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单词 PermutationRepresentation
释义

permutation representation


Let G be a group, and S any finite set on which G acts.

That means that for any g,hG; 𝐯,𝐰S

  • g𝐯V,

  • (gh)𝐯=g(h𝐯),

  • e𝐯=𝐯.

Notice that we almost have what it takes to make S a representationPlanetmathPlanetmath of G, but S is no vector spaceMathworldPlanetmath. We can however obtain a G-module (a vector space carrying a representation of G) as follows.

Let S={𝐬1,𝐬2,,𝐬n}. And let S=[𝐬1,𝐬2,,𝐬n] be the vector space generated by S over . in other words, S is made of all formal linear combinationsMathworldPlanetmathc1𝐬1+c2𝐬2++cn𝐬n with cj.The sum is defined coordinate-wise as is scalar multiplication.

Then the action of G in S can be extended linearly to S as

g(c1𝐬1+c2𝐬2++cn𝐬n)=c1(g𝐬1)+c2(g𝐬2)++cn(g𝐬n)

and then the mapρ:GGL(S) where ρ is such thatρ(g)(𝐯)=g𝐯 makes S into a G-module. The G-module S is known as the permutation representation associated with S.

Example.
If G=Sn acts on S={𝟏,𝟐,,𝐧}, then

S={c1𝟏+c2𝟐++cn𝐧}.

If σSn, the action becomes

σ(c1𝟏+c2𝟐++cn𝐧)=c1σ(𝟏)+c2σ(𝟐)++cnσ(𝐧).

Since S forms a basis for this space, we can compute the matrices corresponding to the defining permutationMathworldPlanetmath and we will see that the corresponding permutation matricesMathworldPlanetmath are obtained.


References.Bruce E. Sagan. The Symmetric GroupMathworldPlanetmathPlanetmath: Representations, Combinatorial Algorithms and Symmetric Functions. 2a Ed. 2000. Graduate Texts in Mathematics. Springer.

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更新时间:2025/5/4 13:10:19