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326 Chapter 4. Determinants<br />

4.III Other Formulas<br />

(This section is optional. Later sections do not depend on this material.)<br />

Determinants are a fount of interesting and amusing formulas. Here is one<br />

that is often seen in calculus classes and used to compute determinants by hand.<br />

4.III.1 Laplace’s Expansion<br />

1.1 Example In this permutation expansion<br />

�<br />

�t1,1<br />

�<br />

�t2,1<br />

�<br />

�t3,1<br />

t1,2<br />

t2,2<br />

t3,2<br />

�<br />

� �<br />

� �<br />

t1,3�<br />

�<br />

�<br />

�1<br />

0 0�<br />

�<br />

�<br />

�1<br />

0 0�<br />

�<br />

t2,3�<br />

� = t1,1t2,2t3,3 �<br />

�0<br />

1 0�<br />

� + t1,1t2,3t3,2 �<br />

�0<br />

0 1�<br />

�<br />

t3,3�<br />

�0<br />

0 1�<br />

�0<br />

1 0�<br />

� �<br />

�<br />

�<br />

�0<br />

1 0�<br />

�<br />

�<br />

�0<br />

1<br />

+ t1,2t2,1t3,3 �<br />

�1<br />

0 0�<br />

� + t1,2t2,3t3,1 �<br />

�0<br />

0<br />

�0<br />

0 1�<br />

�1<br />

0<br />

� �<br />

�<br />

�<br />

�0<br />

0 1�<br />

�<br />

�<br />

�0<br />

0<br />

+ t1,3t2,1t3,2<br />

�<br />

�1<br />

0 0�<br />

� + t1,3t2,2t3,1<br />

�<br />

�0<br />

1<br />

�0<br />

1 0�<br />

�1<br />

0<br />

�<br />

0�<br />

�<br />

1�<br />

�<br />

0�<br />

�<br />

1�<br />

�<br />

0�<br />

�<br />

0�<br />

we can, for instance, factor out the entries from the first row<br />

⎡ � � � � ⎤<br />

�<br />

�1<br />

0 0�<br />

�<br />

� �1<br />

0 0�<br />

�<br />

= t1,1 · ⎣t2,2t3,3<br />

�<br />

�0<br />

1 0�<br />

� + t2,3t3,2<br />

�<br />

�0<br />

0 1�⎦<br />

�<br />

�0<br />

0 1�<br />

�0<br />

1 0�<br />

⎡ � � � � ⎤<br />

�<br />

�0<br />

1 0�<br />

�<br />

� �0<br />

1 0�<br />

�<br />

+ t1,2 · ⎣t2,1t3,3<br />

�<br />

�1<br />

0 0�<br />

� + t2,3t3,1<br />

�<br />

�0<br />

0 1�⎦<br />

�<br />

�0<br />

0 1�<br />

�1<br />

0 0�<br />

⎡ � � � � ⎤<br />

�<br />

�0<br />

0 1�<br />

�<br />

� �0<br />

0 1�<br />

�<br />

+ t1,3 · ⎣t2,1t3,2 �<br />

�1<br />

0 0�<br />

� + t2,2t3,1 �<br />

�0<br />

1 0�⎦<br />

�<br />

�0<br />

1 0�<br />

�1<br />

0 0�<br />

and swap rows in the permutation matrices to get this.<br />

⎡ � � � � ⎤<br />

�<br />

�1<br />

0 0�<br />

�<br />

� �1<br />

0 0�<br />

�<br />

= t1,1 · ⎣t2,2t3,3<br />

�<br />

�0<br />

1 0�<br />

� + t2,3t3,2<br />

�<br />

�0<br />

0 1�⎦<br />

�<br />

�0<br />

0 1�<br />

�0<br />

1 0�<br />

⎡ � � � � ⎤<br />

�<br />

�1<br />

0 0�<br />

�<br />

� �1<br />

0 0�<br />

�<br />

− t1,2 · ⎣t2,1t3,3<br />

�<br />

�0<br />

1 0�<br />

� + t2,3t3,1<br />

�<br />

�0<br />

0 1�⎦<br />

�<br />

�0<br />

0 1�<br />

�0<br />

1 0�<br />

⎡ � � � � ⎤<br />

�<br />

�1<br />

0 0�<br />

�<br />

� �1<br />

0 0�<br />

�<br />

+ t1,3 · ⎣t2,1t3,2<br />

�<br />

�0<br />

1 0�<br />

� + t2,2t3,1<br />

�<br />

�0<br />

0 1�⎦<br />

�<br />

�0<br />

0 1�<br />

�0<br />

1 0�

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