### view filter.agda @ 339:feb0fcc430a9

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author Shinji KONO Sun, 12 Jul 2020 19:55:37 +0900 12071f79f3cf aad9249d1e8f
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open import Level
open import Ordinals
module filter {n : Level } (O : Ordinals {n})   where

open import zf
open import logic
import OD

open import Relation.Nullary
open import Relation.Binary
open import Data.Empty
open import Relation.Binary
open import Relation.Binary.Core
open import  Relation.Binary.PropositionalEquality
open import Data.Nat renaming ( zero to Zero ; suc to Suc ;  ℕ to Nat ; _⊔_ to _n⊔_ )
import BAlgbra

open BAlgbra O

open inOrdinal O
open OD O
open OD.OD
open ODAxiom odAxiom

import ODC

open _∧_
open _∨_
open Bool

-- Kunen p.76 and p.53, we use ⊆
record Filter  ( L : HOD  ) : Set (suc n) where
field
filter : HOD
f⊆PL :  filter ⊆ Power L
filter1 : { p q : HOD } →  q ⊆ L  → filter ∋ p →  p ⊆ q  → filter ∋ q
filter2 : { p q : HOD } → filter ∋ p →  filter ∋ q  → filter ∋ (p ∩ q)

open Filter

record prime-filter { L : HOD } (P : Filter L) : Set (suc (suc n)) where
field
proper  : ¬ (filter P ∋ od∅)
prime   : {p q : HOD } →  filter P ∋ (p ∪ q) → ( filter P ∋ p ) ∨ ( filter P ∋ q )

record ultra-filter { L : HOD } (P : Filter L) : Set (suc (suc n)) where
field
proper  : ¬ (filter P ∋ od∅)
ultra   : {p : HOD } → p ⊆ L →  ( filter P ∋ p ) ∨ (  filter P ∋ ( L ＼ p) )

open _⊆_

trans-⊆ :  { A B C : HOD} → A ⊆ B → B ⊆ C → A ⊆ C
trans-⊆ A⊆B B⊆C = record { incl = λ x → incl B⊆C (incl A⊆B x) }

power→⊆ :  ( A t : HOD) → Power A ∋ t → t ⊆ A
power→⊆ A t  PA∋t = record { incl = λ {x} t∋x → ODC.double-neg-eilm O (t1 t∋x) } where
t1 : {x : HOD }  → t ∋ x → ¬ ¬ (A ∋ x)
t1 = zf.IsZF.power→ isZF A t PA∋t

∈-filter : {L p : HOD} → (P : Filter L ) → filter P ∋ p → p ⊆ L
∈-filter {L} {p} P lt = power→⊆ L p ( incl (f⊆PL P) lt )

∪-lemma1 : {L p q : HOD } → (p ∪ q)  ⊆ L → p ⊆ L
∪-lemma1 {L} {p} {q} lt = record { incl = λ {x} p∋x → incl lt (case1 p∋x) }

∪-lemma2 : {L p q : HOD } → (p ∪ q)  ⊆ L → q ⊆ L
∪-lemma2 {L} {p} {q} lt = record { incl = λ {x} p∋x → incl lt (case2 p∋x) }

q∩q⊆q : {p q : HOD } → (q ∩ p) ⊆ q
q∩q⊆q = record { incl = λ lt → proj1 lt }

open HOD
_=h=_ : (x y : HOD) → Set n
x =h= y  = od x == od y

-----
--
--  ultra filter is prime
--

filter-lemma1 :  {L : HOD} → (P : Filter L)  → ∀ {p q : HOD } → ultra-filter P  → prime-filter P
filter-lemma1 {L} P u = record {
proper = ultra-filter.proper u
; prime = lemma3
} where
lemma3 : {p q : HOD} → filter P ∋ (p ∪ q) → ( filter P ∋ p ) ∨ ( filter P ∋ q )
lemma3 {p} {q} lt with ultra-filter.ultra u (∪-lemma1 (∈-filter P lt) )
... | case1 p∈P  = case1 p∈P
... | case2 ¬p∈P = case2 (filter1 P {q ∩ (L ＼ p)} (∪-lemma2 (∈-filter P lt)) lemma7 lemma8) where
lemma5 : ((p ∪ q ) ∩ (L ＼ p)) =h=  (q ∩ (L ＼ p))
lemma5 = record { eq→ = λ {x} lt → record { proj1 = lemma4 x lt ; proj2 = proj2 lt  }
;  eq← = λ {x} lt → record { proj1 = case2 (proj1 lt) ; proj2 = proj2 lt }
} where
lemma4 : (x : Ordinal ) → odef ((p ∪ q) ∩ (L ＼ p)) x → odef q x
lemma4 x lt with proj1 lt
lemma4 x lt | case1 px = ⊥-elim ( proj2 (proj2 lt) px )
lemma4 x lt | case2 qx = qx
lemma6 : filter P ∋ ((p ∪ q ) ∩ (L ＼ p))
lemma6 = filter2 P lt ¬p∈P
lemma7 : filter P ∋ (q ∩ (L ＼ p))
lemma7 =  subst (λ k → filter P ∋ k ) (==→o≡ lemma5 ) lemma6
lemma8 : (q ∩ (L ＼ p)) ⊆ q
lemma8 = q∩q⊆q

-----
--
--  if Filter contains L, prime filter is ultra
--

filter-lemma2 :  {L : HOD} → (P : Filter L)  → filter P ∋ L → prime-filter P → ultra-filter P
filter-lemma2 {L} P f∋L prime = record {
proper = prime-filter.proper prime
; ultra = λ {p}  p⊆L → prime-filter.prime prime (lemma p  p⊆L)
} where
open _==_
p+1-p=1 : {p : HOD} → p ⊆ L → L =h= (p ∪ (L ＼ p))
eq→ (p+1-p=1 {p} p⊆L) {x} lt with ODC.decp O (odef p x)
eq→ (p+1-p=1 {p} p⊆L) {x} lt | yes p∋x = case1 p∋x
eq→ (p+1-p=1 {p} p⊆L) {x} lt | no ¬p = case2 (record { proj1 = lt ; proj2 = ¬p })
eq← (p+1-p=1 {p} p⊆L) {x} ( case1 p∋x ) = subst (λ k → odef L k ) diso (incl p⊆L ( subst (λ k → odef p k) (sym diso) p∋x  ))
eq← (p+1-p=1 {p} p⊆L) {x} ( case2 ¬p  ) = proj1 ¬p
lemma : (p : HOD) → p ⊆ L   →  filter P ∋ (p ∪ (L ＼ p))
lemma p p⊆L = subst (λ k → filter P ∋ k ) (==→o≡ (p+1-p=1 p⊆L)) f∋L

record Dense  (P : HOD ) : Set (suc n) where
field
dense : HOD
incl :  dense ⊆ P
dense-f : HOD → HOD
dense-d :  { p : HOD} → P ∋ p  → dense ∋ dense-f p
dense-p :  { p : HOD} → P ∋ p  →  p ⊆ (dense-f p)

--    the set of finite partial functions from ω to 2
--
--   ph2 : Nat → Set → 2
--   ph2 : Nat → Maybe 2
--
-- Hω2 : Filter (Power (Power infinite))

record Ideal  ( L : HOD  ) : Set (suc n) where
field
ideal : HOD
i⊆PL :  ideal ⊆ Power L
ideal1 : { p q : HOD } →  q ⊆ L  → ideal ∋ p →  q ⊆ p  → ideal ∋ q
ideal2 : { p q : HOD } → ideal ∋ p →  ideal ∋ q  → ideal ∋ (p ∪ q)

open Ideal

proper-ideal : {L : HOD} → (P : Ideal L ) → {p : HOD} → Set n
proper-ideal {L} P {p} = ideal P ∋ od∅

prime-ideal : {L : HOD} → Ideal L → ∀ {p q : HOD } → Set n
prime-ideal {L} P {p} {q} =  ideal P ∋ ( p ∩ q) → ( ideal P ∋ p ) ∨ ( ideal P ∋ q )

```