{-# OPTIONS --cubical --no-import-sorts --safe #-} module Cubical.Algebra.Ring.Kernel where open import Cubical.Foundations.Prelude open import Cubical.Foundations.Equiv open import Cubical.Foundations.Structure open import Cubical.Foundations.HLevels open import Cubical.Foundations.Powerset open import Cubical.Algebra.Ring.Base open import Cubical.Algebra.Ring.Properties open import Cubical.Algebra.Ring.Ideal private variable ℓ : Level module _ {{R S : Ring {ℓ}}} (f′ : RingHom R S) where open RingHom f′ renaming (map to f) open HomTheory f′ open RingStr ⦃...⦄ open isIdeal open Theory private instance _ = R _ = S _ = snd R _ = snd S kernelSubtype : ⟨ R ⟩ → hProp ℓ kernelSubtype x = (f x ≡ 0r) , isSetRing S _ _ kernelIsIdeal : isIdeal R kernelSubtype +-closed kernelIsIdeal = λ fx≡0 fy≡0 → f (_ + _) ≡⟨ isHom+ _ _ ⟩ f _ + f _ ≡⟨ cong (λ u → u + f _) fx≡0 ⟩ 0r + f _ ≡⟨ cong (λ u → 0r + u) fy≡0 ⟩ 0r + 0r ≡⟨ 0Idempotent S ⟩ 0r ∎ -closed kernelIsIdeal = λ fx≡0 → f (- _) ≡⟨ -commutesWithHom _ ⟩ - f _ ≡⟨ cong -_ fx≡0 ⟩ - 0r ≡⟨ 0Selfinverse S ⟩ 0r ∎ 0r-closed kernelIsIdeal = f 0r ≡⟨ homPres0 ⟩ 0r ∎ ·-closedLeft kernelIsIdeal = λ r fx≡0 → f (r · _) ≡⟨ isHom· _ _ ⟩ f r · f (_) ≡⟨ cong (λ u → f r · u) fx≡0 ⟩ f r · 0r ≡⟨ 0RightAnnihilates S _ ⟩ 0r ∎ ·-closedRight kernelIsIdeal = λ r fx≡0 → f (_ · r) ≡⟨ isHom· _ _ ⟩ f _ · f r ≡⟨ cong (λ u → u · f r) fx≡0 ⟩ 0r · f r ≡⟨ 0LeftAnnihilates S _ ⟩ 0r ∎ kernel : {{R S : Ring {ℓ}}} (f′ : RingHom R S) → IdealsIn R kernel f′ = kernelSubtype f′ , kernelIsIdeal f′