23.07.2012 Views

Linear Algebra

Linear Algebra

Linear Algebra

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

212 Chapter 3. Maps Between Spaces<br />

Representing a scalar multiple of a map works the same way.<br />

1.2 Example If t is a transformation represented by<br />

�<br />

1<br />

RepB,D(t) =<br />

1<br />

�<br />

0<br />

1<br />

so that<br />

� �<br />

v1<br />

�v =<br />

�<br />

↦→<br />

v1<br />

B,D<br />

then the scalar multiple map 5t acts in this way.<br />

� �<br />

v1<br />

�v =<br />

v2<br />

↦−→<br />

� �<br />

5v1<br />

5v1 +5v2<br />

Therefore, this is the matrix representing 5t.<br />

� �<br />

5 0<br />

RepB,D(5t) =<br />

5 5<br />

B<br />

v2<br />

D<br />

B<br />

B,D<br />

=5· t(�v)<br />

v1 + v2<br />

�<br />

D<br />

= t(�v)<br />

1.3 Definition The sum of two same-sized matrices is their entry-by-entry<br />

sum. The scalar multiple of a matrix is the result of entry-by-entry scalar<br />

multiplication.<br />

1.4 Remark These extend the vector addition and scalar multiplication operations<br />

that we defined in the first chapter.<br />

1.5 Theorem Let h, g : V → W be linear maps represented with respect to<br />

bases B,D by the matrices H and G, and let r be a scalar. Then the map<br />

h + g : V → W is represented with respect to B,D by H + G, and the map<br />

r · h: V → W is represented with respect to B,D by rH.<br />

Proof. Exercise 8; generalize the examples above. QED<br />

A notable special case of scalar multiplication is multiplication by zero. For<br />

any map 0 · h is the zero homomorphism and for any matrix 0 · H is the zero<br />

matrix.<br />

1.6 Example The zero map from any three-dimensional space to any twodimensional<br />

space is represented by the 2×3 zero matrix<br />

� �<br />

0 0 0<br />

Z =<br />

0 0 0<br />

no matter which domain and codomain bases are used.<br />

Exercises<br />

� 1.7 Perform � the indicated � � operations, � if defined.<br />

5 −1 2 2 1 4<br />

(a)<br />

+<br />

6 1 1 3 0 5<br />

� �<br />

2 −1 −1<br />

(b) 6 ·<br />

1 2 3

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!