Module: Mathlib.MeasureTheory.Integral.RieszMarkovKakutani.Basic

Goal state before:
pp: X : Type u_1
inst✝² : TopologicalSpace X
inst✝¹ : T2Space X
inst✝ : LocallyCompactSpace X
s₀ s₁ t : Set X
s₀_compact : IsCompact s₀
s₁_compact : IsCompact s₁
t_compact : IsCompact t
disj : Disjoint s₀ s₁
hst : s₀ ∪ s₁ ⊆ t
⊢ ∃ f₀ f₁, EqOn (⇑f₀) 1 s₀ ∧ EqOn (⇑f₁) 1 s₁ ∧ EqOn (⇑(f₀ + f₁)) 1 t

Tactic:
set so : Fin 2 → Set X := fun j => if j = 0 then s₀ᶜ else s₁ᶜ with hso
  have soopen (j : Fin 2) : IsOpen (so j) := by
    fin_cases j
    · simp only [hso, Fin.zero_eta, Fin.isValue, ↓reduceIte, isOpen_compl_iff]
      exact IsCompact.isClosed <| s₀_compact
    · simp only [hso, Fin.isValue, Fin.mk_one, one_ne_zero, ↓reduceIte, isOpen_compl_iff]
      exact IsCompact.isClosed <| s₁_compact
  have hsot : t ⊆ ⋃ j, so j := by
    rw [hso]
    simp only [Fin.isValue]
    intro x hx
    rw [mem_iUnion]
    rw [← subset_compl_iff_disjoint_right, ← compl_compl s₀, compl_subset_iff_union] at disj
    have h : x ∈ s₀ᶜ ∨ x ∈ s₁ᶜ := by
      rw [← mem_union, disj]
      exact mem_univ _
    apply Or.elim h
    · intro h0
      use 0
      simp only [Fin.isValue, ↓reduceIte]
      exact h0
    · intro h1
      use 1
      simp only [Fin.isValue, one_ne_zero, ↓reduceIte]
      exact h1
  obtain ⟨f, f_supp_in_so, sum_f_one_on_t, f_in_icc, f_hcs⟩ :=
    exists_continuous_sum_one_of_isOpen_isCompact soopen t_compact hsot
  use (nnrealPart (⟨f 1, f_hcs 1⟩ : C_c(X, ℝ))),
    (nnrealPart (⟨f 0, f_hcs 0⟩ : C_c(X, ℝ)))
  simp only [Fin.isValue, CompactlySupportedContinuousMap.coe_add]
  have sum_one_x (x : X) (hx : x ∈ t) : (f 0) x + (f 1) x = 1 := by
    simpa only [Finset.sum_apply, Fin.sum_univ_two, Fin.isValue, Pi.one_apply]
      using sum_f_one_on_t hx
  refine ⟨?_, ?_, ?_⟩
  · intro x hx
    simp only [Fin.isValue, nnrealPart_apply,
      CompactlySupportedContinuousMap.coe_mk, Pi.one_apply, Real.toNNReal_eq_one]
    have : (f 0) x = 0 := by
      rw [← notMem_support]
      have : s₀ ⊆ (tsupport (f 0))ᶜ := by
        apply subset_trans _ (compl_subset_compl.mpr (f_supp_in_so 0))
        rw [hso]
        simp only [Fin.isValue, ↓reduceIte, compl_compl, subset_refl]
      apply notMem_of_mem_compl
      exact mem_of_subset_of_mem (subset_trans this (compl_subset_compl_of_subset subset_closure))
        hx
    rw [union_subset_iff] at hst
    rw [← sum_one_x x (mem_of_subset_of_mem hst.1 hx), this]
    exact Eq.symm (AddZeroClass.zero_add ((f 1) x))
  · intro x hx
    simp only [Fin.isValue, nnrealPart_apply,
      CompactlySupportedContinuousMap.coe_mk, Pi.one_apply, Real.toNNReal_eq_one]
    have : (f 1) x = 0 := by
      rw [← notMem_support]
      have : s₁ ⊆ (tsupport (f 1))ᶜ := by
        apply subset_trans _ (compl_subset_compl.mpr (f_supp_in_so 1))
        rw [hso]
        simp only [Fin.isValue, one_ne_zero, ↓reduceIte, compl_compl, subset_refl]
      apply notMem_of_mem_compl
      exact mem_of_subset_of_mem (subset_trans this (compl_subset_compl_of_subset subset_closure))
        hx
    rw [union_subset_iff] at hst
    rw [← sum_one_x x (mem_of_subset_of_mem hst.2 hx), this]
    exact Eq.symm (AddMonoid.add_zero ((f 0) x))
  · intro x hx
    simp only [Fin.isValue, Pi.add_apply, nnrealPart_apply,
      CompactlySupportedContinuousMap.coe_mk, Pi.one_apply]
    rw [Real.toNNReal_add_toNNReal (f_in_icc 1 x).1 (f_in_icc 0 x).1, add_comm]
    simp only [Fin.isValue, Real.toNNReal_eq_one]
    exact sum_one_x x hx

Goal state after: