Equivalences for Fin n

def finZeroEquiv : Fin 0 ≃ Empty

Equivalence between Fin 0 and Empty.

Equations

• finZeroEquiv = Equiv.equivEmpty (Fin 0)

def finZeroEquiv' : Fin 0 ≃ PEmpty.{u}

Equivalence between Fin 0 and PEmpty.

Equations

• finZeroEquiv' = Equiv.equivPEmpty (Fin 0)

def finOneEquiv : Fin 1 ≃ Unit

Equivalence between Fin 1 and Unit.

Equations

• finOneEquiv = Equiv.equivPUnit (Fin 1)

def finTwoEquiv : Fin 2 ≃ Bool

Equivalence between Fin 2 and Bool.

Equations

• finTwoEquiv = { toFun := ![false, true], invFun := fun (b : Bool) => Bool.casesOn b 0 1, left_inv := finTwoEquiv.proof_2, right_inv := finTwoEquiv.proof_3 }

@[simp]

theorem piFinTwoEquiv_symm_apply (α : Fin 2 → Type u) : ⇑(piFinTwoEquiv α).symm = fun (p : α 0 × α 1) => Fin.cons p.1 (Fin.cons p.2 finZeroElim)

@[simp]

theorem piFinTwoEquiv_apply (α : Fin 2 → Type u) : ⇑(piFinTwoEquiv α) = fun (f : (i : Fin 2) → α i) => (f 0, f 1)

def piFinTwoEquiv (α : Fin 2 → Type u) : ((i : Fin 2) → α i) ≃ α 0 × α 1

Π i : Fin 2, α i is equivalent to α 0 × α 1. See also finTwoArrowEquiv for a non-dependent version and prodEquivPiFinTwo for a version with inputs α β : Type u.

Equations

• piFinTwoEquiv α = { toFun := fun (f : (i : Fin 2) → α i) => (f 0, f 1), invFun := fun (p : α 0 × α 1) => Fin.cons p.1 (Fin.cons p.2 finZeroElim), left_inv := ⋯, right_inv := ⋯ }

theorem Fin.preimage_apply_01_prod {α : Fin 2 → Type u} (s : Set (α 0)) (t : Set (α 1)) : (fun (f : (i : Fin 2) → α i) => (f 0, f 1)) ⁻¹' s ×ˢ t = Set.univ.pi (Fin.cons s (Fin.cons t finZeroElim))

theorem Fin.preimage_apply_01_prod' {α : Type u} (s : Set α) (t : Set α) : (fun (f : Fin 2 → α) => (f 0, f 1)) ⁻¹' s ×ˢ t = Set.univ.pi ![s, t]

@[simp]

theorem prodEquivPiFinTwo_symm_apply (α : Type u) (β : Type u) : ⇑(prodEquivPiFinTwo α β).symm = fun (f : (i : Fin 2) → Fin.cons α (Fin.cons β finZeroElim) i) => (f 0, f 1)

@[simp]

theorem prodEquivPiFinTwo_apply (α : Type u) (β : Type u) : ⇑(prodEquivPiFinTwo α β) = fun (p : Fin.cons α (Fin.cons β finZeroElim) 0 × Fin.cons α (Fin.cons β finZeroElim) 1) => Fin.cons p.1 (Fin.cons p.2 finZeroElim)

def prodEquivPiFinTwo (α : Type u) (β : Type u) : α × β ≃ ((i : Fin 2) → ![α, β] i)

A product space α × β is equivalent to the space Π i : Fin 2, γ i, where γ = Fin.cons α (Fin.cons β finZeroElim). See also piFinTwoEquiv and finTwoArrowEquiv.

Equations

• prodEquivPiFinTwo α β = (piFinTwoEquiv (Fin.cons α (Fin.cons β finZeroElim))).symm

@[simp]

theorem finTwoArrowEquiv_apply (α : Type u_1) : ⇑(finTwoArrowEquiv α) = (piFinTwoEquiv fun (x : Fin 2) => α).toFun

@[simp]

theorem finTwoArrowEquiv_symm_apply (α : Type u_1) : ⇑(finTwoArrowEquiv α).symm = fun (x : α × α) => ![x.1, x.2]

def finTwoArrowEquiv (α : Type u_1) : (Fin 2 → α) ≃ α × α

The space of functions Fin 2 → α is equivalent to α × α. See also piFinTwoEquiv and prodEquivPiFinTwo.

Equations

• finTwoArrowEquiv α = let __src := piFinTwoEquiv fun (x : Fin 2) => α; { toFun := __src.toFun, invFun := fun (x : α × α) => ![x.1, x.2], left_inv := ⋯, right_inv := ⋯ }

def OrderIso.piFinTwoIso (α : Fin 2 → Type u) [(i : Fin 2) → Preorder (α i)] : ((i : Fin 2) → α i) ≃o α 0 × α 1

Π i : Fin 2, α i is order equivalent to α 0 × α 1. See also OrderIso.finTwoArrowEquiv for a non-dependent version.

Equations

• OrderIso.piFinTwoIso α = { toEquiv := piFinTwoEquiv α, map_rel_iff' := ⋯ }

def OrderIso.finTwoArrowIso (α : Type u_1) [Preorder α] : (Fin 2 → α) ≃o α × α

The space of functions Fin 2 → α is order equivalent to α × α. See also OrderIso.piFinTwoIso.

Equations

• OrderIso.finTwoArrowIso α = let __src := OrderIso.piFinTwoIso fun (x : Fin 2) => α; { toEquiv := finTwoArrowEquiv α, map_rel_iff' := ⋯ }

def finSuccEquiv' {n : ℕ} (i : Fin (n + 1)) : Fin (n + 1) ≃ Option (Fin n)

An equivalence that removes i and maps it to none. This is a version of Fin.predAbove that produces Option (Fin n) instead of mapping both i.cast_succ and i.succ to i.

Equations

• finSuccEquiv' i = { toFun := i.insertNth none some, invFun := fun (x : Option (Fin n)) => x.casesOn' i i.succAbove, left_inv := ⋯, right_inv := ⋯ }

@[simp]

theorem finSuccEquiv'_at {n : ℕ} (i : Fin (n + 1)) : (finSuccEquiv' i) i = none

@[simp]

theorem finSuccEquiv'_succAbove {n : ℕ} (i : Fin (n + 1)) (j : Fin n) : (finSuccEquiv' i) (i.succAbove j) = some j

theorem finSuccEquiv'_below {n : ℕ} {i : Fin (n + 1)} {m : Fin n} (h : m.castSucc < i) : (finSuccEquiv' i) m.castSucc = some m

theorem finSuccEquiv'_above {n : ℕ} {i : Fin (n + 1)} {m : Fin n} (h : i ≤ m.castSucc) : (finSuccEquiv' i) m.succ = some m

@[simp]

theorem finSuccEquiv'_symm_none {n : ℕ} (i : Fin (n + 1)) : (finSuccEquiv' i).symm none = i

@[simp]

theorem finSuccEquiv'_symm_some {n : ℕ} (i : Fin (n + 1)) (j : Fin n) : (finSuccEquiv' i).symm (some j) = i.succAbove j

theorem finSuccEquiv'_symm_some_below {n : ℕ} {i : Fin (n + 1)} {m : Fin n} (h : m.castSucc < i) : (finSuccEquiv' i).symm (some m) = m.castSucc

theorem finSuccEquiv'_symm_some_above {n : ℕ} {i : Fin (n + 1)} {m : Fin n} (h : i ≤ m.castSucc) : (finSuccEquiv' i).symm (some m) = m.succ

theorem finSuccEquiv'_symm_coe_below {n : ℕ} {i : Fin (n + 1)} {m : Fin n} (h : m.castSucc < i) : (finSuccEquiv' i).symm (some m) = m.castSucc

theorem finSuccEquiv'_symm_coe_above {n : ℕ} {i : Fin (n + 1)} {m : Fin n} (h : i ≤ m.castSucc) : (finSuccEquiv' i).symm (some m) = m.succ

def finSuccEquiv (n : ℕ) : Fin (n + 1) ≃ Option (Fin n)

Equivalence between Fin (n + 1) and Option (Fin n). This is a version of Fin.pred that produces Option (Fin n) instead of requiring a proof that the input is not 0.

Equations

• finSuccEquiv n = finSuccEquiv' 0

@[simp]

theorem finSuccEquiv_zero {n : ℕ} : (finSuccEquiv n) 0 = none

@[simp]

theorem finSuccEquiv_succ {n : ℕ} (m : Fin n) : (finSuccEquiv n) m.succ = some m

@[simp]

theorem finSuccEquiv_symm_none {n : ℕ} : (finSuccEquiv n).symm none = 0

@[simp]

theorem finSuccEquiv_symm_some {n : ℕ} (m : Fin n) : (finSuccEquiv n).symm (some m) = m.succ

theorem finSuccEquiv'_zero {n : ℕ} : finSuccEquiv' 0 = finSuccEquiv n

The equiv version of Fin.predAbove_zero.

theorem finSuccEquiv'_last_apply_castSucc {n : ℕ} (i : Fin n) : (finSuccEquiv' (Fin.last n)) i.castSucc = some i

theorem finSuccEquiv'_last_apply {n : ℕ} {i : Fin (n + 1)} (h : i ≠ Fin.last n) : (finSuccEquiv' (Fin.last n)) i = some (i.castLT ⋯)

theorem finSuccEquiv'_ne_last_apply {n : ℕ} {i : Fin (n + 1)} {j : Fin (n + 1)} (hi : i ≠ Fin.last n) (hj : j ≠ i) : (finSuccEquiv' i) j = some ((i.castLT ⋯).predAbove j)

def finSuccAboveEquiv {n : ℕ} (p : Fin (n + 1)) : Fin n ≃o { x : Fin (n + 1) // x ≠ p }

Fin.succAbove as an order isomorphism between Fin n and {x : Fin (n + 1) // x ≠ p}.

Equations

• finSuccAboveEquiv p = let __src := (Equiv.optionSubtype p) ⟨(finSuccEquiv' p).symm, ⋯⟩; { toEquiv := __src, map_rel_iff' := ⋯ }

theorem finSuccAboveEquiv_apply {n : ℕ} (p : Fin (n + 1)) (i : Fin n) : (finSuccAboveEquiv p) i = ⟨p.succAbove i, ⋯⟩

theorem finSuccAboveEquiv_symm_apply_last {n : ℕ} (x : { x : Fin (n + 1) // x ≠ Fin.last n }) : (finSuccAboveEquiv (Fin.last n)).symm x = (↑x).castLT ⋯

theorem finSuccAboveEquiv_symm_apply_ne_last {n : ℕ} {p : Fin (n + 1)} (h : p ≠ Fin.last n) (x : { x : Fin (n + 1) // x ≠ p }) : (finSuccAboveEquiv p).symm x = (p.castLT ⋯).predAbove ↑x

def finSuccEquivLast {n : ℕ} : Fin (n + 1) ≃ Option (Fin n)

Equiv between Fin (n + 1) and Option (Fin n) sending Fin.last n to none

Equations

• finSuccEquivLast = finSuccEquiv' (Fin.last n)

@[simp]

theorem finSuccEquivLast_castSucc {n : ℕ} (i : Fin n) : finSuccEquivLast i.castSucc = some i

@[simp]

theorem finSuccEquivLast_last {n : ℕ} : finSuccEquivLast (Fin.last n) = none

@[simp]

theorem finSuccEquivLast_symm_some {n : ℕ} (i : Fin n) : finSuccEquivLast.symm (some i) = i.castSucc

@[simp]

theorem finSuccEquivLast_symm_none {n : ℕ} : finSuccEquivLast.symm none = Fin.last n

@[simp]

theorem Equiv.piFinSuccAbove_symm_apply {n : ℕ} (α : Fin (n + 1) → Type u) (i : Fin (n + 1)) : ⇑(Equiv.piFinSuccAbove α i).symm = fun (f : α i × ((j : Fin n) → α (i.succAbove j))) => i.insertNth f.1 f.2

@[simp]

theorem Equiv.piFinSuccAbove_apply {n : ℕ} (α : Fin (n + 1) → Type u) (i : Fin (n + 1)) : ⇑(Equiv.piFinSuccAbove α i) = fun (f : (j : Fin (n + 1)) → α j) => (f i, i.removeNth f)

def Equiv.piFinSuccAbove {n : ℕ} (α : Fin (n + 1) → Type u) (i : Fin (n + 1)) : ((j : Fin (n + 1)) → α j) ≃ α i × ((j : Fin n) → α (i.succAbove j))

Equivalence between Π j : Fin (n + 1), α j and α i × Π j : Fin n, α (Fin.succAbove i j).

Equations

• One or more equations did not get rendered due to their size.

def OrderIso.piFinSuccAboveIso {n : ℕ} (α : Fin (n + 1) → Type u) [(i : Fin (n + 1)) → LE (α i)] (i : Fin (n + 1)) : ((j : Fin (n + 1)) → α j) ≃o α i × ((j : Fin n) → α (i.succAbove j))

Order isomorphism between Π j : Fin (n + 1), α j and α i × Π j : Fin n, α (Fin.succAbove i j).

Equations

• OrderIso.piFinSuccAboveIso α i = { toEquiv := Equiv.piFinSuccAbove α i, map_rel_iff' := ⋯ }

@[simp]

theorem Equiv.piFinSucc_apply (n : ℕ) (β : Type u) : ⇑(Equiv.piFinSucc n β) = fun (f : Fin (n + 1) → β) => (f 0, Fin.tail f)

@[simp]

theorem Equiv.piFinSucc_symm_apply (n : ℕ) (β : Type u) : ⇑(Equiv.piFinSucc n β).symm = fun (f : β × (Fin n → β)) => Fin.cons f.1 f.2

def Equiv.piFinSucc (n : ℕ) (β : Type u) : (Fin (n + 1) → β) ≃ β × (Fin n → β)

Equivalence between Fin (n + 1) → β and β × (Fin n → β).

Equations

• Equiv.piFinSucc n β = Equiv.piFinSuccAbove (fun (x : Fin (n + 1)) => β) 0

def Equiv.embeddingFinSucc (n : ℕ) (ι : Type u_1) : (Fin (n + 1) ↪ ι) ≃ (e : Fin n ↪ ι) × { i : ι // i ∉ Set.range ⇑e }

An embedding e : Fin (n+1) ↪ ι corresponds to an embedding f : Fin n ↪ ι (corresponding the last n coordinates of e) together with a value not taken by f (corresponding to e 0).

Equations

• Equiv.embeddingFinSucc n ι = ((finSuccEquiv n).embeddingCongr (Equiv.refl ι)).trans (Function.Embedding.optionEmbeddingEquiv (Fin n) ι)

@[simp]

theorem Equiv.embeddingFinSucc_fst {n : ℕ} {ι : Type u_1} (e : Fin (n + 1) ↪ ι) : ⇑((Equiv.embeddingFinSucc n ι) e).fst = ⇑e ∘ Fin.succ

@[simp]

theorem Equiv.embeddingFinSucc_snd {n : ℕ} {ι : Type u_1} (e : Fin (n + 1) ↪ ι) : ↑((Equiv.embeddingFinSucc n ι) e).snd = e 0

@[simp]

theorem Equiv.coe_embeddingFinSucc_symm {n : ℕ} {ι : Type u_1} (f : (e : Fin n ↪ ι) × { i : ι // i ∉ Set.range ⇑e }) : ⇑((Equiv.embeddingFinSucc n ι).symm f) = Fin.cons ↑f.snd ⇑f.fst

@[simp]

theorem Equiv.piFinCastSucc_apply (n : ℕ) (β : Type u) : ⇑(Equiv.piFinCastSucc n β) = fun (f : Fin (n + 1) → β) => (f (Fin.last n), Fin.init f)

@[simp]

theorem Equiv.piFinCastSucc_symm_apply (n : ℕ) (β : Type u) : ⇑(Equiv.piFinCastSucc n β).symm = fun (f : β × (Fin n → β)) => Fin.snoc f.2 f.1

def Equiv.piFinCastSucc (n : ℕ) (β : Type u) : (Fin (n + 1) → β) ≃ β × (Fin n → β)

Equivalence between Fin (n + 1) → β and β × (Fin n → β) which separates out the last element of the tuple.

Equations

• Equiv.piFinCastSucc n β = Equiv.piFinSuccAbove (fun (x : Fin (n + 1)) => β) (Fin.last n)

def finSumFinEquiv {m : ℕ} {n : ℕ} : Fin m ⊕ Fin n ≃ Fin (m + n)

Equivalence between Fin m ⊕ Fin n and Fin (m + n)

Equations

• finSumFinEquiv = { toFun := Sum.elim (Fin.castAdd n) (Fin.natAdd m), invFun := fun (i : Fin (m + n)) => Fin.addCases Sum.inl Sum.inr i, left_inv := ⋯, right_inv := ⋯ }

@[simp]

theorem finSumFinEquiv_apply_left {m : ℕ} {n : ℕ} (i : Fin m) : finSumFinEquiv (Sum.inl i) = Fin.castAdd n i

@[simp]

theorem finSumFinEquiv_apply_right {m : ℕ} {n : ℕ} (i : Fin n) : finSumFinEquiv (Sum.inr i) = Fin.natAdd m i

@[simp]

theorem finSumFinEquiv_symm_apply_castAdd {m : ℕ} {n : ℕ} (x : Fin m) : finSumFinEquiv.symm (Fin.castAdd n x) = Sum.inl x

@[simp]

theorem finSumFinEquiv_symm_apply_natAdd {m : ℕ} {n : ℕ} (x : Fin n) : finSumFinEquiv.symm (Fin.natAdd m x) = Sum.inr x

@[simp]

theorem finSumFinEquiv_symm_last {n : ℕ} : finSumFinEquiv.symm (Fin.last n) = Sum.inr 0

def finAddFlip {m : ℕ} {n : ℕ} : Fin (m + n) ≃ Fin (n + m)

The equivalence between Fin (m + n) and Fin (n + m) which rotates by n.

Equations

• finAddFlip = (finSumFinEquiv.symm.trans (Equiv.sumComm (Fin m) (Fin n))).trans finSumFinEquiv

@[simp]

theorem finAddFlip_apply_castAdd {m : ℕ} (k : Fin m) (n : ℕ) : finAddFlip (Fin.castAdd n k) = Fin.natAdd n k

@[simp]

theorem finAddFlip_apply_natAdd {n : ℕ} (k : Fin n) (m : ℕ) : finAddFlip (Fin.natAdd m k) = Fin.castAdd m k

@[simp]

theorem finAddFlip_apply_mk_left {m : ℕ} {n : ℕ} {k : ℕ} (h : k < m) (hk : optParam (k < m + n) ⋯) (hnk : optParam (n + k < n + m) ⋯) : finAddFlip ⟨k, hk⟩ = ⟨n + k, hnk⟩

@[simp]

theorem finAddFlip_apply_mk_right {m : ℕ} {n : ℕ} {k : ℕ} (h₁ : m ≤ k) (h₂ : k < m + n) : finAddFlip ⟨k, h₂⟩ = ⟨k - m, ⋯⟩

def finRotate (n : ℕ) : Equiv.Perm (Fin n)

Rotate Fin n one step to the right.

Equations

• finRotate x = match x with | 0 => Equiv.refl (Fin 0) | n.succ => finAddFlip.trans (finCongr ⋯)

@[simp]

theorem finRotate_zero : finRotate 0 = Equiv.refl (Fin 0)

theorem finRotate_succ (n : ℕ) : finRotate (n + 1) = finAddFlip.trans (finCongr ⋯)

theorem finRotate_of_lt {n : ℕ} {k : ℕ} (h : k < n) : (finRotate (n + 1)) ⟨k, ⋯⟩ = ⟨k + 1, ⋯⟩

theorem finRotate_last' {n : ℕ} : (finRotate (n + 1)) ⟨n, ⋯⟩ = ⟨0, ⋯⟩

theorem finRotate_last {n : ℕ} : (finRotate (n + 1)) (Fin.last n) = 0

theorem Fin.snoc_eq_cons_rotate {n : ℕ} {α : Type u_1} (v : Fin n → α) (a : α) : Fin.snoc v a = fun (i : Fin (n + 1)) => Fin.cons a v ((finRotate (n + 1)) i)

@[simp]

theorem finRotate_one : finRotate 1 = Equiv.refl (Fin 1)

@[simp]

theorem finRotate_succ_apply {n : ℕ} (i : Fin (n + 1)) : (finRotate (n + 1)) i = i + 1

theorem finRotate_apply_zero {n : ℕ} : (finRotate n.succ) 0 = 1

theorem coe_finRotate_of_ne_last {n : ℕ} {i : Fin n.succ} (h : i ≠ Fin.last n) : ↑((finRotate (n + 1)) i) = ↑i + 1

theorem coe_finRotate {n : ℕ} (i : Fin n.succ) : ↑((finRotate n.succ) i) = if i = Fin.last n then 0 else ↑i + 1

@[simp]

theorem finProdFinEquiv_symm_apply {m : ℕ} {n : ℕ} (x : Fin (m * n)) : finProdFinEquiv.symm x = (x.divNat, x.modNat)

@[simp]

theorem finProdFinEquiv_apply_val {m : ℕ} {n : ℕ} (x : Fin m × Fin n) : ↑(finProdFinEquiv x) = ↑x.2 + n * ↑x.1

def finProdFinEquiv {m : ℕ} {n : ℕ} : Fin m × Fin n ≃ Fin (m * n)

Equivalence between Fin m × Fin n and Fin (m * n)

Equations

• finProdFinEquiv = { toFun := fun (x : Fin m × Fin n) => ⟨↑x.2 + n * ↑x.1, ⋯⟩, invFun := fun (x : Fin (m * n)) => (x.divNat, x.modNat), left_inv := ⋯, right_inv := ⋯ }

@[simp]

theorem Nat.divModEquiv_apply (n : ℕ) [NeZero n] (a : ℕ) : n.divModEquiv a = (a / n, ↑a)

@[simp]

theorem Nat.divModEquiv_symm_apply (n : ℕ) [NeZero n] (p : ℕ × Fin n) : n.divModEquiv.symm p = p.1 * n + ↑p.2

def Nat.divModEquiv (n : ℕ) [NeZero n] : ℕ ≃ ℕ × Fin n

The equivalence induced by a ↦ (a / n, a % n) for nonzero n. This is like finProdFinEquiv.symm but with m infinite. See Nat.div_mod_unique for a similar propositional statement.

Equations

• n.divModEquiv = { toFun := fun (a : ℕ) => (a / n, ↑a), invFun := fun (p : ℕ × Fin n) => p.1 * n + ↑p.2, left_inv := ⋯, right_inv := ⋯ }

@[simp]

theorem Int.divModEquiv_symm_apply (n : ℕ) [NeZero n] (p : ℤ × Fin n) : (Int.divModEquiv n).symm p = p.1 * ↑n + ↑↑p.2

@[simp]

theorem Int.divModEquiv_apply (n : ℕ) [NeZero n] (a : ℤ) : (Int.divModEquiv n) a = (a / ↑n, ↑(a.natMod ↑n))

def Int.divModEquiv (n : ℕ) [NeZero n] : ℤ ≃ ℤ × Fin n

The equivalence induced by a ↦ (a / n, a % n) for nonzero n. See Int.ediv_emod_unique for a similar propositional statement.

Equations

• Int.divModEquiv n = { toFun := fun (a : ℤ) => (a / ↑n, ↑(a.natMod ↑n)), invFun := fun (p : ℤ × Fin n) => p.1 * ↑n + ↑↑p.2, left_inv := ⋯, right_inv := ⋯ }

@[simp]

theorem Fin.castLEOrderIso_apply {n : ℕ} {m : ℕ} (h : n ≤ m) (i : Fin n) : (Fin.castLEOrderIso h) i = ⟨Fin.castLE h i, ⋯⟩

@[simp]

theorem Fin.castLEOrderIso_symm_apply {n : ℕ} {m : ℕ} (h : n ≤ m) (i : { i : Fin m // ↑i < n }) : (RelIso.symm (Fin.castLEOrderIso h)) i = ⟨↑↑i, ⋯⟩

def Fin.castLEOrderIso {n : ℕ} {m : ℕ} (h : n ≤ m) : Fin n ≃o { i : Fin m // ↑i < n }

Promote a Fin n into a larger Fin m, as a subtype where the underlying values are retained. This is the OrderIso version of Fin.castLE.

Equations

• Fin.castLEOrderIso h = { toFun := fun (i : Fin n) => ⟨Fin.castLE h i, ⋯⟩, invFun := fun (i : { i : Fin m // ↑i < n }) => ⟨↑↑i, ⋯⟩, left_inv := ⋯, right_inv := ⋯, map_rel_iff' := ⋯ }

instance subsingleton_fin_zero : Subsingleton (Fin 0)

Fin 0 is a subsingleton.

Equations

• subsingleton_fin_zero = ⋯

instance subsingleton_fin_one : Subsingleton (Fin 1)

Fin 1 is a subsingleton.

Equations

• subsingleton_fin_one = ⋯