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CorollaryStatement: Literature-sourcedProof: AI-adaptedSession-authored (Fable 5 assisted)precheck passaudited 2026-08-11
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Every finite group has a finite presentation from its multiplication table

Statement

Every finite group GG has the finite multiplication-table presentation

Gxg (gG) | xgxhxgh1 (g,hG).G\cong\left\langle x_g\ (g\in G)\ \middle|\ x_gx_hx_{gh}^{-1}\ (g,h\in G)\right\rangle.

Facts & Assumptions

Given: A finite group GG and a distinct formal symbol xgx_g for each gGg\in G.

[L1]

A map u:XHu:X\to H that sends every relator in RR to the identity extends uniquely to a homomorphism XRH\langle X\mid R\rangle\to H (Von Dyck's theorem: maps of generators that satisfy the relators extend uniquely from a presented group).

[F1]

A presentation XR\langle X\mid R\rangle is finite when both XX and RR are finite (Relators and relations; finitely generated, finitely related, and finite presentations).

[F2]

A set is finite when it is in bijection with a natural number; and if AA is finite and f:ABf:A\to B is a bijection, then BB is finite (The cardinality A\lvert A\rvert of a finite set).

[F4]

Every nonempty subset of N\mathbb N has a least element (The well-ordering principle).

[F5]

In XR\langle X\mid R\rangle every relator of RR becomes the identity (Group presentation by generators and relations).

Proof

technique · constructive
1.1

Let X={xg:gG}X=\{x_g:g\in G\} and R={xgxhxgh1:(g,h)G×G}R=\{x_gx_hx_{gh}^{-1}:(g,h)\in G\times G\}. The map gxgg\mapsto x_g is a bijection, so [F2] makes XX finite. By [L2], G×GG\times G is finite, so by [F2] fix a bijection c:G×Gnc:G\times G\to n for some nNn\in\mathbb N, and let qq send (g,h)(g,h) to xgxhxgh1x_gx_hx_{gh}^{-1}, so that RR is the image of qq. Sending each rRr\in R to the least element of the nonempty set {k<n:q(c1(k))=r}\{k<n:q(c^{-1}(k))=r\}, which exists by [F4], is an injection of RR into nn; it is a bijection onto its image, that image is finite by [F3], and [F2] transports finiteness back, so RR is finite.

F2F3F4L2givenconstruct
2.1

The assignment xggx_g\mapsto g sends each relator xgxhxgh1x_gx_hx_{gh}^{-1} to gh(gh)1=eGgh(gh)^{-1}=e_G, so [L1] gives a homomorphism π:P:=XRG\pi:P:=\langle X\mid R\rangle\to G.

L1step 1.1construct
3.1

By [F5] every relator of RR is the identity in PP, so [xg][xh][xgh]1=e[x_g][x_h][x_{gh}]^{-1}=e and hence [xg][xh]=[xgh][x_g][x_h]=[x_{gh}]; therefore σ:GP\sigma:G\to P, σ(g)=[xg]\sigma(g)=[x_g], is a homomorphism.

F5step 1.1step 2.1construct
4.1

The composite πσ\pi\circ\sigma fixes every gGg\in G; the composite σπ\sigma\circ\pi fixes every generator class [xg][x_g], and uniqueness in [L1] makes it the identity on PP. Thus π\pi and σ\sigma are inverse isomorphisms.

L1step 2.1step 3.1
5.1

Both XX and RR are finite and PGP\cong G, so [F1] shows that GG has the displayed finite presentation, including when GG is the one-element group.

F1step 1.1step 4.1discharge-construct

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