수학/선형대수학

[선형대수학] 선형 사상의 값에 대응되는 행렬 표현 (Matrix Representation Corresponding to The Value of Linear Map)

xeskin 2020. 8. 27. 14:39
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Thm_Thm––––

AMm×n,BMn×pAMm×n,BMn×p

Let AB=[u1u2up],B=[v1v2vp] 

Then

uj=Avj,vj=Bej(B=BIp)

 

Thm_

Let V,W be finite dimensional vector spaces with ordered bases β,γ respectively. Let T be linear. Then

[T(u)]γ=[T]γβ[u]βuV

 

Proof_

Let β={v1,,vn},γ={w1,,wm}.

Let [u]β=[a1an]; that is, u=a1v1++anvn.

[T]γβ=(bij)m×n; that is, T(vj)=ni=1bijwi(1jn).

Then

T(u)=T(nj=1ajvj)=nj=1ajmi=1bijwi=mi=1(nj=1bijaj)wi

Hence,

[T(u)]γ=[nj=1b1jajnj=1bmjaj]=(bij)m×n[a1an]=[T]γβ[u]β

 

Ex_

Let T:P3(R)P2(R) be the linear map given by T(f(x))=f(x).

β={1,x,x2x3},γ={1,x,x2}.

Note

[T]γβ=[010000200003]

Let P(x)=24x+x2+3x3; then

[P(x)]β=[2413],T(P(x))=4+2x+9x2

so, [T(P(x))]γ=[429].

On the other hand,

[T]γβ[P(x)]β=[010000200003][2413]=[420]

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