Documentation

Mathlib.GroupTheory.Torsion

Torsion groups #

This file defines torsion groups, i.e. groups where all elements have finite order.

Main definitions #

Implementation #

All torsion monoids are really groups (which is proven here as Monoid.IsTorsion.group), but since the definition can be stated on monoids it is implemented on Monoid to match other declarations in the group theory library.

Tags #

periodic group, aperiodic group, torsion subgroup, torsion abelian group

Future work #

A predicate on an additive monoid saying that all elements are of finite order.

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    def Monoid.IsTorsion (G : Type u_1) [Monoid G] :

    A predicate on a monoid saying that all elements are of finite order.

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      @[simp]

      An additive monoid is not a torsion monoid if it has an element of infinite order.

      @[simp]

      A monoid is not a torsion monoid if it has an element of infinite order.

      theorem IsTorsion.addGroup.proof_2 {G : Type u_1} [AddMonoid G] :
      ∀ (a : G), zsmulRec 0 a = zsmulRec 0 a
      theorem IsTorsion.addGroup.proof_3 {G : Type u_1} [AddMonoid G] :
      ∀ (n : ) (a : G), zsmulRec (Int.ofNat n.succ) a = zsmulRec (Int.ofNat n.succ) a
      theorem IsTorsion.addGroup.proof_4 {G : Type u_1} [AddMonoid G] :
      ∀ (n : ) (a : G), zsmulRec (Int.negSucc n) a = zsmulRec (Int.negSucc n) a
      theorem IsTorsion.addGroup.proof_5 {G : Type u_1} [AddMonoid G] (tG : AddMonoid.IsTorsion G) (g : G) :
      -g + g = 0
      noncomputable def IsTorsion.addGroup {G : Type u_1} [AddMonoid G] (tG : AddMonoid.IsTorsion G) :

      Torsion additive monoids are really additive groups

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        theorem IsTorsion.addGroup.proof_1 {G : Type u_1} [AddMonoid G] :
        ∀ (a b : G), a - b = a - b
        noncomputable def IsTorsion.group {G : Type u_1} [Monoid G] (tG : Monoid.IsTorsion G) :

        Torsion monoids are really groups.

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          Subgroups of additive torsion groups are additive torsion groups.

          theorem IsTorsion.subgroup {G : Type u_1} [Group G] (tG : Monoid.IsTorsion G) (H : Subgroup G) :

          Subgroups of torsion groups are torsion groups.

          theorem AddIsTorsion.of_surjective {G : Type u_1} {H : Type u_2} [AddGroup G] [AddGroup H] {f : G →+ H} (hf : Function.Surjective f) (tG : AddMonoid.IsTorsion G) :

          The image of a surjective additive torsion group homomorphism is torsion.

          theorem IsTorsion.of_surjective {G : Type u_1} {H : Type u_2} [Group G] [Group H] {f : G →* H} (hf : Function.Surjective f) (tG : Monoid.IsTorsion G) :

          The image of a surjective torsion group homomorphism is torsion.

          theorem AddIsTorsion.extension_closed {G : Type u_1} {H : Type u_2} [AddGroup G] {N : AddSubgroup G} [AddGroup H] {f : G →+ H} (hN : N = f.ker) (tH : AddMonoid.IsTorsion H) (tN : AddMonoid.IsTorsion N) :

          Additive torsion groups are closed under extensions.

          theorem IsTorsion.extension_closed {G : Type u_1} {H : Type u_2} [Group G] {N : Subgroup G} [Group H] {f : G →* H} (hN : N = f.ker) (tH : Monoid.IsTorsion H) (tN : Monoid.IsTorsion N) :

          Torsion groups are closed under extensions.

          theorem AddIsTorsion.quotient_iff {G : Type u_1} {H : Type u_2} [AddGroup G] {N : AddSubgroup G} [AddGroup H] {f : G →+ H} (hf : Function.Surjective f) (hN : N = f.ker) (tN : AddMonoid.IsTorsion N) :

          The image of a quotient is additively torsion iff the group is torsion.

          theorem IsTorsion.quotient_iff {G : Type u_1} {H : Type u_2} [Group G] {N : Subgroup G} [Group H] {f : G →* H} (hf : Function.Surjective f) (hN : N = f.ker) (tN : Monoid.IsTorsion N) :

          The image of a quotient is torsion iff the group is torsion.

          If a group exponent exists, the group is additively torsion.

          If a group exponent exists, the group is torsion.

          theorem IsAddTorsion.exponentExists {G : Type u_1} [AddGroup G] (tG : AddMonoid.IsTorsion G) (bounded : (Set.range fun (g : G) => addOrderOf g).Finite) :

          The group exponent exists for any bounded additive torsion group.

          theorem IsTorsion.exponentExists {G : Type u_1} [Group G] (tG : Monoid.IsTorsion G) (bounded : (Set.range fun (g : G) => orderOf g).Finite) :

          The group exponent exists for any bounded torsion group.

          Finite additive groups are additive torsion groups.

          Finite groups are torsion groups.

          A module whose scalars are additively torsion is additively torsion.

          A module with a finite ring of scalars is additively torsion.

          The torsion submonoid of an additive commutative monoid.

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            The torsion submonoid of a commutative monoid.

            (Note that by Monoid.IsTorsion.group torsion monoids are truthfully groups.)

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              Additive torsion submonoids are additively torsion.

              Torsion submonoids are torsion.

              theorem AddCommMonoid.primaryComponent.proof_1 (G : Type u_1) [AddCommMonoid G] (p : ) [hp : Fact (Nat.Prime p)] :
              ∀ {a b : G}, a {g : G | ∃ (n : ), addOrderOf g = p ^ n}b {g : G | ∃ (n : ), addOrderOf g = p ^ n}∃ (k : ), addOrderOf (a + b) = p ^ k

              The p-primary component is the submonoid of elements with additive order prime-power of p.

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                theorem AddCommMonoid.primaryComponent.proof_2 (G : Type u_1) [AddCommMonoid G] (p : ) :
                ∃ (n : ), addOrderOf 0 = p ^ n
                @[simp]
                theorem CommMonoid.primaryComponent_coe (G : Type u_1) [CommMonoid G] (p : ) [hp : Fact (Nat.Prime p)] :
                (CommMonoid.primaryComponent G p) = {g : G | ∃ (n : ), orderOf g = p ^ n}
                @[simp]
                theorem AddCommMonoid.primaryComponent_coe (G : Type u_1) [AddCommMonoid G] (p : ) [hp : Fact (Nat.Prime p)] :
                (AddCommMonoid.primaryComponent G p) = {g : G | ∃ (n : ), addOrderOf g = p ^ n}
                def CommMonoid.primaryComponent (G : Type u_1) [CommMonoid G] (p : ) [hp : Fact (Nat.Prime p)] :

                The p-primary component is the submonoid of elements with order prime-power of p.

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                  Elements of the p-primary component have additive order p^n for some n

                  Elements of the p-primary component have order p^n for some n.

                  The p- and q-primary components are disjoint for p ≠ q.

                  theorem CommMonoid.primaryComponent.disjoint {G : Type u_1} [CommMonoid G] {p : } [hp : Fact (Nat.Prime p)] {p' : } [hp' : Fact (Nat.Prime p')] (hne : p p') :

                  The p- and q-primary components are disjoint for p ≠ q.

                  @[simp]

                  The additive torsion submonoid of an additive torsion monoid is .

                  @[simp]

                  The torsion submonoid of a torsion monoid is .

                  An additive torsion monoid is isomorphic to its torsion submonoid.

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                    A torsion monoid is isomorphic to its torsion submonoid.

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                      theorem Monoid.IsTorsion.torsionMulEquiv_apply {G : Type u_1} [CommMonoid G] (tG : Monoid.IsTorsion G) (a : (CommMonoid.torsion G)) :
                      tG.torsionMulEquiv a = ((MulEquiv.submonoidCongr ) a)
                      theorem AddMonoid.IsTorsion.torsionAddEquiv_symm_apply_coe {G : Type u_1} [AddCommMonoid G] (tG : AddMonoid.IsTorsion G) (a : G) :
                      tG.torsionAddEquiv.symm a = (AddSubmonoid.topEquiv.symm a),
                      theorem Monoid.IsTorsion.torsionMulEquiv_symm_apply_coe {G : Type u_1} [CommMonoid G] (tG : Monoid.IsTorsion G) (a : G) :
                      tG.torsionMulEquiv.symm a = (Submonoid.topEquiv.symm a),

                      Additive torsion submonoids of an additive torsion submonoid are isomorphic to the submonoid.

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                        Torsion submonoids of a torsion submonoid are isomorphic to the submonoid.

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                          theorem AddCommGroup.torsion.proof_1 (G : Type u_1) [AddCommGroup G] :
                          ∀ {x : G}, x (AddCommMonoid.addTorsion G).carrierIsOfFinAddOrder (-x)

                          The torsion subgroup of an additive abelian group.

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                            The torsion subgroup of an abelian group.

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                              The additive torsion submonoid of an abelian group equals the torsion subgroup as a submonoid.

                              The torsion submonoid of an abelian group equals the torsion subgroup as a submonoid.

                              The p-primary component is the subgroup of elements with additive order prime-power of p.

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                                @[simp]
                                theorem AddCommGroup.primaryComponent_coe (G : Type u_1) [AddCommGroup G] (p : ) [hp : Fact (Nat.Prime p)] :
                                (AddCommGroup.primaryComponent G p) = {g : G | ∃ (n : ), addOrderOf g = p ^ n}
                                @[simp]
                                theorem CommGroup.primaryComponent_coe (G : Type u_1) [CommGroup G] (p : ) [hp : Fact (Nat.Prime p)] :
                                (CommGroup.primaryComponent G p) = {g : G | ∃ (n : ), orderOf g = p ^ n}
                                def CommGroup.primaryComponent (G : Type u_1) [CommGroup G] (p : ) [hp : Fact (Nat.Prime p)] :

                                The p-primary component is the subgroup of elements with order prime-power of p.

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                                  The p-primary component is a p group.

                                  A predicate on an additive monoid saying that only 0 is of finite order.

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                                    A predicate on a monoid saying that only 1 is of finite order.

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                                      @[simp]

                                      An additive monoid is not torsion free if any nontrivial element has finite order.

                                      @[simp]

                                      A nontrivial monoid is not torsion-free if any nontrivial element has finite order.

                                      A nontrivial additive torsion group is not torsion-free.

                                      A nontrivial torsion group is not torsion-free.

                                      A nontrivial torsion-free additive group is not torsion.

                                      A nontrivial torsion-free group is not torsion.

                                      Subgroups of additive torsion-free groups are additively torsion-free.

                                      Subgroups of torsion-free groups are torsion-free.

                                      theorem AddMonoid.IsTorsionFree.prod {η : Type u_3} {Gs : ηType u_4} [(i : η) → AddGroup (Gs i)] (tfGs : ∀ (i : η), AddMonoid.IsTorsionFree (Gs i)) :
                                      AddMonoid.IsTorsionFree ((i : η) → Gs i)

                                      Direct products of additive torsion free groups are torsion free.

                                      theorem IsTorsionFree.prod {η : Type u_3} {Gs : ηType u_4} [(i : η) → Group (Gs i)] (tfGs : ∀ (i : η), Monoid.IsTorsionFree (Gs i)) :
                                      Monoid.IsTorsionFree ((i : η) → Gs i)

                                      Direct products of torsion free groups are torsion free.

                                      Quotienting a group by its additive torsion subgroup yields an additive torsion free group.

                                      Quotienting a group by its torsion subgroup yields a torsion free group.