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Section III. Computing <strong>Linear</strong> Maps 197<br />

showing that this is the matrix representing h with respect to the bases.<br />

Rep B,D(h) =<br />

�<br />

−1/2 1<br />

�<br />

2<br />

−1/2 −1 −2<br />

B,D<br />

We will use lower case letters for a map, upper case for the matrix, and<br />

lower case again for the entries of the matrix. Thus for the map h, the matrix<br />

representing it is H, with entries hi,j.<br />

1.4 Theorem Assume that V and W are vector spaces of dimensions m and<br />

n with bases B and D, and that h: V → W is a linear map. If h is represented<br />

by<br />

⎛<br />

⎞<br />

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

⎜ h2,1 h2,2 ⎜<br />

... h2,n ⎟<br />

RepB,D(h) = ⎜ .<br />

⎝ .<br />

⎟<br />

.<br />

⎠<br />

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

and �v ∈ V is represented by<br />

⎛ ⎞<br />

c1<br />

⎜c2⎟<br />

⎜ ⎟<br />

RepB(�v) = ⎜<br />

⎝ .<br />

⎟<br />

. ⎠<br />

cn<br />

B<br />

B,D<br />

then the representation of the image of �v is this.<br />

⎛<br />

⎞<br />

h1,1c1 + h1,2c2 + ···+ h1,ncn<br />

⎜ h2,1c1 ⎜ + h2,2c2 + ···+ h2,ncn ⎟<br />

RepD( h(�v))= ⎜<br />

⎝<br />

.<br />

⎟<br />

.<br />

⎠<br />

hm,1c1 + hm,2c2 + ···+ hm,ncn<br />

Proof. Exercise 28. QED<br />

We will think of the matrix Rep B,D(h) and the vector Rep B(�v) as combining<br />

to make the vector Rep D(h(�v)).<br />

1.5 Definition The matrix-vector product of a m×n matrix and a n×1 vector<br />

is this.<br />

⎛<br />

⎞ ⎛<br />

⎞<br />

a1,1 a1,2 ... a1,n ⎛ ⎞ a1,1c1 + a1,2c2 + ···+ a1,ncn<br />

c1<br />

⎜ a2,1 a2,2 ⎜<br />

... a2,n ⎟ ⎜ a2,1c1 ⎟ ⎜ .<br />

⎜ .<br />

⎝ .<br />

⎟ ⎝ .<br />

⎟ ⎜ + a2,2c2 + ···+ a2,ncn ⎟<br />

.<br />

⎠ . ⎠ = ⎜<br />

.<br />

⎝<br />

.<br />

⎟<br />

.<br />

⎠<br />

cn<br />

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

am,1c1 + am,2c2 + ···+ am,ncn<br />

D

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