{ localUrl: '../page/free_group_universal_property.html', arbitalUrl: 'https://arbital.com/p/free_group_universal_property', rawJsonUrl: '../raw/6gd.json', likeableId: '3650', likeableType: 'page', myLikeValue: '0', likeCount: '1', dislikeCount: '0', likeScore: '1', individualLikes: [ 'EricBruylant' ], pageId: 'free_group_universal_property', edit: '1', editSummary: '', prevEdit: '0', currentEdit: '1', wasPublished: 'true', type: 'wiki', title: 'Free group universal property', clickbait: '', textLength: '3532', alias: 'free_group_universal_property', externalUrl: '', sortChildrenBy: 'likes', hasVote: 'false', voteType: '', votesAnonymous: 'false', editCreatorId: 'PatrickStevens', editCreatedAt: '2016-10-23 18:25:51', pageCreatorId: 'PatrickStevens', pageCreatedAt: '2016-10-23 18:25:51', seeDomainId: '0', editDomainId: 'AlexeiAndreev', submitToDomainId: '0', isAutosave: 'false', isSnapshot: 'false', isLiveEdit: 'true', isMinorEdit: 'false', indirectTeacher: 'false', todoCount: '0', isEditorComment: 'false', isApprovedComment: 'true', isResolved: 'false', snapshotText: '', anchorContext: '', anchorText: '', anchorOffset: '0', mergedInto: '', isDeleted: 'false', viewCount: '30', text: '[summary: The [-5kg] may be defined by a [-600], allowing [-4c7] to talk about free groups. The universal property is a helpful way to think about free groups even without any category theory: it opens the way to considering [5j9 group presentations], which are very interesting objects in their own right (for example, because they're easy to compute with).]\n\nThe [-600] of the [-5kg] basically tells us that "the definition of the free group doesn't depend (up to isomorphism) on the exact details of the set $X$ we picked; only on its [-4w5]", which is morally a very useful thing to know.\nYou may skip down to the next subheading if you might be scared of category theory, but the property itself doesn't need category theory and is helpful.\n\nThe universal property is the technical [4c7 category-theoretic] fact that [free_group_functor_left_adjoint_to_forgetful the free-group functor is left adjoint to the forgetful functor], and it is not so immediately useful as the other more concrete properties on this page, but it is exceedingly important in category theory as a very natural example of a [adjoint_functor pair of adjoint functors] and as an example for the [-general_adjoint_functor_theorem].\n\n# Statement and explanation\nThe universal property which characterises the free group is:\n\n> The free group $FX$ on the set $X$ is the group, unique up to isomorphism, such that for any group $G$ and any [3jy function of sets] $f: X \\to G$ %%note:Here we're slightly abusing notation: we've written $G$ for the [-3gz] of the group $G$ here.%%, there is a unique [-47t] $\\overline{f}: FX \\to G$ such that $\\overline{f}(\\rho_{a_1} \\rho_{a_2} \\dots \\rho_{a_n}) = f(a_1) \\cdot f(a_2) \\cdot \\dots \\cdot f(a_n)$.\n\nThis looks very opaque at first sight, but what it says is that $FX$ is the unique group such that:\n\n> Given any target group $G$, we can extend any map $f: X \\to G$ to a unique homomorphism $FX \\to G$, in the sense that whenever we're given the image of each generator (that is, member of $X$) by $f$, the laws of a group homomorphism force exactly where every other element of $FX$ must go.\nThat is, we can specify homomorphisms from $FX$ by specifying where the generators go, and moreover, *every* possible such specification does indeed correspond to a homomorphism.\n\n# Why is this a non-trivial property?\n\nConsider the cyclic group $C_3$ with three elements; say $\\{ e, a, b\\}$ with $e$ the identity and $a + a = b$, $a+b = e = b+a$, and $b+b = a$.\nThen this group is generated by the element $a$, because $a=a$, $a+a = b$, and $a+a+a = e$.\nLet us pick $G = (\\mathbb{Z}, +)$.\nWe'll try and define a map $f: C_3 \\to \\mathbb{Z}$ by $a \\mapsto 1$.\n\nIf $C_3$ had the universal property of the free group on $\\{ e, a, b\\}$, then we would be able to find a homomorphism $\\overline{f}: C_3 \\to \\mathbb{Z}$, such that $\\overline{f}(a) = 1$ (that is, mimicking the action of the set-function $f$).\nBut in fact, no such homomorphism can exist, because if $\\overline{f}$ were such a homomorphism, then $\\overline{f}(e) = \\overline{f}(a+a+a) = 1+1+1 = 3$ so $\\overline{f}(e) = 3$, which contradicts that [49z the image of the identity under a group homomorphism is the identity].\n\nIn essence, $C_3$ "has extra relations" (namely that $a+a+a = e$) which the free group doesn't have, and which can thwart the attempt to define $\\overline{f}$; this is reflected in the fact that $C_3$ fails to have the universal property.\n\nA proof of the universal property may be found [free_group_satisfies_universal_property elsewhere].', metaText: '', isTextLoaded: 'true', isSubscribedToDiscussion: 'false', isSubscribedToUser: 'false', isSubscribedAsMaintainer: 'false', discussionSubscriberCount: '1', maintainerCount: '1', userSubscriberCount: '0', lastVisit: '', hasDraft: 'false', votes: [], voteSummary: [ '0', '0', '0', '0', '0', '0', '0', '0', '0', '0' ], muVoteSummary: '0', voteScaling: '0', currentUserVote: '-2', voteCount: '0', lockedVoteType: '', maxEditEver: '0', redLinkCount: '0', lockedBy: '', lockedUntil: '', nextPageId: '', prevPageId: '', usedAsMastery: 'false', proposalEditNum: '0', permissions: { edit: { has: 'false', reason: 'You don't have domain permission to edit this page' }, proposeEdit: { has: 'true', reason: '' }, delete: { has: 'false', reason: 'You don't have domain permission to delete this page' }, comment: { has: 'false', reason: 'You can't comment in this domain because you are not a member' }, proposeComment: { has: 'true', reason: '' } }, summaries: {}, creatorIds: [ 'PatrickStevens' ], childIds: [], parentIds: [ 'free_group' ], commentIds: [], questionIds: [], tagIds: [], relatedIds: [], markIds: [], explanations: [], learnMore: [], requirements: [], subjects: [], lenses: [], lensParentId: 'free_group', pathPages: [], learnMoreTaughtMap: {}, learnMoreCoveredMap: {}, learnMoreRequiredMap: {}, editHistory: {}, domainSubmissions: {}, answers: [], answerCount: '0', commentCount: '0', newCommentCount: '0', linkedMarkCount: '0', changeLogs: [ { likeableId: '0', likeableType: 'changeLog', myLikeValue: '0', likeCount: '0', dislikeCount: '0', likeScore: '0', individualLikes: [], id: '20269', pageId: 'free_group_universal_property', userId: 'PatrickStevens', edit: '0', type: 'newParent', createdAt: '2016-10-23 18:25:52', auxPageId: 'free_group', oldSettingsValue: '', newSettingsValue: '' }, { likeableId: '3648', likeableType: 'changeLog', myLikeValue: '0', likeCount: '1', dislikeCount: '0', likeScore: '1', individualLikes: [], id: '20267', pageId: 'free_group_universal_property', userId: 'PatrickStevens', edit: '1', type: 'newEdit', createdAt: '2016-10-23 18:25:51', auxPageId: '', oldSettingsValue: '', newSettingsValue: '' } ], feedSubmissions: [], searchStrings: {}, hasChildren: 'false', hasParents: 'true', redAliases: {}, improvementTagIds: [], nonMetaTagIds: [], todos: [], slowDownMap: 'null', speedUpMap: 'null', arcPageIds: 'null', contentRequests: {} }