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Section III. Computing <strong>Linear</strong> Maps 205<br />

Proof. For the matrix<br />

⎛<br />

⎜<br />

H = ⎜<br />

⎝<br />

h1,1 h1,2 ... h1,n<br />

h2,1 h2,2 ... h2,n<br />

.<br />

hm,1 hm,2 ... hm,n<br />

fix any n-dimensional domain space V and any m-dimensional codomain space<br />

W . Also fix bases B = 〈 � β1,..., � βn〉 and D = 〈 �δ1,..., �δm〉 for those spaces.<br />

Define a function h: V → W by: where �v in the domain is represented as<br />

⎛ ⎞<br />

Rep B(�v) =<br />

then its image h(�v) is the member the codomain represented by<br />

⎛<br />

⎞<br />

h1,1v1 + ···+ h1,nvn<br />

⎜<br />

RepD( h(�v))=<br />

.<br />

⎝ .<br />

⎟<br />

. ⎠<br />

hm,1v1 + ···+ hm,nvn<br />

that is, h(�v) =h(v1 � β1 + ···+ vn � βn) is defined to be (h1,1v1 + ···+ h1,nvn) · �δ1 +<br />

···+(hm,1v1 + ···+ hm,nvn) · �δm. (This is well-defined by the uniqueness of the<br />

representation RepB(�v).) Observe that h has simply been defined to make it the map that is represented<br />

with respect to B,D by the matrix H. So to finish, we need only check<br />

that h is linear. If �v,�u ∈ V are such that<br />

⎛ ⎞<br />

⎛ ⎞<br />

Rep B(�v) =<br />

⎜<br />

⎝<br />

v1<br />

.<br />

vn<br />

and c, d ∈ R then the calculation<br />

⎜<br />

⎝<br />

v1<br />

.<br />

vn<br />

⎟<br />

⎠<br />

B<br />

⎟<br />

⎠ and Rep B(�u) =<br />

h(c�v + d�u) = � h1,1(cv1 + du1)+···+ h1,n(cvn + dun) � · �δ1 +<br />

···+ � hm,1(cv1 + du1)+···+ hm,n(cvn + dun) � · �δm = c · h(�v)+d · h(�u)<br />

provides this verification. QED<br />

2.2 Example Which map the matrix represents depends on which bases are<br />

used. If<br />

� �<br />

� � � �<br />

� � � �<br />

1 0<br />

1 0<br />

0 1<br />

H = , B1 = D1 = 〈 , 〉, and B2 = D2 = 〈 , 〉,<br />

0 0<br />

0 1<br />

1 0<br />

⎞<br />

⎟<br />

⎠<br />

⎜<br />

⎝<br />

u1<br />

.<br />

un<br />

D<br />

⎟<br />

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