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Culegere de probleme de Analiz˘a numeric˘a

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4.3. Sisteme <strong>de</strong> ecuat¸ii 61<br />

obt¸inem<br />

akkxk = −<br />

n<br />

akjxj ⇒ |akk||xk| ≤<br />

j=1<br />

j=i<br />

|akk| ≤<br />

n<br />

j=1<br />

j=k<br />

|akj| |xj|<br />

|xk| ≤<br />

n<br />

j=1<br />

j=k<br />

n<br />

|akj||xj|<br />

j=1<br />

j=k<br />

|akj|<br />

Observat¸ia 4.2.7 În acest caz EG se face pără permutări.<br />

Dacă lii = 1 avem factorizare Doolittle, iar dacă vii = 1 avem factorizare<br />

Crout.<br />

4.3 Sisteme <strong>de</strong> ecuat¸ii<br />

Problema 4.3.1 Arătat¸i că m-norma<br />

este naturală.<br />

Solut¸ie. Vom arăta că<br />

Fie x ∈ R n astfel încât<br />

Am = max<br />

i<br />

n<br />

j=1<br />

|aij|<br />

Am = max Ax∞<br />

x∞=1<br />

x∞ = max<br />

1≤i≤n |xi| = 1<br />

Ax∞ = max<br />

1≤i≤n |(Ax)i|<br />

<br />

n<br />

<br />

= max <br />

1≤i≤n<br />

≤ max<br />

1≤i≤n<br />

n<br />

j=1<br />

|aij| max<br />

1≤j≤n |xj| = max<br />

1≤i≤n<br />

= max<br />

1≤i≤n<br />

n<br />

j=1<br />

|aij|<br />

j=1<br />

aijxj<br />

<br />

<br />

<br />

<br />

≤<br />

n<br />

|aij|x∞ =<br />

j=1

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