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1 module Symmetric where
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3 open import Level hiding ( suc ; zero )
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4 open import Algebra
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5 open import Algebra.Structures
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6 open import Data.Fin hiding ( _<_ ; _≤_ ; _-_ ; _+_ )
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7 open import Data.Fin.Properties hiding ( <-trans ; ≤-trans ) renaming ( <-cmp to <-fcmp )
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8 open import Data.Product
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9 open import Data.Fin.Permutation
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10 open import Function hiding (id ; flip)
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11 open import Function.Inverse as Inverse using (_↔_; Inverse; _InverseOf_)
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12 open import Function.LeftInverse using ( _LeftInverseOf_ )
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13 open import Function.Equality using (Π)
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14 open import Data.Nat -- using (ℕ; suc; zero; s≤s ; z≤n )
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15 open import Data.Nat.Properties -- using (<-trans)
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16 open import Relation.Binary.PropositionalEquality
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17 open import Data.List using (List; []; _∷_ ; length ; _++_ ; head ) renaming (reverse to rev )
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18 open import nat
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19
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20 fid : {p : ℕ } → Fin p → Fin p
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21 fid x = x
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22
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23 -- Data.Fin.Permutation.id
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24 pid : {p : ℕ } → Permutation p p
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25 pid = permutation fid fid record { left-inverse-of = λ x → refl ; right-inverse-of = λ x → refl }
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26
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27 -- Data.Fin.Permutation.flip
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28 pinv : {p : ℕ } → Permutation p p → Permutation p p
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29 pinv {p} P = permutation (_⟨$⟩ˡ_ P) (_⟨$⟩ʳ_ P ) record { left-inverse-of = λ x → inverseʳ P ; right-inverse-of = λ x → inverseˡ P }
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31 record _=p=_ {p : ℕ } ( x y : Permutation p p ) : Set where
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32 field
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33 peq : ( q : Fin p ) → x ⟨$⟩ʳ q ≡ y ⟨$⟩ʳ q
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34
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35 open _=p=_
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37 prefl : {p : ℕ } { x : Permutation p p } → x =p= x
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38 peq (prefl {p} {x}) q = refl
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40 psym : {p : ℕ } { x y : Permutation p p } → x =p= y → y =p= x
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41 peq (psym {p} {x} {y} eq ) q = sym (peq eq q)
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42
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43 ptrans : {p : ℕ } { x y z : Permutation p p } → x =p= y → y =p= z → x =p= z
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44 peq (ptrans {p} {x} {y} x=y y=z ) q = trans (peq x=y q) (peq y=z q)
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46 Symmetric : ℕ → Group Level.zero Level.zero
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47 Symmetric p = record {
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48 Carrier = Permutation p p
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49 ; _≈_ = _=p=_
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50 ; _∙_ = _∘ₚ_
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51 ; ε = pid
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52 ; _⁻¹ = pinv
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53 ; isGroup = record { isMonoid = record { isSemigroup = record { isMagma = record {
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54 isEquivalence = record {refl = prefl ; trans = ptrans ; sym = psym }
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55 ; ∙-cong = presp }
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56 ; assoc = passoc }
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57 ; identity = ( (λ q → record { peq = λ q → refl } ) , (λ q → record { peq = λ q → refl } )) }
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58 ; inverse = ( (λ x → record { peq = λ q → inverseʳ x} ) , (λ x → record { peq = λ q → inverseˡ x} ))
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59 ; ⁻¹-cong = λ i=j → record { peq = λ q → p-inv i=j q }
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60 }
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61 } where
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62 presp : {x y u v : Permutation p p } → x =p= y → u =p= v → (x ∘ₚ u) =p= (y ∘ₚ v)
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63 presp {x} {y} {u} {v} x=y u=v = record { peq = λ q → lemma4 q } where
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64 lemma4 : (q : Fin p) → ((x ∘ₚ u) ⟨$⟩ʳ q) ≡ ((y ∘ₚ v) ⟨$⟩ʳ q)
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65 lemma4 q = trans (cong (λ k → Inverse.to u Π.⟨$⟩ k) (peq x=y q) ) (peq u=v _ )
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66 passoc : (x y z : Permutation p p) → ((x ∘ₚ y) ∘ₚ z) =p= (x ∘ₚ (y ∘ₚ z))
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67 passoc x y z = record { peq = λ q → refl }
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68 p-inv : {i j : Permutation p p} → i =p= j → (q : Fin p) → pinv i ⟨$⟩ʳ q ≡ pinv j ⟨$⟩ʳ q
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69 p-inv {i} {j} i=j q = begin
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70 i ⟨$⟩ˡ q ≡⟨ cong (λ k → i ⟨$⟩ˡ k) (sym (inverseʳ j) ) ⟩
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71 i ⟨$⟩ˡ ( j ⟨$⟩ʳ ( j ⟨$⟩ˡ q )) ≡⟨ cong (λ k → i ⟨$⟩ˡ k) (sym (peq i=j _ )) ⟩
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72 i ⟨$⟩ˡ ( i ⟨$⟩ʳ ( j ⟨$⟩ˡ q )) ≡⟨ inverseˡ i ⟩
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73 j ⟨$⟩ˡ q
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74 ∎ where open ≡-Reasoning
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