Module: Mathlib.Algebra.MvPolynomial.SchwartzZippel

Goal state before:
pp: case h₂
R : Type u_1
inst✝² : CommRing R
inst✝¹ : IsDomain R
inst✝ : DecidableEq R
n : ℕ
p : MvPolynomial (Fin (n + 1)) R
hp : p ≠ 0
S : Fin (n + 1) → Finset R
p' : Polynomial (MvPolynomial (Fin n) R) := (finSuccEquiv R n) p
hp' : p' = (finSuccEquiv R n) p
k : ℕ := p'.natDegree
hk : k = p'.natDegree
pₖ : MvPolynomial (Fin n) R := p'.leadingCoeff
hpₖ : pₖ = p'.leadingCoeff
hp'₀ : p' ≠ 0
hpₖ₀ : pₖ ≠ 0
⊢ ↑(#({x ∈ piFinset fun i ↦ S i | (eval x) p = 0 ∧ (eval (tail x)) pₖ ≠ 0})) / ∏ i, ↑(#(S i)) ≤
    ↑(degreeOf 0 p) / ↑(#(S 0))

Tactic:
· -- We bound the second set by noting that if `x` is in it, then `x₀` is the root of
        -- the univariate polynomial`pₓ` obtained by evaluating each (multivariate polynomial)
        -- coefficient at `xₜ`. Since `pₓ` has degree `k`, there are at most `k` such `x₀` for
        -- each `xₜ`, which gives the result.
        calc
          #{x ∈ S ^^ (n + 1) | eval x p = 0 ∧ eval (tail x) pₖ ≠ 0} / ∏ i, (#(S i) : ℚ≥0)
            ≤ ↑(p.degreeOf 0 * ∏ i, #(S (.succ i))) / ∏ i, (#(S i) : ℚ≥0) := ?_
          _ = p.degreeOf 0 * (∏ i, #(S (.succ i))) / (#(S 0) * ∏ i, #(S (.succ i))) := by
            norm_cast; rw [prod_univ_succ]
          _ ≤ (p.degreeOf 0 / #(S 0) : ℚ≥0) := mul_div_mul_right_le (by positivity)
        gcongr
        calc
          #{x ∈ S ^^ (n + 1) | eval x p = 0 ∧ eval (tail x) pₖ ≠ 0}
            = #{x ∈ S ^^ (n + 1) | eval (tail x) pₖ ≠ 0 ∧ eval x p = 0} := by simp_rw [and_comm]
          _ = #({xₜ ∈ tail S ^^ n | eval xₜ pₖ ≠ 0}.biUnion fun xₜ ↦ image (fun x₀ ↦ (x₀, xₜ))
                {x₀ ∈ S 0 | eval (cons x₀ xₜ) p = 0}) := by
            rw [← filter_filter, filter_piFinset_eq_map_consEquiv S (fun r ↦ eval r pₖ ≠ 0),
              filter_map, card_map, product_eq_biUnion_right, filter_biUnion]
            simp [filter_image]
            rfl
          _ ≤ ∑ xₜ ∈ tail S ^^ n with eval xₜ pₖ ≠ 0,
                #(image (fun x₀ ↦ (x₀, xₜ)) {x₀ ∈ S 0 | eval (cons x₀ xₜ) p = 0}) :=
            card_biUnion_le
          _ ≤ ∑ xₜ ∈ tail S ^^ n with eval xₜ pₖ ≠ 0, #{x₀ ∈ S 0 | eval (cons x₀ xₜ) p = 0} := by
            gcongr; exact card_image_le
          _ ≤ ∑ xₜ ∈ tail S ^^ n with eval xₜ pₖ ≠ 0, p.degreeOf 0 := ?_
          _ ≤ ∑ _xₜ ∈ tail S ^^ n, p.degreeOf 0 := by gcongr; exact filter_subset ..
          _ = p.degreeOf 0 * ∏ i, #(S (.succ i)) := by simp [mul_comm, tail]
        gcongr with xₜ hxₜ
        set pₓ := p'.map (eval xₜ) with hpₓ
        have hpₓdeg : pₓ.natDegree = k := by
          rw [hpₓ, hk, Polynomial.natDegree_map_of_leadingCoeff_ne_zero _ (mem_filter.1 hxₜ).2]
        have hpₓ₀ : pₓ ≠ 0 := fun h ↦ (mem_filter.1 hxₜ).2 <| by
          rw [hpₖ, Polynomial.leadingCoeff, ← hk, ← hpₓdeg, h, Polynomial.natDegree_zero,
            ← Polynomial.coeff_map, ← hpₓ, h, Polynomial.coeff_zero]
        calc
          #{x₀ ∈ S 0 | eval (cons x₀ xₜ) p = 0} ≤ #pₓ.roots.toFinset := by
            gcongr
            simp +contextual [subset_iff, eval_eq_eval_mv_eval', pₓ, hpₓ₀, p']
          _ ≤ Multiset.card pₓ.roots := pₓ.roots.toFinset_card_le
          _ ≤ pₓ.natDegree := pₓ.card_roots'
          _ = k := hpₓdeg
          _ ≤ p.degreeOf 0 := by
            have :
              (ofLex (AddMonoidAlgebra.supDegree toLex p'.leadingCoeff)).cons k ∈ p.support := by
              rwa [← mem_support_coeff_finSuccEquiv, mem_support_iff, ← hp', hk,
                ← Polynomial.leadingCoeff, ← hpₖ, ← leadingCoeff_toLex,
                AddMonoidAlgebra.leadingCoeff_ne_zero toLex.injective]
            simpa using monomial_le_degreeOf 0 this

Goal state after: