-- Mathlib/RingTheory/Ideal/Norm/RelNorm.lean
theorem spanNorm_mul (I J : Ideal S) : spanNorm R (I * J) = spanNorm R I * spanNorm R J

Proof state:
case neg
R : Type u_1
inst✝¹² : CommRing R
inst✝¹¹ : IsDomain R
S : Type u_3
inst✝¹⁰ : CommRing S
inst✝⁹ : IsDomain S
inst✝⁸ : IsIntegrallyClosed R
inst✝⁷ : IsIntegrallyClosed S
inst✝⁶ : Algebra R S
inst✝⁵ : Module.Finite R S
inst✝⁴ : NoZeroSMulDivisors R S
inst✝³ : Algebra.IsSeparable (FractionRing R) (FractionRing S)
inst✝² : IsDedekindDomain R
inst✝¹ : IsDedekindDomain S
I J : Ideal S
inst✝ : Nontrivial R
h✝ : Nontrivial S
P : Ideal R
hP : P.IsMaximal
hP0 : ¬P = ⊥
P' : Submonoid S := Algebra.algebraMapSubmonoid S P.primeCompl
Rₚ : Type u_1 := Localization.AtPrime P
Sₚ : Type u_3 := Localization P'
x✝ : Algebra Rₚ Sₚ := localizationAlgebra P.primeCompl S
this : IsScalarTower R Rₚ Sₚ
h : P' ≤ S⁰
⊢ map (algebraMap R (Localization.AtPrime P)) (spanNorm R (I * J)) =
    map (algebraMap R (Localization.AtPrime P)) (spanNorm R I * spanNorm R J)

Tactic:
have : IsDomain Sₚ := IsLocalization.isDomain_localization h

Result:
case neg
R : Type u_1
inst✝¹² : CommRing R
inst✝¹¹ : IsDomain R
S : Type u_3
inst✝¹⁰ : CommRing S
inst✝⁹ : IsDomain S
inst✝⁸ : IsIntegrallyClosed R
inst✝⁷ : IsIntegrallyClosed S
inst✝⁶ : Algebra R S
inst✝⁵ : Module.Finite R S
inst✝⁴ : NoZeroSMulDivisors R S
inst✝³ : Algebra.IsSeparable (FractionRing R) (FractionRing S)
inst✝² : IsDedekindDomain R
inst✝¹ : IsDedekindDomain S
I J : Ideal S
inst✝ : Nontrivial R
h✝ : Nontrivial S
P : Ideal R
hP : P.IsMaximal
hP0 : ¬P = ⊥
P' : Submonoid S := Algebra.algebraMapSubmonoid S P.primeCompl
Rₚ : Type u_1 := Localization.AtPrime P
Sₚ : Type u_3 := Localization P'
x✝ : Algebra Rₚ Sₚ := localizationAlgebra P.primeCompl S
this✝ : IsScalarTower R Rₚ Sₚ
h : P' ≤ S⁰
this : IsDomain Sₚ
⊢ map (algebraMap R (Localization.AtPrime P)) (spanNorm R (I * J)) =
    map (algebraMap R (Localization.AtPrime P)) (spanNorm R I * spanNorm R J)

Proof state:
case neg
R : Type u_1
inst✝¹² : CommRing R
inst✝¹¹ : IsDomain R
S : Type u_3
inst✝¹⁰ : CommRing S
inst✝⁹ : IsDomain S
inst✝⁸ : IsIntegrallyClosed R
inst✝⁷ : IsIntegrallyClosed S
inst✝⁶ : Algebra R S
inst✝⁵ : Module.Finite R S
inst✝⁴ : NoZeroSMulDivisors R S
inst✝³ : Algebra.IsSeparable (FractionRing R) (FractionRing S)
inst✝² : IsDedekindDomain R
inst✝¹ : IsDedekindDomain S
I J : Ideal S
inst✝ : Nontrivial R
h✝ : Nontrivial S
P : Ideal R
hP : P.IsMaximal
hP0 : ¬P = ⊥
P' : Submonoid S := Algebra.algebraMapSubmonoid S P.primeCompl
Rₚ : Type u_1 := Localization.AtPrime P
Sₚ : Type u_3 := Localization P'
x✝ : Algebra Rₚ Sₚ := localizationAlgebra P.primeCompl S
this✝⁶ : IsScalarTower R Rₚ Sₚ
h : P' ≤ S⁰
this✝⁵ : IsDomain Sₚ
this✝⁴ : IsDedekindDomain Sₚ
this✝³ : IsPrincipalIdealRing Sₚ
this✝² : NoZeroSMulDivisors Rₚ Sₚ
this✝¹ : Module.Finite Rₚ Sₚ
L : Type u_3 := FractionRing S
g : Sₚ →+* L := IsLocalization.map L (RingHom.id S) h
algInst✝ : Algebra Sₚ L := g.toAlgebra
this✝ : IsScalarTower S Sₚ (FractionRing S)
this : IsFractionRing Sₚ (FractionRing S)
⊢ map (algebraMap R (Localization.AtPrime P)) (spanNorm R (I * J)) =
    map (algebraMap R (Localization.AtPrime P)) (spanNorm R I * spanNorm R J)

Tactic:
have : Algebra.IsSeparable (FractionRing Rₚ) (FractionRing Sₚ) := by
  apply Algebra.IsSeparable.of_equiv_equiv
    (FractionRing.algEquiv Rₚ (FractionRing R)).symm.toRingEquiv
    (FractionRing.algEquiv Sₚ (FractionRing S)).symm.toRingEquiv
  apply IsLocalization.ringHom_ext R⁰
  ext
  simp only [AlgEquiv.toRingEquiv_eq_coe, RingHom.coe_comp,
    RingHom.coe_coe, Function.comp_apply, ← IsScalarTower.algebraMap_apply]
  rw [IsScalarTower.algebraMap_apply R Rₚ (FractionRing R), AlgEquiv.coe_ringEquiv,
    AlgEquiv.commutes, IsScalarTower.algebraMap_apply R S L,
    IsScalarTower.algebraMap_apply S Sₚ L, AlgEquiv.coe_ringEquiv, AlgEquiv.commutes]
  simp only [← IsScalarTower.algebraMap_apply]
  rw [IsScalarTower.algebraMap_apply R Rₚ (FractionRing Rₚ),
    ← IsScalarTower.algebraMap_apply Rₚ, ← IsScalarTower.algebraMap_apply]

Result:
case neg
R : Type u_1
inst✝¹² : CommRing R
inst✝¹¹ : IsDomain R
S : Type u_3
inst✝¹⁰ : CommRing S
inst✝⁹ : IsDomain S
inst✝⁸ : IsIntegrallyClosed R
inst✝⁷ : IsIntegrallyClosed S
inst✝⁶ : Algebra R S
inst✝⁵ : Module.Finite R S
inst✝⁴ : NoZeroSMulDivisors R S
inst✝³ : Algebra.IsSeparable (FractionRing R) (FractionRing S)
inst✝² : IsDedekindDomain R
inst✝¹ : IsDedekindDomain S
I J : Ideal S
inst✝ : Nontrivial R
h✝ : Nontrivial S
P : Ideal R
hP : P.IsMaximal
hP0 : ¬P = ⊥
P' : Submonoid S := Algebra.algebraMapSubmonoid S P.primeCompl
Rₚ : Type u_1 := Localization.AtPrime P
Sₚ : Type u_3 := Localization P'
x✝ : Algebra Rₚ Sₚ := localizationAlgebra P.primeCompl S
this✝⁷ : IsScalarTower R Rₚ Sₚ
h : P' ≤ S⁰
this✝⁶ : IsDomain Sₚ
this✝⁵ : IsDedekindDomain Sₚ
this✝⁴ : IsPrincipalIdealRing Sₚ
this✝³ : NoZeroSMulDivisors Rₚ Sₚ
this✝² : Module.Finite Rₚ Sₚ
L : Type u_3 := FractionRing S
g : Sₚ →+* L := IsLocalization.map L (RingHom.id S) h
algInst✝ : Algebra Sₚ L := g.toAlgebra
this✝¹ : IsScalarTower S Sₚ (FractionRing S)
this✝ : IsFractionRing Sₚ (FractionRing S)
this : Algebra.IsSeparable (FractionRing Rₚ) (FractionRing Sₚ)
⊢ map (algebraMap R (Localization.AtPrime P)) (spanNorm R (I * J)) =
    map (algebraMap R (Localization.AtPrime P)) (spanNorm R I * spanNorm R J)