diff --git a/OrderedSemigroups/Basic.lean b/OrderedSemigroups/Basic.lean index 3cac634..68b37eb 100644 --- a/OrderedSemigroups/Basic.lean +++ b/OrderedSemigroups/Basic.lean @@ -6,20 +6,20 @@ universe u variable {α : Type u} -/- The addition of two ℕ+ -/ +/-- Define + notation for the addition of two ℕ+ -/ instance : Add ℕ+ := ⟨fun a b ↦ ⟨a.val + b.val, by have a' : 0 < a.val := by simp have b' : 0 < b.val := by simp omega⟩⟩ -/- The addition of a ℕ+ with a ℕ returning an ℕ+ -/ +/-- Define + notation for the addition of a ℕ+ with a ℕ returning an ℕ+ -/ instance : HAdd ℕ+ ℕ ℕ+ := ⟨fun a b ↦ ⟨a.val + b, by have a' : 0 < a.val := by simp omega⟩⟩ -/- The action of ℕ+ on a type with Mul where +/-- The action of ℕ+ on a type with Mul where nppowRec n a = a * a ⋯ * a (aka a^n)-/ def nppowRec [Mul α] : ℕ+ → α → α | 1, a => a @@ -28,13 +28,13 @@ def nppowRec [Mul α] : ℕ+ → α → α (nppowRec ⟨n+1, by simp⟩ a) * a termination_by x => x -/- A semigroup with an action of ℕ+ on it, by default it is exponentiation -/ +/-- A semigroup with an action of ℕ+ on it, by default it is exponentiation -/ class Semigroup' (α : Type u) extends Semigroup α where nppow : ℕ+ → α → α := nppowRec nppow_one : ∀ x, nppow 1 x = x := by intros; rfl nppow_succ : ∀ (n : ℕ+) (x), nppow (n+1) x = nppow n x * x -/- Use the exponentiation notation for the action of ℕ+ on a semigroup' -/ +/-- Define the exponentiation notation for the action of ℕ+ on a semigroup' -/ instance (α : Type u) [Semigroup' α] : Pow α ℕ+ := ⟨fun x n ↦ Semigroup'.nppow n x⟩ @@ -86,7 +86,7 @@ end LinearOrderedSemigroup class LinearOrderedCancelSemigroup (α : Type u) extends OrderedCancelSemigroup α, LinearOrder α instance (α : Type u) [LinearOrderedCancelSemigroup α] : LinearOrderedSemigroup α where - le_total := fun a b ↦ LinearOrderedCancelSemigroup.le_total a b + le_total := LinearOrderedCancelSemigroup.le_total decidableLE := LinearOrderedCancelSemigroup.decidableLE min_def := LinearOrderedCancelSemigroup.min_def max_def := LinearOrderedCancelSemigroup.max_def @@ -97,27 +97,33 @@ variable [LinearOrderedCancelSemigroup α] theorem LinearOrderedCancelSemigroup.mul_lt_mul_left (a b : α) (h : a < b) (c : α) : c * a < c * b := mul_lt_mul_left' h c -/- Every element of a LinearOrderedCancelSemigroup is either positive, negative, or zero. -/ +lemma pos_right_pos_forall {a b : α} (h : b * a > b) : is_positive a := by + intro x + have : b * a * x > b * x := mul_lt_mul_right' h x + simp [mul_assoc] at this + trivial + +lemma neg_right_neg_forall {a b : α} (h : b * a < b) : is_negative a := by + intro x + have : b * a * x < b * x := mul_lt_mul_right' h x + simp [mul_assoc] at this + trivial + +lemma zero_right_zero_forall {a b : α} (h : b * a = b) : is_zero a := by + intro x + have : b * a * x = b * x := congrFun (congrArg HMul.hMul h) x + simp [mul_assoc] at this + trivial + +/-- Every element of a LinearOrderedCancelSemigroup is either positive, negative, or zero. -/ theorem pos_neg_or_zero : ∀a : α, is_positive a ∨ is_negative a ∨ is_zero a := by intro a rcases le_total (a*a) a with ha | ha <;> rcases LE.le.lt_or_eq ha with ha | ha - · right; left; intro x; - have : a * a * x < a * x := mul_lt_mul_right' ha x - have : a * (a * x) < a * x := by rw [mul_assoc a a x] at this; trivial - simp at this; trivial - · right; right; intro x - have : a * a * x = a * x := congrFun (congrArg HMul.hMul ha) x - have : a * (a * x) = a * x := by rw [mul_assoc a a x] at this; trivial - simp at this; trivial - · left; intro x - have : a * x < a * a * x := mul_lt_mul_right' ha x - have : a * x < a * (a * x) := by rw [mul_assoc a a x] at this; trivial - simp at this; trivial - · right; right; intro x - have : a * a * x = a * x := congrFun (congrArg HMul.hMul (id (Eq.symm ha))) x - have : a * (a * x) = a * x := by rw [mul_assoc a a x] at this; trivial - simp at this; trivial + · right; left; exact neg_right_neg_forall ha + · right; right; exact zero_right_zero_forall ha + · left; exact pos_right_pos_forall ha + · right; right; exact zero_right_zero_forall ha.symm end LinearOrderedCancelSemigroup