Ordinals as games #

We define the canonical map Ordinal → SetTheory.PGame, where every ordinal is mapped to the game whose left set consists of all previous ordinals.

The map to surreals is defined in Ordinal.toSurreal.

Main declarations #

Ordinal.toPGame: The canonical map between ordinals and pre-games.
Ordinal.toPGameEmbedding: The order embedding version of the previous map.

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@[irreducible]

noncomputable def Ordinal.toPGame (o : Ordinal.{u}) :

SetTheory.PGame

Converts an ordinal into the corresponding pre-game.

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@[simp]

theorem Ordinal.toPGame_leftMoves (o : Ordinal.{u_1}) :

o.toPGame.LeftMoves = o.toType

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@[simp]

theorem Ordinal.toPGame_rightMoves (o : Ordinal.{u_1}) :

o.toPGame.RightMoves = PEmpty.{u_1 + 1}

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instance Ordinal.isEmpty_zero_toPGame_leftMoves :

IsEmpty (toPGame 0).LeftMoves

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instance Ordinal.isEmpty_toPGame_rightMoves (o : Ordinal.{u_1}) :

IsEmpty o.toPGame.RightMoves

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noncomputable def Ordinal.toLeftMovesToPGame {o : Ordinal.{u_1}} :

↑(Set.Iio o) ≃ o.toPGame.LeftMoves

Converts an ordinal less than o into a move for the PGame corresponding to o, and vice versa.

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theorem Ordinal.toLeftMovesToPGame_symm_lt {o : Ordinal.{u_1}} (i : o.toPGame.LeftMoves) :

↑(toLeftMovesToPGame.symm i) < o

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theorem Ordinal.toPGame_moveLeft_hEq {o : Ordinal.{u_1}} :

o.toPGame.moveLeft ≍ fun (x : o.toType) => (↑(o.enumIsoToType.symm x)).toPGame

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@[simp]

theorem Ordinal.toPGame_moveLeft' {o : Ordinal.{u_1}} (i : o.toPGame.LeftMoves) :

o.toPGame.moveLeft i = (↑(toLeftMovesToPGame.symm i)).toPGame

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theorem Ordinal.toPGame_moveLeft {o : Ordinal.{u_1}} (i : ↑(Set.Iio o)) :

o.toPGame.moveLeft (toLeftMovesToPGame i) = (↑i).toPGame

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noncomputable def Ordinal.zeroToPGameRelabelling :

(toPGame 0).Relabelling 0

0.toPGame has the same moves as 0.

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theorem Ordinal.toPGame_zero :

toPGame 0 ≈ 0

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noncomputable instance Ordinal.uniqueOneToPGameLeftMoves :

Unique (toPGame 1).LeftMoves

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theorem Ordinal.one_toPGame_leftMoves_default_eq :

default = toLeftMovesToPGame ⟨0, ⋯⟩

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@[simp]

theorem Ordinal.to_leftMoves_one_toPGame_symm (i : (toPGame 1).LeftMoves) :

toLeftMovesToPGame.symm i = ⟨0, ⋯⟩

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theorem Ordinal.one_toPGame_moveLeft (x : (toPGame 1).LeftMoves) :

(toPGame 1).moveLeft x = toPGame 0

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noncomputable def Ordinal.oneToPGameRelabelling :

(toPGame 1).Relabelling 1

1.toPGame has the same moves as 1.

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theorem Ordinal.toPGame_one :

toPGame 1 ≈ 1

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theorem Ordinal.toPGame_lf {a b : Ordinal.{u_1}} (h : a < b) :

a.toPGame.LF b.toPGame

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theorem Ordinal.toPGame_le {a b : Ordinal.{u_1}} (h : a ≤ b) :

a.toPGame ≤ b.toPGame

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theorem Ordinal.toPGame_lt {a b : Ordinal.{u_1}} (h : a < b) :

a.toPGame < b.toPGame

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theorem Ordinal.toPGame_nonneg (a : Ordinal.{u_1}) :

0 ≤ a.toPGame

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theorem Ordinal.toPGame_lf_iff {a b : Ordinal.{u_1}} :

a.toPGame.LF b.toPGame ↔ a < b

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@[simp]

theorem Ordinal.toPGame_le_iff {a b : Ordinal.{u_1}} :

a.toPGame ≤ b.toPGame ↔ a ≤ b

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@[simp]

theorem Ordinal.toPGame_lt_iff {a b : Ordinal.{u_1}} :

a.toPGame < b.toPGame ↔ a < b

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@[simp]

theorem Ordinal.toPGame_equiv_iff {a b : Ordinal.{u_1}} :

a.toPGame ≈ b.toPGame ↔ a = b

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theorem Ordinal.toPGame_injective :

Function.Injective toPGame

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@[simp]

theorem Ordinal.toPGame_inj {a b : Ordinal.{u_1}} :

a.toPGame = b.toPGame ↔ a = b

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noncomputable def Ordinal.toPGameEmbedding :

Ordinal.{u} ↪o SetTheory.PGame

The order embedding version of toPGame.

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theorem Ordinal.toPGameEmbedding_apply (o : Ordinal.{u}) :

toPGameEmbedding o = o.toPGame

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noncomputable def Ordinal.toGame :

Ordinal.{u} ↪o SetTheory.Game

Converts an ordinal into the corresponding game.

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@[simp]

theorem Ordinal.mk_toPGame (o : Ordinal.{u_1}) :

⟦o.toPGame⟧ = toGame o

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@[simp]

theorem Ordinal.toGame_zero :

toGame 0 = 0

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@[simp]

theorem Ordinal.toGame_one :

toGame 1 = 1

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theorem Ordinal.toGame_injective :

Function.Injective ⇑toGame

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@[simp]

theorem Ordinal.toGame_lf_iff {a b : Ordinal.{u_1}} :

(toGame a).LF (toGame b) ↔ a < b

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theorem Ordinal.toGame_le_iff {a b : Ordinal.{u_1}} :

toGame a ≤ toGame b ↔ a ≤ b

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theorem Ordinal.toGame_lt_iff {a b : Ordinal.{u_1}} :

toGame a < toGame b ↔ a < b

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theorem Ordinal.toGame_inj {a b : Ordinal.{u_1}} :

toGame a = toGame b ↔ a = b

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@[irreducible]

theorem Ordinal.toPGame_nadd (a b : Ordinal.{u_1}) :

(a.nadd b).toPGame ≈ a.toPGame + b.toPGame

The natural addition of ordinals corresponds to their sum as games.

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theorem Ordinal.toGame_nadd (a b : Ordinal.{u_1}) :

toGame (a.nadd b) = toGame a + toGame b

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@[irreducible]

theorem Ordinal.toPGame_nmul (a b : Ordinal.{u_1}) :

(a.nmul b).toPGame ≈ a.toPGame * b.toPGame

The natural multiplication of ordinals corresponds to their product as pre-games.

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theorem Ordinal.toGame_nmul (a b : Ordinal.{u_1}) :

toGame (a.nmul b) = ⟦a.toPGame * b.toPGame⟧

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@[simp]

theorem Ordinal.toGame_natCast (n : ℕ) :

toGame ↑n = ↑n

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theorem Ordinal.toPGame_natCast (n : ℕ) :

(↑n).toPGame ≈ ↑n

Documentation

Mathlib.SetTheory.Game.Ordinal

Ordinals as games #

Main declarations #