) !mem_insert H6r,
show false, from H (H7 ▸ H6l)
qed,
calc
card (image f (insert a t)) = card (insert (f a) (image f t)) : image_insert
... = card (image f t) + 1 : card_insert_of_not_mem H4
... = card t + 1 : H3
... = card (insert a t) : card_insert_of_not_mem H)
end card_image
end finset