(* This file is a part of IsarMathLib - a library of formalized mathematics for Isabelle/Isar. Copyright (C) 2005, 2006 Slawomir Kolodynski This program is free software; Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3. The name of the author may not be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. *) section ‹Groups 3› theory Group_ZF_3 imports Group_ZF_2 Finite1 begin text‹In this theory we consider notions in group theory that are useful for the construction of real numbers in the ‹Real_ZF_x› series of theories.› subsection‹Group valued finite range functions› text‹In this section show that the group valued functions $f : X\rightarrow G$, with the property that $f(X)$ is a finite subset of $G$, is a group. Such functions play an important role in the construction of real numbers in the ‹Real_ZF› series.› text‹The following proves the essential condition to show that the set of finite range functions is closed with respect to the lifted group operation.› lemma (in group0) Group_ZF_3_1_L1: assumes A1: "F = P {lifted to function space over} X" and A2: "s ∈ FinRangeFunctions(X,G)" "r ∈ FinRangeFunctions(X,G)" shows "F`⟨ s,r⟩ ∈ FinRangeFunctions(X,G)" proof - let ?q = "F`⟨ s,r⟩" from A2 have T1:"s:X→G" "r:X→G" using FinRangeFunctions_def by auto with A1 have T2:"?q : X→G" using group0_2_L1 monoid0.Group_ZF_2_1_L0 by simp moreover have "?q``(X) ∈ Fin(G)" proof - from A2 have "{s`(x). x∈X} ∈ Fin(G)" "{r`(x). x∈X} ∈ Fin(G)" using Finite1_L18 by auto with A1 T1 T2 show ?thesis using group_oper_fun Finite1_L15 Group_ZF_2_1_L3 func_imagedef by simp qed ultimately show ?thesis using FinRangeFunctions_def by simp qed text‹The set of group valued finite range functions is closed with respect to the lifted group operation.› lemma (in group0) Group_ZF_3_1_L2: assumes A1: "F = P {lifted to function space over} X" shows "FinRangeFunctions(X,G) {is closed under} F" proof - let ?A = "FinRangeFunctions(X,G)" from A1 have "∀x∈?A. ∀y∈?A. F`⟨ x,y⟩ ∈ ?A" using Group_ZF_3_1_L1 by simp then show ?thesis using IsOpClosed_def by simp qed text‹A composition of a finite range function with the group inverse is a finite range function.› lemma (in group0) Group_ZF_3_1_L3: assumes A1: "s ∈ FinRangeFunctions(X,G)" shows "GroupInv(G,P) O s ∈ FinRangeFunctions(X,G)" using groupAssum assms group0_2_T2 Finite1_L20 by simp text‹The set of finite range functions is s subgroup of the lifted group.› theorem Group_ZF_3_1_T1: assumes A1: "IsAgroup(G,P)" and A2: "F = P {lifted to function space over} X" and A3: "X≠0" shows "IsAsubgroup(FinRangeFunctions(X,G),F)" proof - let ?e = "TheNeutralElement(G,P)" let ?S = "FinRangeFunctions(X,G)" from A1 have T1: "group0(G,P)" using group0_def by simp with A1 A2 have T2:"group0(X→G,F)" using group0.Group_ZF_2_1_T2 group0_def by simp moreover have "?S ≠ 0" proof - from T1 A3 have "ConstantFunction(X,?e) ∈ ?S" using group0.group0_2_L1 monoid0.unit_is_neutral Finite1_L17 by simp thus ?thesis by auto qed moreover have "?S ⊆ X→G" using FinRangeFunctions_def by auto moreover from A2 T1 have "?S {is closed under} F" using group0.Group_ZF_3_1_L2 by simp moreover from A1 A2 T1 have "∀s ∈ ?S. GroupInv(X→G,F)`(s) ∈ ?S" using FinRangeFunctions_def group0.Group_ZF_2_1_L6 group0.Group_ZF_3_1_L3 by simp ultimately show ?thesis using group0.group0_3_T3 by simp qed subsection‹Almost homomorphisms› text‹An almost homomorphism is a group valued function defined on a monoid $M$ with the property that the set $\{ f(m+n)-f(m)-f(n)\}_{m,n \in M}$ is finite. This term is used by R. D. Arthan in "The Eudoxus Real Numbers". We use this term in the general group context and use the A`Campo's term "slopes" (see his "A natural construction for the real numbers") to mean an almost homomorphism mapping interegers into themselves. We consider almost homomorphisms because we use slopes to define real numbers in the ‹Real_ZF_x› series.› text‹HomDiff is an acronym for "homomorphism difference". This is the expression $s(mn)(s(m)s(n))^{-1}$, or $s(m+n)-s(m)-s(n)$ in the additive notation. It is equal to the neutral element of the group if $s$ is a homomorphism. › definition "HomDiff(G,f,s,x) ≡ f`⟨s`(f`⟨ fst(x),snd(x)⟩) , (GroupInv(G,f)`(f`⟨ s`(fst(x)),s`(snd(x))⟩))⟩" text‹Almost homomorphisms are defined as those maps $s:G\rightarrow G$ such that the homomorphism difference takes only finite number of values on $G\times G$. › definition "AlmostHoms(G,f) ≡ {s ∈ G→G.{HomDiff(G,f,s,x). x ∈ G×G } ∈ Fin(G)}" text‹AlHomOp1$(G,f)$ is the group operation on almost homomorphisms defined in a natural way by $(s\cdot r)(n) = s(n)\cdot r(n)$. In the terminology defined in func1.thy this is the group operation $f$ (on $G$) lifted to the function space $G\rightarrow G$ and restricted to the set AlmostHoms$(G,f)$.› definition "AlHomOp1(G,f) ≡ restrict(f {lifted to function space over} G, AlmostHoms(G,f)×AlmostHoms(G,f))" text‹We also define a composition (binary) operator on almost homomorphisms in a natural way. We call that operator ‹AlHomOp2› - the second operation on almost homomorphisms. Composition of almost homomorphisms is used to define multiplication of real numbers in ‹Real_ZF› series. › definition "AlHomOp2(G,f) ≡ restrict(Composition(G),AlmostHoms(G,f)×AlmostHoms(G,f))" text‹This lemma provides more readable notation for the HomDiff definition. Not really intended to be used in proofs, but just to see the definition in the notation defined in the group0 locale.› lemma (in group0) HomDiff_notation: shows "HomDiff(G,P,s,⟨ m,n⟩) = s`(m⋅n)⋅(s`(m)⋅s`(n))¯" using HomDiff_def by simp text‹The next lemma shows the set from the definition of almost homomorphism in a different form.› lemma (in group0) Group_ZF_3_2_L1A: shows "{HomDiff(G,P,s,x). x ∈ G×G } = {s`(m⋅n)⋅(s`(m)⋅s`(n))¯. ⟨ m,n⟩ ∈ G×G}" proof - have "∀m∈G.∀n∈G. HomDiff(G,P,s,⟨ m,n⟩) = s`(m⋅n)⋅(s`(m)⋅s`(n))¯" using HomDiff_notation by simp then show ?thesis by (rule ZF1_1_L4A) qed text‹Let's define some notation. We inherit the notation and assumptions from the group0 context (locale) and add some. We will use AH to denote the set of almost homomorphisms. $\sim$ is the inverse (negative if the group is the group of integers) of almost homomorphisms, $(\sim p)(n)= p(n)^{-1}$. $\delta $ will denote the homomorphism difference specific for the group (HomDiff$(G,f)$). The notation $s \approx r$ will mean that $s,r$ are almost equal, that is they are in the equivalence relation defined by the group of finite range functions (that is a normal subgroup of almost homomorphisms, if the group is abelian). We show that this is equivalent to the set $\{ s(n)\cdot r(n)^{-1}: n\in G\}$ being finite. We also add an assumption that the $G$ is abelian as many needed properties do not hold without that.› locale group1 = group0 + assumes isAbelian: "P {is commutative on} G" fixes AH defines AH_def [simp]: "AH ≡ AlmostHoms(G,P)" fixes Op1 defines Op1_def [simp]: "Op1 ≡ AlHomOp1(G,P)" fixes Op2 defines Op2_def [simp]: "Op2 ≡ AlHomOp2(G,P)" fixes FR defines FR_def [simp]: "FR ≡ FinRangeFunctions(G,G)" fixes neg ("∼_" [90] 91) defines neg_def [simp]: "∼s ≡ GroupInv(G,P) O s" fixes δ defines δ_def [simp]: "δ(s,x) ≡ HomDiff(G,P,s,x)" fixes AHprod (infix "∙" 69) defines AHprod_def [simp]: "s ∙ r ≡ AlHomOp1(G,P)`⟨s,r⟩" fixes AHcomp (infix "∘" 70) defines AHcomp_def [simp]: "s ∘ r ≡ AlHomOp2(G,P)`⟨s,r⟩" fixes AlEq (infix "≅" 68) defines AlEq_def [simp]: "s ≅ r ≡ ⟨s,r⟩ ∈ QuotientGroupRel(AH,Op1,FR)" text‹HomDiff is a homomorphism on the lifted group structure.› lemma (in group1) Group_ZF_3_2_L1: assumes A1: "s:G→G" "r:G→G" and A2: "x ∈ G×G" and A3: "F = P {lifted to function space over} G" shows "δ(F`⟨ s,r⟩,x) = δ(s,x)⋅δ(r,x)" proof - let ?p = "F`⟨ s,r⟩" from A2 obtain m n where D1: "x = ⟨ m,n⟩" "m∈G" "n∈G" by auto then have T1:"m⋅n ∈ G" using group0_2_L1 monoid0.group0_1_L1 by simp with A1 D1 have T2: "s`(m)∈G" "s`(n)∈G" "r`(m)∈G" "r`(n)∈G" "s`(m⋅n)∈G" "r`(m⋅n)∈G" using apply_funtype by auto from A3 A1 have T3:"?p : G→G" using group0_2_L1 monoid0.Group_ZF_2_1_L0 by simp from D1 T3 have "δ(?p,x) = ?p`(m⋅n)⋅((?p`(n))¯⋅(?p`(m))¯)" using HomDiff_notation apply_funtype group_inv_of_two by simp also from A3 A1 D1 T1 isAbelian T2 have "… = δ(s,x)⋅ δ(r,x)" using Group_ZF_2_1_L3 group0_4_L3 HomDiff_notation by simp finally show ?thesis by simp qed text‹The group operation lifted to the function space over $G$ preserves almost homomorphisms.› lemma (in group1) Group_ZF_3_2_L2: assumes A1: "s ∈ AH" "r ∈ AH" and A2: "F = P {lifted to function space over} G" shows "F`⟨ s,r⟩ ∈ AH" proof - let ?p = "F`⟨ s,r⟩" from A1 A2 have "?p : G→G" using AlmostHoms_def group0_2_L1 monoid0.Group_ZF_2_1_L0 by simp moreover have "{δ(?p,x). x ∈ G×G} ∈ Fin(G)" proof - from A1 have "{δ(s,x). x ∈ G×G } ∈ Fin(G)" "{δ(r,x). x ∈ G×G } ∈ Fin(G)" using AlmostHoms_def by auto with groupAssum A1 A2 show ?thesis using IsAgroup_def IsAmonoid_def IsAssociative_def Finite1_L15 AlmostHoms_def Group_ZF_3_2_L1 by auto qed ultimately show ?thesis using AlmostHoms_def by simp qed text‹The set of almost homomorphisms is closed under the lifted group operation.› lemma (in group1) Group_ZF_3_2_L3: assumes "F = P {lifted to function space over} G" shows "AH {is closed under} F" using assms IsOpClosed_def Group_ZF_3_2_L2 by simp text‹The terms in the homomorphism difference for a function are in the group.› lemma (in group1) Group_ZF_3_2_L4: assumes "s:G→G" and "m∈G" "n∈G" shows "m⋅n ∈ G" "s`(m⋅n) ∈ G" "s`(m) ∈ G" "s`(n) ∈ G" "δ(s,⟨ m,n⟩) ∈ G" "s`(m)⋅s`(n) ∈ G" using assms group_op_closed inverse_in_group apply_funtype HomDiff_def by auto text‹It is handy to have a version of ‹ Group_ZF_3_2_L4› specifically for almost homomorphisms.› corollary (in group1) Group_ZF_3_2_L4A: assumes "s ∈ AH" and "m∈G" "n∈G" shows "m⋅n ∈ G" "s`(m⋅n) ∈ G" "s`(m) ∈ G" "s`(n) ∈ G" "δ(s,⟨ m,n⟩) ∈ G" "s`(m)⋅s`(n) ∈ G" using assms AlmostHoms_def Group_ZF_3_2_L4 by auto text‹The terms in the homomorphism difference are in the group, a different form.› lemma (in group1) Group_ZF_3_2_L4B: assumes A1:"s ∈ AH" and A2:"x∈G×G" shows "fst(x)⋅snd(x) ∈ G" "s`(fst(x)⋅snd(x)) ∈ G" "s`(fst(x)) ∈ G" "s`(snd(x)) ∈ G" "δ(s,x) ∈ G" "s`(fst(x))⋅s`(snd(x)) ∈ G" proof - let ?m = "fst(x)" let ?n = "snd(x)" from A1 A2 show "?m⋅?n ∈ G" "s`(?m⋅?n) ∈ G" "s`(?m) ∈ G" "s`(?n) ∈ G" "s`(?m)⋅s`(?n) ∈ G" using Group_ZF_3_2_L4A by auto from A1 A2 have "δ(s,⟨ ?m,?n⟩) ∈ G" using Group_ZF_3_2_L4A by simp moreover from A2 have "⟨ ?m,?n⟩ = x" by auto ultimately show "δ(s,x) ∈ G" by simp qed text‹What are the values of the inverse of an almost homomorphism?› lemma (in group1) Group_ZF_3_2_L5: assumes "s ∈ AH" and "n∈G" shows "(∼s)`(n) = (s`(n))¯" using assms AlmostHoms_def comp_fun_apply by auto text‹Homomorphism difference commutes with the inverse for almost homomorphisms.› lemma (in group1) Group_ZF_3_2_L6: assumes A1:"s ∈ AH" and A2:"x∈G×G" shows "δ(∼s,x) = (δ(s,x))¯" proof - let ?m = "fst(x)" let ?n = "snd(x)" have "δ(∼s,x) = (∼s)`(?m⋅?n)⋅((∼s)`(?m)⋅(∼s)`(?n))¯" using HomDiff_def by simp from A1 A2 isAbelian show ?thesis using Group_ZF_3_2_L4B HomDiff_def Group_ZF_3_2_L5 group0_4_L4A by simp qed text‹The inverse of an almost homomorphism maps the group into itself.› lemma (in group1) Group_ZF_3_2_L7: assumes "s ∈ AH" shows "∼s : G→G" using groupAssum assms AlmostHoms_def group0_2_T2 comp_fun by auto text‹The inverse of an almost homomorphism is an almost homomorphism.› lemma (in group1) Group_ZF_3_2_L8: assumes A1: "F = P {lifted to function space over} G" and A2: "s ∈ AH" shows "GroupInv(G→G,F)`(s) ∈ AH" proof - from A2 have "{δ(s,x). x ∈ G×G} ∈ Fin(G)" using AlmostHoms_def by simp with groupAssum have "GroupInv(G,P)``{δ(s,x). x ∈ G×G} ∈ Fin(G)" using group0_2_T2 Finite1_L6A by blast moreover have "GroupInv(G,P)``{δ(s,x). x ∈ G×G} = {(δ(s,x))¯. x ∈ G×G}" proof - from groupAssum have "GroupInv(G,P) : G→G" using group0_2_T2 by simp moreover from A2 have "∀x∈G×G. δ(s,x)∈G" using Group_ZF_3_2_L4B by simp ultimately show ?thesis using func1_1_L17 by simp qed ultimately have "{(δ(s,x))¯. x ∈ G×G} ∈ Fin(G)" by simp moreover from A2 have "{(δ(s,x))¯. x ∈ G×G} = {δ(∼s,x). x ∈ G×G}" using Group_ZF_3_2_L6 by simp ultimately have "{δ(∼s,x). x ∈ G×G} ∈ Fin(G)" by simp with A2 groupAssum A1 show ?thesis using Group_ZF_3_2_L7 AlmostHoms_def Group_ZF_2_1_L6 by simp qed text‹The function that assigns the neutral element everywhere is an almost homomorphism.› lemma (in group1) Group_ZF_3_2_L9: shows "ConstantFunction(G,𝟭) ∈ AH" and "AH≠0" proof - let ?z = "ConstantFunction(G,𝟭)" have "G×G≠0" using group0_2_L1 monoid0.group0_1_L3A by blast moreover have "∀x∈G×G. δ(?z,x) = 𝟭" proof fix x assume A1:"x ∈ G × G" then obtain m n where "x = ⟨ m,n⟩" "m∈G" "n∈G" by auto then show "δ(?z,x) = 𝟭" using group0_2_L1 monoid0.group0_1_L1 func1_3_L2 HomDiff_def group0_2_L2 group_inv_of_one by simp qed ultimately have "{δ(?z,x). x∈G×G} = {𝟭}" by (rule ZF1_1_L5) then show "?z ∈ AH" using group0_2_L2 Finite1_L16 func1_3_L1 group0_2_L2 AlmostHoms_def by simp then show "AH≠0" by auto qed text‹If the group is abelian, then almost homomorphisms form a subgroup of the lifted group.› lemma Group_ZF_3_2_L10: assumes A1: "IsAgroup(G,P)" and A2: "P {is commutative on} G" and A3: "F = P {lifted to function space over} G" shows "IsAsubgroup(AlmostHoms(G,P),F)" proof - let ?AH = "AlmostHoms(G,P)" from A2 A1 have T1: "group1(G,P)" using group1_axioms.intro group0_def group1_def by simp from A1 A3 have "group0(G→G,F)" using group0_def group0.Group_ZF_2_1_T2 by simp moreover from T1 have "?AH≠0" using group1.Group_ZF_3_2_L9 by simp moreover have T2:"?AH ⊆ G→G" using AlmostHoms_def by auto moreover from T1 A3 have "?AH {is closed under} F" using group1.Group_ZF_3_2_L3 by simp moreover from T1 A3 have "∀s∈?AH. GroupInv(G→G,F)`(s) ∈ ?AH" using group1.Group_ZF_3_2_L8 by simp ultimately show "IsAsubgroup(AlmostHoms(G,P),F)" using group0.group0_3_T3 by simp qed text‹If the group is abelian, then almost homomorphisms form a group with the first operation, hence we can use theorems proven in group0 context aplied to this group.› lemma (in group1) Group_ZF_3_2_L10A: shows "IsAgroup(AH,Op1)" "group0(AH,Op1)" using groupAssum isAbelian Group_ZF_3_2_L10 IsAsubgroup_def AlHomOp1_def group0_def by auto text‹The group of almost homomorphisms is abelian› lemma Group_ZF_3_2_L11: assumes A1: "IsAgroup(G,f)" and A2: "f {is commutative on} G" shows "IsAgroup(AlmostHoms(G,f),AlHomOp1(G,f))" "AlHomOp1(G,f) {is commutative on} AlmostHoms(G,f)" proof- let ?AH = "AlmostHoms(G,f)" let ?F = "f {lifted to function space over} G" from A1 A2 have "IsAsubgroup(?AH,?F)" using Group_ZF_3_2_L10 by simp then show "IsAgroup(?AH,AlHomOp1(G,f))" using IsAsubgroup_def AlHomOp1_def by simp from A1 have "?F : (G→G)×(G→G)→(G→G)" using IsAgroup_def monoid0_def monoid0.Group_ZF_2_1_L0A by simp moreover have "?AH ⊆ G→G" using AlmostHoms_def by auto moreover from A1 A2 have "?F {is commutative on} (G→G)" using group0_def group0.Group_ZF_2_1_L7 by simp ultimately show "AlHomOp1(G,f){is commutative on} ?AH" using func_ZF_4_L1 AlHomOp1_def by simp qed text‹The first operation on homomorphisms acts in a natural way on its operands.› lemma (in group1) Group_ZF_3_2_L12: assumes "s∈AH" "r∈AH" and "n∈G" shows "(s∙r)`(n) = s`(n)⋅r`(n)" using assms AlHomOp1_def restrict AlmostHoms_def Group_ZF_2_1_L3 by simp text‹What is the group inverse in the group of almost homomorphisms?› lemma (in group1) Group_ZF_3_2_L13: assumes A1: "s∈AH" shows "GroupInv(AH,Op1)`(s) = GroupInv(G,P) O s" "GroupInv(AH,Op1)`(s) ∈ AH" "GroupInv(G,P) O s ∈ AH" proof - let ?F = "P {lifted to function space over} G" from groupAssum isAbelian have "IsAsubgroup(AH,?F)" using Group_ZF_3_2_L10 by simp with A1 show I: "GroupInv(AH,Op1)`(s) = GroupInv(G,P) O s" using AlHomOp1_def Group_ZF_2_1_L6A by simp from A1 show "GroupInv(AH,Op1)`(s) ∈ AH" using Group_ZF_3_2_L10A group0.inverse_in_group by simp with I show "GroupInv(G,P) O s ∈ AH" by simp qed text‹The group inverse in the group of almost homomorphisms acts in a natural way on its operand.› lemma (in group1) Group_ZF_3_2_L14: assumes "s∈AH" and "n∈G" shows "(GroupInv(AH,Op1)`(s))`(n) = (s`(n))¯" using isAbelian assms Group_ZF_3_2_L13 AlmostHoms_def comp_fun_apply by auto text‹The next lemma states that if $s,r$ are almost homomorphisms, then $s\cdot r^{-1}$ is also an almost homomorphism.› lemma Group_ZF_3_2_L15: assumes "IsAgroup(G,f)" and "f {is commutative on} G" and "AH = AlmostHoms(G,f)" "Op1 = AlHomOp1(G,f)" and "s ∈ AH" "r ∈ AH" shows "Op1`⟨ s,r⟩ ∈ AH" "GroupInv(AH,Op1)`(r) ∈ AH" "Op1`⟨ s,GroupInv(AH,Op1)`(r)⟩ ∈ AH" using assms group0_def group1_axioms.intro group1_def group1.Group_ZF_3_2_L10A group0.group0_2_L1 monoid0.group0_1_L1 group0.inverse_in_group by auto text‹A version of ‹Group_ZF_3_2_L15› formulated in notation used in ‹group1› context. States that the product of almost homomorphisms is an almost homomorphism and the the product of an almost homomorphism with a (pointwise) inverse of an almost homomorphism is an almost homomorphism.› corollary (in group1) Group_ZF_3_2_L16: assumes "s ∈ AH" "r ∈ AH" shows "s∙r ∈ AH" "s∙(∼r) ∈ AH" using assms isAbelian group0_def group1_axioms group1_def Group_ZF_3_2_L15 Group_ZF_3_2_L13 by auto subsection‹The classes of almost homomorphisms› text‹In the ‹Real_ZF› series we define real numbers as a quotient of the group of integer almost homomorphisms by the integer finite range functions. In this section we setup the background for that in the general group context.› text‹Finite range functions are almost homomorphisms.› lemma (in group1) Group_ZF_3_3_L1: shows "FR ⊆ AH" proof fix s assume A1:"s ∈ FR" then have T1:"{s`(n). n ∈ G} ∈ Fin(G)" "{s`(fst(x)). x∈G×G} ∈ Fin(G)" "{s`(snd(x)). x∈G×G} ∈ Fin(G)" using Finite1_L18 Finite1_L6B by auto have "{s`(fst(x)⋅snd(x)). x ∈ G×G} ∈ Fin(G)" proof - have "∀x∈G×G. fst(x)⋅snd(x) ∈ G" using group0_2_L1 monoid0.group0_1_L1 by simp moreover from T1 have "{s`(n). n ∈ G} ∈ Fin(G)" by simp ultimately show ?thesis by (rule Finite1_L6B) qed moreover have "{(s`(fst(x))⋅s`(snd(x)))¯. x∈G×G} ∈ Fin(G)" proof - have "∀g∈G. g¯ ∈ G" using inverse_in_group by simp moreover from T1 have "{s`(fst(x))⋅s`(snd(x)). x∈G×G} ∈ Fin(G)" using group_oper_fun Finite1_L15 by simp ultimately show ?thesis by (rule Finite1_L6C) qed ultimately have "{δ(s,x). x∈G×G} ∈ Fin(G)" using HomDiff_def Finite1_L15 group_oper_fun by simp with A1 show "s ∈ AH" using FinRangeFunctions_def AlmostHoms_def by simp qed text‹Finite range functions valued in an abelian group form a normal subgroup of almost homomorphisms.› lemma Group_ZF_3_3_L2: assumes A1:"IsAgroup(G,f)" and A2:"f {is commutative on} G" shows "IsAsubgroup(FinRangeFunctions(G,G),AlHomOp1(G,f))" "IsAnormalSubgroup(AlmostHoms(G,f),AlHomOp1(G,f), FinRangeFunctions(G,G))" proof - let ?H1 = "AlmostHoms(G,f)" let ?H2 = "FinRangeFunctions(G,G)" let ?F = "f {lifted to function space over} G" from A1 A2 have T1:"group0(G,f)" "monoid0(G,f)" "group1(G,f)" using group0_def group0.group0_2_L1 group1_axioms.intro group1_def by auto with A1 A2 have "IsAgroup(G→G,?F)" "IsAsubgroup(?H1,?F)" "IsAsubgroup(?H2,?F)" using group0.Group_ZF_2_1_T2 Group_ZF_3_2_L10 monoid0.group0_1_L3A Group_ZF_3_1_T1 by auto then have "IsAsubgroup(?H1∩?H2,restrict(?F,?H1×?H1))" using group0_3_L7 by simp moreover from T1 have "?H1∩?H2 = ?H2" using group1.Group_ZF_3_3_L1 by auto ultimately show "IsAsubgroup(?H2,AlHomOp1(G,f))" using AlHomOp1_def by simp with A1 A2 show "IsAnormalSubgroup(AlmostHoms(G,f),AlHomOp1(G,f), FinRangeFunctions(G,G))" using Group_ZF_3_2_L11 Group_ZF_2_4_L6 by simp qed text‹The group of almost homomorphisms divided by the subgroup of finite range functions is an abelian group.› theorem (in group1) Group_ZF_3_3_T1: shows "IsAgroup(AH//QuotientGroupRel(AH,Op1,FR),QuotientGroupOp(AH,Op1,FR))" and "QuotientGroupOp(AH,Op1,FR) {is commutative on} (AH//QuotientGroupRel(AH,Op1,FR))" using groupAssum isAbelian Group_ZF_3_3_L2 Group_ZF_3_2_L10A Group_ZF_2_4_T1 Group_ZF_3_2_L10A Group_ZF_3_2_L11 Group_ZF_3_3_L2 IsAnormalSubgroup_def Group_ZF_2_4_L6 by auto text‹It is useful to have a direct statement that the quotient group relation is an equivalence relation for the group of AH and subgroup FR.› lemma (in group1) Group_ZF_3_3_L3: shows "QuotientGroupRel(AH,Op1,FR) ⊆ AH × AH" and "equiv(AH,QuotientGroupRel(AH,Op1,FR))" using groupAssum isAbelian QuotientGroupRel_def Group_ZF_3_3_L2 Group_ZF_3_2_L10A group0.Group_ZF_2_4_L3 by auto text‹The "almost equal" relation is symmetric.› lemma (in group1) Group_ZF_3_3_L3A: assumes A1: "s≅r" shows "r≅s" proof - let ?R = "QuotientGroupRel(AH,Op1,FR)" from A1 have "equiv(AH,?R)" and "⟨s,r⟩ ∈ ?R" using Group_ZF_3_3_L3 by simp_all then have "⟨r,s⟩ ∈ ?R" by (rule equiv_is_sym) then show "r≅s" by simp qed text‹Although we have bypassed this fact when proving that group of almost homomorphisms divided by the subgroup of finite range functions is a group, it is still useful to know directly that the first group operation on AH is congruent with respect to the quotient group relation.› lemma (in group1) Group_ZF_3_3_L4: shows "Congruent2(QuotientGroupRel(AH,Op1,FR),Op1)" using groupAssum isAbelian Group_ZF_3_2_L10A Group_ZF_3_3_L2 Group_ZF_2_4_L5A by simp text‹The class of an almost homomorphism $s$ is the neutral element of the quotient group of almost homomorphisms iff $s$ is a finite range function.› lemma (in group1) Group_ZF_3_3_L5: assumes "s ∈ AH" and "r = QuotientGroupRel(AH,Op1,FR)" and "TheNeutralElement(AH//r,QuotientGroupOp(AH,Op1,FR)) = e" shows "r``{s} = e ⟷ s ∈ FR" using groupAssum isAbelian assms Group_ZF_3_2_L11 group0_def Group_ZF_3_3_L2 group0.Group_ZF_2_4_L5E by simp text‹The group inverse of a class of an almost homomorphism $f$ is the class of the inverse of $f$.› lemma (in group1) Group_ZF_3_3_L6: assumes A1: "s ∈ AH" and "r = QuotientGroupRel(AH,Op1,FR)" and "F = ProjFun2(AH,r,Op1)" shows "r``{∼s} = GroupInv(AH//r,F)`(r``{s})" proof - from groupAssum isAbelian assms have "r``{GroupInv(AH, Op1)`(s)} = GroupInv(AH//r,F)`(r `` {s})" using Group_ZF_3_2_L10A Group_ZF_3_3_L2 QuotientGroupOp_def group0.Group_ZF_2_4_L7 by simp with A1 show ?thesis using Group_ZF_3_2_L13 by simp qed subsection‹Compositions of almost homomorphisms› text‹The goal of this section is to establish some facts about composition of almost homomorphisms that are needed for the real numbers construction in ‹Real_ZF_x› series. In particular we show that the set of almost homomorphisms is closed under composition and that composition is congruent with respect to the equivalence relation defined by the group of finite range functions (a normal subgroup of almost homomorphisms).› text‹The next formula restates the definition of the homomorphism difference to express the value an almost homomorphism on a product.› lemma (in group1) Group_ZF_3_4_L1: assumes "s∈AH" and "m∈G" "n∈G" shows "s`(m⋅n) = s`(m)⋅s`(n)⋅δ(s,⟨ m,n⟩)" using isAbelian assms Group_ZF_3_2_L4A HomDiff_def group0_4_L5 by simp text‹What is the value of a composition of almost homomorhisms?› lemma (in group1) Group_ZF_3_4_L2: assumes "s∈AH" "r∈AH" and "m∈G" shows "(s∘r)`(m) = s`(r`(m))" "s`(r`(m)) ∈ G" using assms AlmostHoms_def func_ZF_5_L3 restrict AlHomOp2_def apply_funtype by auto text‹What is the homomorphism difference of a composition?› lemma (in group1) Group_ZF_3_4_L3: assumes A1: "s∈AH" "r∈AH" and A2: "m∈G" "n∈G" shows "δ(s∘r,⟨ m,n⟩) = δ(s,⟨ r`(m),r`(n)⟩)⋅s`(δ(r,⟨ m,n⟩))⋅δ(s,⟨ r`(m)⋅r`(n),δ(r,⟨ m,n⟩)⟩)" proof - from A1 A2 have T1: "s`(r`(m))⋅ s`(r`(n)) ∈ G" "δ(s,⟨ r`(m),r`(n)⟩)∈ G" "s`(δ(r,⟨ m,n⟩)) ∈G" "δ(s,⟨ (r`(m)⋅r`(n)),δ(r,⟨ m,n⟩)⟩) ∈ G" using Group_ZF_3_4_L2 AlmostHoms_def apply_funtype Group_ZF_3_2_L4A group0_2_L1 monoid0.group0_1_L1 by auto from A1 A2 have "δ(s∘r,⟨ m,n⟩) = s`(r`(m)⋅r`(n)⋅δ(r,⟨ m,n⟩))⋅(s`((r`(m)))⋅s`(r`(n)))¯" using HomDiff_def group0_2_L1 monoid0.group0_1_L1 Group_ZF_3_4_L2 Group_ZF_3_4_L1 by simp moreover from A1 A2 have "s`(r`(m)⋅r`(n)⋅δ(r,⟨ m,n⟩)) = s`(r`(m)⋅r`(n))⋅s`(δ(r,⟨ m,n⟩))⋅δ(s,⟨ (r`(m)⋅r`(n)),δ(r,⟨ m,n⟩)⟩)" "s`(r`(m)⋅r`(n)) = s`(r`(m))⋅s`(r`(n))⋅δ(s,⟨ r`(m),r`(n)⟩)" using Group_ZF_3_2_L4A Group_ZF_3_4_L1 by auto moreover from T1 isAbelian have "s`(r`(m))⋅s`(r`(n))⋅δ(s,⟨ r`(m),r`(n)⟩)⋅ s`(δ(r,⟨ m,n⟩))⋅δ(s,⟨ (r`(m)⋅r`(n)),δ(r,⟨ m,n⟩)⟩)⋅ (s`((r`(m)))⋅s`(r`(n)))¯ = δ(s,⟨ r`(m),r`(n)⟩)⋅s`(δ(r,⟨ m,n⟩))⋅δ(s,⟨ (r`(m)⋅r`(n)),δ(r,⟨ m,n⟩)⟩)" using group0_4_L6C by simp ultimately show ?thesis by simp qed text‹What is the homomorphism difference of a composition (another form)? Here we split the homomorphism difference of a composition into a product of three factors. This will help us in proving that the range of homomorphism difference for the composition is finite, as each factor has finite range.› lemma (in group1) Group_ZF_3_4_L4: assumes A1: "s∈AH" "r∈AH" and A2: "x ∈ G×G" and A3: "A = δ(s,⟨ r`(fst(x)),r`(snd(x))⟩)" "B = s`(δ(r,x))" "C = δ(s,⟨ (r`(fst(x))⋅r`(snd(x))),δ(r,x)⟩)" shows "δ(s∘r,x) = A⋅B⋅C" proof - let ?m = "fst(x)" let ?n = "snd(x)" note A1 moreover from A2 have "?m∈G" "?n∈G" by auto ultimately have "δ(s∘r,⟨ ?m,?n⟩) = δ(s,⟨ r`(?m),r`(?n)⟩)⋅s`(δ(r,⟨ ?m,?n⟩))⋅ δ(s,⟨ (r`(?m)⋅r`(?n)),δ(r,⟨ ?m,?n⟩)⟩)" by (rule Group_ZF_3_4_L3) with A1 A2 A3 show ?thesis by auto qed text‹The range of the homomorphism difference of a composition of two almost homomorphisms is finite. This is the essential condition to show that a composition of almost homomorphisms is an almost homomorphism.› lemma (in group1) Group_ZF_3_4_L5: assumes A1: "s∈AH" "r∈AH" shows "{δ(Composition(G)`⟨ s,r⟩,x). x ∈ G×G} ∈ Fin(G)" proof - from A1 have "∀x∈G×G. ⟨ r`(fst(x)),r`(snd(x))⟩ ∈ G×G" using Group_ZF_3_2_L4B by simp moreover from A1 have "{δ(s,x). x∈G×G} ∈ Fin(G)" using AlmostHoms_def by simp ultimately have "{δ(s,⟨ r`(fst(x)),r`(snd(x))⟩). x∈G×G} ∈ Fin(G)" by (rule Finite1_L6B) moreover have "{s`(δ(r,x)). x∈G×G} ∈ Fin(G)" proof - from A1 have "∀m∈G. s`(m) ∈ G" using AlmostHoms_def apply_funtype by auto moreover from A1 have "{δ(r,x). x∈G×G} ∈ Fin(G)" using AlmostHoms_def by simp ultimately show ?thesis by (rule Finite1_L6C) qed ultimately have "{δ(s,⟨ r`(fst(x)),r`(snd(x))⟩)⋅s`(δ(r,x)). x∈G×G} ∈ Fin(G)" using group_oper_fun Finite1_L15 by simp moreover have "{δ(s,⟨ (r`(fst(x))⋅r`(snd(x))),δ(r,x)⟩). x∈G×G} ∈ Fin(G)" proof - from A1 have "∀x∈G×G. ⟨ (r`(fst(x))⋅r`(snd(x))),δ(r,x)⟩ ∈ G×G" using Group_ZF_3_2_L4B by simp moreover from A1 have "{δ(s,x). x∈G×G} ∈ Fin(G)" using AlmostHoms_def by simp ultimately show ?thesis by (rule Finite1_L6B) qed ultimately have "{δ(s,⟨ r`(fst(x)),r`(snd(x))⟩)⋅s`(δ(r,x))⋅ δ(s,⟨ (r`(fst(x))⋅r`(snd(x))),δ(r,x)⟩). x∈G×G} ∈ Fin(G)" using group_oper_fun Finite1_L15 by simp moreover from A1 have "{δ(s∘r,x). x∈G×G} = {δ(s,⟨ r`(fst(x)),r`(snd(x))⟩)⋅s`(δ(r,x))⋅ δ(s,⟨ (r`(fst(x))⋅r`(snd(x))),δ(r,x)⟩). x∈G×G}" using Group_ZF_3_4_L4 by simp ultimately have "{δ(s∘r,x). x∈G×G} ∈ Fin(G)" by simp with A1 show ?thesis using restrict AlHomOp2_def by simp qed text‹Composition of almost homomorphisms is an almost homomorphism.› theorem (in group1) Group_ZF_3_4_T1: assumes A1: "s∈AH" "r∈AH" shows "Composition(G)`⟨ s,r⟩ ∈ AH" "s∘r ∈ AH" proof - from A1 have "⟨ s,r⟩ ∈ (G→G)×(G→G)" using AlmostHoms_def by simp then have "Composition(G)`⟨ s,r⟩ : G→G" using func_ZF_5_L1 apply_funtype by blast with A1 show "Composition(G)`⟨ s,r⟩ ∈ AH" using Group_ZF_3_4_L5 AlmostHoms_def by simp with A1 show "s∘r ∈ AH" using AlHomOp2_def restrict by simp qed text‹The set of almost homomorphisms is closed under composition. The second operation on almost homomorphisms is associative.› lemma (in group1) Group_ZF_3_4_L6: shows "AH {is closed under} Composition(G)" "AlHomOp2(G,P) {is associative on} AH" proof - show "AH {is closed under} Composition(G)" using Group_ZF_3_4_T1 IsOpClosed_def by simp moreover have "AH ⊆ G→G" using AlmostHoms_def by auto moreover have "Composition(G) {is associative on} (G→G)" using func_ZF_5_L5 by simp ultimately show "AlHomOp2(G,P) {is associative on} AH" using func_ZF_4_L3 AlHomOp2_def by simp qed text‹Type information related to the situation of two almost homomorphisms.› lemma (in group1) Group_ZF_3_4_L7: assumes A1: "s∈AH" "r∈AH" and A2: "n∈G" shows "s`(n) ∈ G" "(r`(n))¯ ∈ G" "s`(n)⋅(r`(n))¯ ∈ G" "s`(r`(n)) ∈ G" proof - from A1 A2 show "s`(n) ∈ G" "(r`(n))¯ ∈ G" "s`(r`(n)) ∈ G" "s`(n)⋅(r`(n))¯ ∈ G" using AlmostHoms_def apply_type group0_2_L1 monoid0.group0_1_L1 inverse_in_group by auto qed text‹Type information related to the situation of three almost homomorphisms.› lemma (in group1) Group_ZF_3_4_L8: assumes A1: "s∈AH" "r∈AH" "q∈AH" and A2: "n∈G" shows "q`(n)∈G" "s`(r`(n)) ∈ G" "r`(n)⋅(q`(n))¯ ∈ G" "s`(r`(n)⋅(q`(n))¯) ∈ G" "δ(s,⟨ q`(n),r`(n)⋅(q`(n))¯⟩) ∈ G" proof - from A1 A2 show "q`(n)∈ G" "s`(r`(n)) ∈ G" "r`(n)⋅(q`(n))¯ ∈ G" using AlmostHoms_def apply_type group0_2_L1 monoid0.group0_1_L1 inverse_in_group by auto with A1 A2 show "s`(r`(n)⋅(q`(n))¯) ∈ G" "δ(s,⟨ q`(n),r`(n)⋅(q`(n))¯⟩) ∈ G" using AlmostHoms_def apply_type Group_ZF_3_2_L4A by auto qed text‹A formula useful in showing that the composition of almost homomorphisms is congruent with respect to the quotient group relation.› lemma (in group1) Group_ZF_3_4_L9: assumes A1: "s1 ∈ AH" "r1 ∈ AH" "s2 ∈ AH" "r2 ∈ AH" and A2: "n∈G" shows "(s1∘r1)`(n)⋅((s2∘r2)`(n))¯ = s1`(r2`(n))⋅ (s2`(r2`(n)))¯⋅s1`(r1`(n)⋅(r2`(n))¯)⋅ δ(s1,⟨ r2`(n),r1`(n)⋅(r2`(n))¯⟩)" proof - from A1 A2 isAbelian have "(s1∘r1)`(n)⋅((s2∘r2)`(n))¯ = s1`(r2`(n)⋅(r1`(n)⋅(r2`(n))¯))⋅(s2`(r2`(n)))¯" using Group_ZF_3_4_L2 Group_ZF_3_4_L7 group0_4_L6A group_oper_assoc by simp with A1 A2 have "(s1∘r1)`(n)⋅((s2∘r2)`(n))¯ = s1`(r2`(n))⋅ s1`(r1`(n)⋅(r2`(n))¯)⋅δ(s1,⟨ r2`(n),r1`(n)⋅(r2`(n))¯⟩)⋅ (s2`(r2`(n)))¯" using Group_ZF_3_4_L8 Group_ZF_3_4_L1 by simp with A1 A2 isAbelian show ?thesis using Group_ZF_3_4_L8 group0_4_L7 by simp qed text‹The next lemma shows a formula that translates an expression in terms of the first group operation on almost homomorphisms and the group inverse in the group of almost homomorphisms to an expression using only the underlying group operations.› lemma (in group1) Group_ZF_3_4_L10: assumes A1: "s ∈ AH" "r ∈ AH" and A2: "n ∈ G" shows "(s∙(GroupInv(AH,Op1)`(r)))`(n) = s`(n)⋅(r`(n))¯" proof - from A1 A2 show ?thesis using isAbelian Group_ZF_3_2_L13 Group_ZF_3_2_L12 Group_ZF_3_2_L14 by simp qed text‹A neccessary condition for two a. h. to be almost equal.› lemma (in group1) Group_ZF_3_4_L11: assumes A1: "s≅r" shows "{s`(n)⋅(r`(n))¯. n∈G} ∈ Fin(G)" proof - from A1 have "s∈AH" "r∈AH" using QuotientGroupRel_def by auto moreover from A1 have "{(s∙(GroupInv(AH,Op1)`(r)))`(n). n∈G} ∈ Fin(G)" using QuotientGroupRel_def Finite1_L18 by simp ultimately show ?thesis using Group_ZF_3_4_L10 by simp qed text‹A sufficient condition for two a. h. to be almost equal.› lemma (in group1) Group_ZF_3_4_L12: assumes A1: "s∈AH" "r∈AH" and A2: "{s`(n)⋅(r`(n))¯. n∈G} ∈ Fin(G)" shows "s≅r" proof - from groupAssum isAbelian A1 A2 show ?thesis using Group_ZF_3_2_L15 AlmostHoms_def Group_ZF_3_4_L10 Finite1_L19 QuotientGroupRel_def by simp qed text‹Another sufficient consdition for two a.h. to be almost equal. It is actually just an expansion of the definition of the quotient group relation.› lemma (in group1) Group_ZF_3_4_L12A: assumes "s∈AH" "r∈AH" and "s∙(GroupInv(AH,Op1)`(r)) ∈ FR" shows "s≅r" "r≅s" proof - from assms show "s≅r" using assms QuotientGroupRel_def by simp then show "r≅s" by (rule Group_ZF_3_3_L3A) qed text‹Another necessary condition for two a.h. to be almost equal. It is actually just an expansion of the definition of the quotient group relation.› lemma (in group1) Group_ZF_3_4_L12B: assumes "s≅r" shows "s∙(GroupInv(AH,Op1)`(r)) ∈ FR" using assms QuotientGroupRel_def by simp text‹The next lemma states the essential condition for the composition of a. h. to be congruent with respect to the quotient group relation for the subgroup of finite range functions.› lemma (in group1) Group_ZF_3_4_L13: assumes A1: "s1≅s2" "r1≅r2" shows "(s1∘r1) ≅ (s2∘r2)" proof - have "{s1`(r2`(n))⋅ (s2`(r2`(n)))¯. n∈G} ∈ Fin(G)" proof - from A1 have "∀n∈G. r2`(n) ∈ G" using QuotientGroupRel_def AlmostHoms_def apply_funtype by auto moreover from A1 have "{s1`(n)⋅(s2`(n))¯. n∈G} ∈ Fin(G)" using Group_ZF_3_4_L11 by simp ultimately show ?thesis by (rule Finite1_L6B) qed moreover have "{s1`(r1`(n)⋅(r2`(n))¯). n ∈ G} ∈ Fin(G)" proof - from A1 have "∀n∈G. s1`(n)∈G" using QuotientGroupRel_def AlmostHoms_def apply_funtype by auto moreover from A1 have "{r1`(n)⋅(r2`(n))¯. n∈G} ∈ Fin(G)" using Group_ZF_3_4_L11 by simp ultimately show ?thesis by (rule Finite1_L6C) qed ultimately have "{s1`(r2`(n))⋅ (s2`(r2`(n)))¯⋅s1`(r1`(n)⋅(r2`(n))¯). n∈G} ∈ Fin(G)" using group_oper_fun Finite1_L15 by simp moreover have "{δ(s1,⟨ r2`(n),r1`(n)⋅(r2`(n))¯⟩). n∈G} ∈ Fin(G)" proof - from A1 have "∀n∈G. ⟨ r2`(n),r1`(n)⋅(r2`(n))¯⟩ ∈ G×G" using QuotientGroupRel_def Group_ZF_3_4_L7 by auto moreover from A1 have "{δ(s1,x). x ∈ G×G} ∈ Fin(G)" using QuotientGroupRel_def AlmostHoms_def by simp ultimately show ?thesis by (rule Finite1_L6B) qed ultimately have "{s1`(r2`(n))⋅ (s2`(r2`(n)))¯⋅s1`(r1`(n)⋅(r2`(n))¯)⋅ δ(s1,⟨ r2`(n),r1`(n)⋅(r2`(n))¯⟩). n∈G} ∈ Fin(G)" using group_oper_fun Finite1_L15 by simp with A1 show ?thesis using QuotientGroupRel_def Group_ZF_3_4_L9 Group_ZF_3_4_T1 Group_ZF_3_4_L12 by simp qed text‹Composition of a. h. to is congruent with respect to the quotient group relation for the subgroup of finite range functions. Recall that if an operation say "$\circ $" on $X$ is congruent with respect to an equivalence relation $R$ then we can define the operation on the quotient space $X/R$ by $[s]_R\circ [r]_R := [s\circ r]_R$ and this definition will be correct i.e. it will not depend on the choice of representants for the classes $[x]$ and $[y]$. This is why we want it here.› lemma (in group1) Group_ZF_3_4_L13A: shows "Congruent2(QuotientGroupRel(AH,Op1,FR),Op2)" proof - show ?thesis using Group_ZF_3_4_L13 Congruent2_def by simp qed text‹The homomorphism difference for the identity function is equal to the neutral element of the group (denoted $e$ in the group1 context).› lemma (in group1) Group_ZF_3_4_L14: assumes A1: "x ∈ G×G" shows "δ(id(G),x) = 𝟭" proof - from A1 show ?thesis using group0_2_L1 monoid0.group0_1_L1 HomDiff_def id_conv group0_2_L6 by simp qed text‹The identity function ($I(x) = x$) on $G$ is an almost homomorphism.› lemma (in group1) Group_ZF_3_4_L15: shows "id(G) ∈ AH" proof - have "G×G ≠ 0" using group0_2_L1 monoid0.group0_1_L3A by blast then show ?thesis using Group_ZF_3_4_L14 group0_2_L2 id_type AlmostHoms_def by simp qed text‹Almost homomorphisms form a monoid with composition. The identity function on the group is the neutral element there.› lemma (in group1) Group_ZF_3_4_L16: shows "IsAmonoid(AH,Op2)" "monoid0(AH,Op2)" "id(G) = TheNeutralElement(AH,Op2)" proof- let ?i = "TheNeutralElement(G→G,Composition(G))" have "IsAmonoid(G→G,Composition(G))" "monoid0(G→G,Composition(G))" using monoid0_def Group_ZF_2_5_L2 by auto moreover have "AH {is closed under} Composition(G)" using Group_ZF_3_4_L6 by simp moreover have "AH ⊆ G→G" using AlmostHoms_def by auto moreover have "?i ∈ AH" using Group_ZF_2_5_L2 Group_ZF_3_4_L15 by simp moreover have "id(G) = ?i" using Group_ZF_2_5_L2 by simp ultimately show "IsAmonoid(AH,Op2)" "monoid0(AH,Op2)" "id(G) = TheNeutralElement(AH,Op2)" using monoid0.group0_1_T1 group0_1_L6 AlHomOp2_def monoid0_def by auto qed text‹We can project the monoid of almost homomorphisms with composition to the group of almost homomorphisms divided by the subgroup of finite range functions. The class of the identity function is the neutral element of the quotient (monoid).› theorem (in group1) Group_ZF_3_4_T2: assumes A1: "R = QuotientGroupRel(AH,Op1,FR)" shows "IsAmonoid(AH//R,ProjFun2(AH,R,Op2))" "R``{id(G)} = TheNeutralElement(AH//R,ProjFun2(AH,R,Op2))" proof - have "group0(AH,Op1)" using Group_ZF_3_2_L10A group0_def by simp with A1 groupAssum isAbelian show "IsAmonoid(AH//R,ProjFun2(AH,R,Op2))" "R``{id(G)} = TheNeutralElement(AH//R,ProjFun2(AH,R,Op2))" using Group_ZF_3_3_L2 group0.Group_ZF_2_4_L3 Group_ZF_3_4_L13A Group_ZF_3_4_L16 monoid0.Group_ZF_2_2_T1 Group_ZF_2_2_L1 by auto qed subsection‹Shifting almost homomorphisms› text‹In this this section we consider what happens if we multiply an almost homomorphism by a group element. We show that the resulting function is also an a. h., and almost equal to the original one. This is used only for slopes (integer a.h.) in ‹Int_ZF_2› where we need to correct a positive slopes by adding a constant, so that it is at least $2$ on positive integers.› text‹If $s$ is an almost homomorphism and $c$ is some constant from the group, then $s\cdot c$ is an almost homomorphism.› lemma (in group1) Group_ZF_3_5_L1: assumes A1: "s ∈ AH" and A2: "c∈G" and A3: "r = {⟨x,s`(x)⋅c⟩. x∈G}" shows "∀x∈G. r`(x) = s`(x)⋅c" "r ∈ AH" "s ≅ r" proof - from A1 A2 A3 have I: "r:G→G" using AlmostHoms_def apply_funtype group_op_closed ZF_fun_from_total by auto with A3 show II: "∀x∈G. r`(x) = s`(x)⋅c" using ZF_fun_from_tot_val by simp with isAbelian A1 A2 have III: "∀p ∈ G×G. δ(r,p) = δ(s,p)⋅c¯" using group_op_closed AlmostHoms_def apply_funtype HomDiff_def group0_4_L7 by auto have "{δ(r,p). p ∈ G×G} ∈ Fin(G)" proof - from A1 A2 have "{δ(s,p). p ∈ G×G} ∈ Fin(G)" "c¯∈G" using AlmostHoms_def inverse_in_group by auto then have "{δ(s,p)⋅c¯. p ∈ G×G} ∈ Fin(G)" using group_oper_fun Finite1_L16AA by simp moreover from III have "{δ(r,p). p ∈ G×G} = {δ(s,p)⋅c¯. p ∈ G×G}" by (rule ZF1_1_L4B) ultimately show ?thesis by simp qed with I show IV: "r ∈ AH" using AlmostHoms_def by simp from isAbelian A1 A2 I II have "∀n ∈ G. s`(n)⋅(r`(n))¯ = c¯" using AlmostHoms_def apply_funtype group0_4_L6AB by auto then have "{s`(n)⋅(r`(n))¯. n∈G} = {c¯. n∈G}" by (rule ZF1_1_L4B) with A1 A2 IV show "s ≅ r" using group0_2_L1 monoid0.group0_1_L3A inverse_in_group Group_ZF_3_4_L12 by simp qed end