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feat(set_theory/zfc/ordinal): transitive sets (#15288)
We define transitive sets, as an initial development towards von Neumann ordinals.
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/- | ||
Copyright (c) 2022 Violeta Hernández Palacios. All rights reserved. | ||
Released under Apache 2.0 license as described in the file LICENSE. | ||
Authors: Violeta Hernández Palacios | ||
-/ | ||
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import set_theory.zfc.basic | ||
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/-! | ||
# Von Neumann ordinals | ||
This file works towards the development of von Neumann ordinals, i.e. transitive sets, well-ordered | ||
under `∈`. We currently only have an initial development of transitive sets. | ||
Further development can be found on the branch `von_neumann_v2`. | ||
## Definitions | ||
- `Set.is_transitive` means that every element of a set is a subset. | ||
## Todo | ||
- Define von Neumann ordinals. | ||
- Define the basic arithmetic operations on ordinals from a purely set-theoretic perspective. | ||
- Prove the equivalences between these definitions and those provided in | ||
`set_theory/ordinal/arithmetic.lean`. | ||
-/ | ||
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namespace Set | ||
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/-- A transitive set is one where every element is a subset. -/ | ||
def is_transitive (x : Set) : Prop := ∀ y ∈ x, y ⊆ x | ||
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theorem is_transitive.subset_of_mem {x y : Set} (h : x.is_transitive) : y ∈ x → y ⊆ x := h y | ||
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theorem is_transitive_iff_mem_trans {z : Set} : | ||
z.is_transitive ↔ ∀ {x y : Set}, x ∈ y → y ∈ z → x ∈ z := | ||
⟨λ h x y hx hy, h.subset_of_mem hy hx, λ H x hx y hy, H hy hx⟩ | ||
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alias is_transitive_iff_mem_trans ↔ is_transitive.mem_trans _ | ||
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theorem is_transitive.sUnion {x : Set} (h : x.is_transitive) : (⋃₀ x).is_transitive := | ||
λ y hy z hz, begin | ||
rcases mem_sUnion.1 hy with ⟨w, hw, hw'⟩, | ||
exact mem_sUnion_of_mem hz (h.mem_trans hw' hw) | ||
end | ||
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theorem is_transitive_iff_sUnion_subset {x : Set} : x.is_transitive ↔ ⋃₀ x ⊆ x := | ||
⟨λ h y hy, by { rcases mem_sUnion.1 hy with ⟨z, hz, hz'⟩, exact h.mem_trans hz' hz }, | ||
λ H y hy z hz, H $ mem_sUnion_of_mem hz hy⟩ | ||
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alias is_transitive_iff_sUnion_subset ↔ is_transitive.sUnion_subset _ | ||
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theorem is_transitive_iff_subset_powerset {x : Set} : x.is_transitive ↔ x ⊆ powerset x := | ||
⟨λ h y hy, mem_powerset.2 $ h.subset_of_mem hy, λ H y hy z hz, mem_powerset.1 (H hy) hz⟩ | ||
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alias is_transitive_iff_subset_powerset ↔ is_transitive.subset_powerset _ | ||
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end Set |