-- Module: Mathlib.RingTheory.Spectrum.Prime.LTSeries
Goal:
R : Type u_1
inst : CommRing R
inst_1 : IsNoetherianRing R
x : R
n : ℕ
hn :
  ∀ (p : LTSeries (PrimeSpectrum R))
    (hx :
      x ∈
        (RelSeries.last (r :=
            {x : PrimeSpectrum R × PrimeSpectrum R |
              match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
              | (a, b) => a < b})
            p).asIdeal)
    (hp :
      p.length (r :=
          {x : PrimeSpectrum R × PrimeSpectrum R |
            match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
            | (a, b) => a < b}) =
        n),
    ∃ (q : LTSeries (PrimeSpectrum R)),
      x ∈
          (q.toFun (r :=
              {x : PrimeSpectrum R × PrimeSpectrum R |
                match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                | (a, b) => a < b})
              1).asIdeal ∧
        n =
            q.length (r :=
              {x : PrimeSpectrum R × PrimeSpectrum R |
                match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                | (a, b) => a < b}) ∧
          RelSeries.head (r :=
                {x : PrimeSpectrum R × PrimeSpectrum R |
                  match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                  | (a, b) => a < b})
                p =
              RelSeries.head (r :=
                {x : PrimeSpectrum R × PrimeSpectrum R |
                  match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                  | (a, b) => a < b})
                q ∧
            RelSeries.last (r :=
                {x : PrimeSpectrum R × PrimeSpectrum R |
                  match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                  | (a, b) => a < b})
                p =
              RelSeries.last (r :=
                {x : PrimeSpectrum R × PrimeSpectrum R |
                  match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                  | (a, b) => a < b})
                q
p : LTSeries (PrimeSpectrum R)
hx :
  x ∈
    (RelSeries.last (r :=
        {x : PrimeSpectrum R × PrimeSpectrum R |
          match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
          | (a, b) => a < b})
        p).asIdeal
hp :
  p.length (r :=
      {x : PrimeSpectrum R × PrimeSpectrum R |
        match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
        | (a, b) => a < b}) =
    n + 1
h0 : ¬n = 0
neg :
  ∀ (q : PrimeSpectrum R) (hxq : x ∈ q.asIdeal)
    (h2 :
      p.toFun (r :=
          {x : PrimeSpectrum R × PrimeSpectrum R |
            match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
            | (a, b) => a < b})
          (⟨p.length (r :=
                  {x : PrimeSpectrum R × PrimeSpectrum R |
                    match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                    | (a, b) => a < b}) -
                2,
              ⋯⟩ :
            Fin
              (p.length (r :=
                  {x : PrimeSpectrum R × PrimeSpectrum R |
                    match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                    | (a, b) => a < b}) +
                1)) <
        q)
    (hq :
      q <
        RelSeries.last (r :=
          {x : PrimeSpectrum R × PrimeSpectrum R |
            match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
            | (a, b) => a < b})
          p),
    ∃ (q : LTSeries (PrimeSpectrum R)),
      x ∈
          (q.toFun (r :=
              {x : PrimeSpectrum R × PrimeSpectrum R |
                match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                | (a, b) => a < b})
              1).asIdeal ∧
        n + 1 =
            q.length (r :=
              {x : PrimeSpectrum R × PrimeSpectrum R |
                match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                | (a, b) => a < b}) ∧
          RelSeries.head (r :=
                {x : PrimeSpectrum R × PrimeSpectrum R |
                  match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                  | (a, b) => a < b})
                p =
              RelSeries.head (r :=
                {x : PrimeSpectrum R × PrimeSpectrum R |
                  match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                  | (a, b) => a < b})
                q ∧
            RelSeries.last (r :=
                {x : PrimeSpectrum R × PrimeSpectrum R |
                  match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                  | (a, b) => a < b})
                p =
              RelSeries.last (r :=
                {x : PrimeSpectrum R × PrimeSpectrum R |
                  match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                  | (a, b) => a < b})
                q
⊢ ∃ (q : LTSeries (PrimeSpectrum R)),
    x ∈
        (q.toFun (r :=
            {x : PrimeSpectrum R × PrimeSpectrum R |
              match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
              | (a, b) => a < b})
            1).asIdeal ∧
      n + 1 =
          q.length (r :=
            {x : PrimeSpectrum R × PrimeSpectrum R |
              match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
              | (a, b) => a < b}) ∧
        RelSeries.head (r :=
              {x : PrimeSpectrum R × PrimeSpectrum R |
                match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                | (a, b) => a < b})
              p =
            RelSeries.head (r :=
              {x : PrimeSpectrum R × PrimeSpectrum R |
                match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                | (a, b) => a < b})
              q ∧
          RelSeries.last (r :=
              {x : PrimeSpectrum R × PrimeSpectrum R |
                match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                | (a, b) => a < b})
              p =
            RelSeries.last (r :=
              {x : PrimeSpectrum R × PrimeSpectrum R |
                match (motive := PrimeSpectrum R × PrimeSpectrum R → Prop) x with
                | (a, b) => a < b})
              q

Tactic:
obtain ⟨q, hxq, h2, hq⟩ : ∃ q : PrimeSpectrum R, x ∈ q.1 ∧ p ⟨p.length - 2, _⟩ < q ∧ q < p.last :=
    (p ⟨p.length - 1, p.length.sub_lt_succ 1⟩).exist_mem_one_of_mem_two
      (p.strictMono (Nat.pred_lt (by simpa [hp]))) (p.strictMono (Nat.pred_lt (by simp [hp]))) hx

Premises used: RelSeries.last, HSub.hSub, Fin.mk, RelSeries.length, PrimeSpectrum.exist_mem_one_of_mem_two, LTSeries.strictMono, PrimeSpectrum, RelSeries.toFun, OfNat.ofNat