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Web skærmformat. - Aarhus Universitet

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V. Matricer – 2. Lineære afbildninger 391<br />

Eksempel<br />

En diagonal n × n-matrix<br />

⎛<br />

λ1<br />

⎜<br />

Λ = ⎜<br />

⎝<br />

0<br />

.<br />

0<br />

. ..<br />

0<br />

⎞<br />

...<br />

⎟<br />

0 ⎟<br />

⎠<br />

λn<br />

med alle diagonal indgange λi = 0 er invertibel.<br />

Den inverse er<br />

Λ −1 ⎛<br />

λ<br />

⎜<br />

= ⎜<br />

⎝<br />

−1<br />

1<br />

0<br />

0<br />

. ..<br />

⎞<br />

...<br />

⎟<br />

0 ⎟<br />

⎠<br />

. 0 λ −1<br />

n<br />

2.12. Inverter produkt ☞ [LA] 4 Inverse matricer<br />

Sætning<br />

Lad A,B vœre invertible n × n-matricer. Så er AB invertibel og der gœlder<br />

"Pas på rœkkefølgen."<br />

Bevis<br />

(AB) −1 = B −1 A −1<br />

(AB)(B −1 A −1 ) = A(BB −1 )A −1

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