41 lines
1.1 KiB
Plaintext
41 lines
1.1 KiB
Plaintext
import Mathlib.Data.Set.Basic
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/-! # Common.Set.Basic
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Additional theorems and definitions useful in the context of `Set`s.
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-/
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namespace Set
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/-
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The Minkowski sum of two sets `s` and `t` is the set
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`s + t = { a + b : a ∈ s, b ∈ t }`.
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-/
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def minkowskiSum {α : Type u} [Add α] (s t : Set α) :=
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{ x | ∃ a ∈ s, ∃ b ∈ t, x = a + b }
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/--
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The sum of two sets is nonempty **iff** the summands are nonempty.
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-/
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theorem nonempty_minkowski_sum_iff_nonempty_add_nonempty {α : Type u} [Add α]
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{s t : Set α}
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: (minkowskiSum s t).Nonempty ↔ s.Nonempty ∧ t.Nonempty := by
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apply Iff.intro
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· intro h
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have ⟨x, hx⟩ := h
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have ⟨a, ⟨ha, ⟨b, ⟨hb, _⟩⟩⟩⟩ := hx
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apply And.intro
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· exact ⟨a, ha⟩
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· exact ⟨b, hb⟩
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· intro ⟨⟨a, ha⟩, ⟨b, hb⟩⟩
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exact ⟨a + b, ⟨a, ⟨ha, ⟨b, ⟨hb, rfl⟩⟩⟩⟩⟩
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/--
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The characteristic function of a set `S`.
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It returns `1` if the specified input belongs to `S` and `0` otherwise.
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-/
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def characteristic (S : Set α) (x : α) [Decidable (x ∈ S)] : Nat :=
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if x ∈ S then 1 else 0
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end Set |