-- Mathlib/Algebra/AlgebraicCard.lean
theorem cardinalMk_lift_le_mul :
    Cardinal.lift.{u} #{ x : A // IsAlgebraic R x } ≤ Cardinal.lift.{v} #R[X] * ℵ₀

Proof state:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
⊢ lift.{u, v} #{ x // IsAlgebraic R x } ≤ lift.{v, u} #R[X] * ℵ₀

Tactic:
rw [← mk_uLift, ← mk_uLift]

Result:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
⊢ #(ULift.{u, v} { x // IsAlgebraic R x }) ≤ #(ULift.{v, u} R[X]) * ℵ₀

Proof state:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
⊢ #(ULift.{u, v} { x // IsAlgebraic R x }) ≤ #(ULift.{v, u} R[X]) * ℵ₀

Tactic:
refine lift_mk_le_lift_mk_mul_of_lift_mk_preimage_le g fun f => ?_

Result:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
f : R[X]
⊢ lift.{u, v} #↑(g ⁻¹' {f}) ≤ ℵ₀

Proof state:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
f : R[X]
⊢ lift.{u, v} #↑(g ⁻¹' {f}) ≤ ℵ₀

Tactic:
rw [lift_le_aleph0, le_aleph0_iff_set_countable]

Result:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
f : R[X]
⊢ (g ⁻¹' {f}).Countable

Proof state:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
x : ↑{x | IsAlgebraic R x}
⊢ ↑x ∈ (g x).rootSet A

Tactic:
exact mem_rootSet.2 ⟨hg₁ x, hg₂ x⟩

Result:
no goals

Proof state:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
⊢ #(ULift.{u, v} { x // IsAlgebraic R x }) ≤ #(ULift.{v, u} R[X]) * ℵ₀

Tactic:
choose g hg₁ hg₂ using fun x : { x : A | IsAlgebraic R x } => x.coe_prop

Result:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
⊢ #(ULift.{u, v} { x // IsAlgebraic R x }) ≤ #(ULift.{v, u} R[X]) * ℵ₀

Proof state:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
f : R[X]
⊢ (g ⁻¹' {f}).Countable

Tactic:
suffices MapsTo (↑) (g ⁻¹' {f}) (f.rootSet A) from
  this.countable_of_injOn Subtype.coe_injective.injOn (f.rootSet_finite A).countable

Result:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
f : R[X]
⊢ MapsTo Subtype.val (g ⁻¹' {f}) (f.rootSet A)

Proof state:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
f : R[X]
⊢ MapsTo Subtype.val (g ⁻¹' {f}) (f.rootSet A)

Tactic:
rintro x (rfl : g x = f)

Result:
R : Type u
A : Type v
inst✝⁴ : CommRing R
inst✝³ : CommRing A
inst✝² : IsDomain A
inst✝¹ : Algebra R A
inst✝ : NoZeroSMulDivisors R A
g : ↑{x | IsAlgebraic R x} → R[X]
hg₁ : ∀ (x : ↑{x | IsAlgebraic R x}), g x ≠ 0
hg₂ : ∀ (x : ↑{x | IsAlgebraic R x}), (aeval ↑x) (g x) = 0
x : ↑{x | IsAlgebraic R x}
⊢ ↑x ∈ (g x).rootSet A