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Culegere de probleme de Analiz˘a numeric˘a

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4.3. Sisteme <strong>de</strong> ecuat¸ii 63<br />

adică<br />

=<br />

n<br />

|xj|<br />

j=1<br />

n<br />

i=1<br />

|aij| ≤<br />

n<br />

j=1<br />

|xj| max<br />

1≤j≤n<br />

Al ≤ max<br />

1≤j≤n<br />

Fie p ∈ N, 1 ≤ p ≤ n astfel încât<br />

max<br />

1≤j≤n<br />

n<br />

|aij| =<br />

i=1<br />

n<br />

i=1<br />

|aij| = x1 max<br />

1≤j≤n<br />

n<br />

|aij|.<br />

i=1<br />

n<br />

i=1<br />

s¸ix ∈ Rn astfel încât xi = δip. Avemx1 = 1.<br />

<br />

n n n <br />

<br />

Al ≥ Ax1 = |(Ax)i| = aijxj<br />

=<br />

i=1<br />

i=1<br />

j=1<br />

|aip|<br />

n<br />

i=1<br />

n<br />

|aij|,<br />

i=1<br />

|aipxp| = max<br />

1≤j≤n<br />

n<br />

i=1<br />

|aij|<br />

Problema 4.3.3 Arătat¸i că norma euclidiană, l-norma s¸i m-norma sunt norme<br />

matriciale.<br />

Problema 4.3.4 Rezolvat¸i sistemul<br />

⎧<br />

⎨<br />

⎩<br />

5x1 +x2 +x3 = 7<br />

x1 +5x2 +x3 = 7<br />

x1 +x2 +5x3 = 7<br />

utilizând metoda lui Jacobi s¸i metoda Gauss-Sei<strong>de</strong>l.<br />

De câte iterat¸ii este nevoie pentru a se putea atinge o precizie dorităε?<br />

Solut¸ie.<br />

x (k)<br />

i<br />

x (k)<br />

i<br />

= 1<br />

aii<br />

<br />

1<br />

= bi −<br />

aii<br />

⎛<br />

⎜<br />

⎝ bi −<br />

i−1<br />

j=1<br />

n<br />

j=1<br />

j=i<br />

aijx (k)<br />

j −<br />

x (0) = (0,0,0) T<br />

x (1) =<br />

aijx (k−1)<br />

j<br />

<br />

7 7 7<br />

, ,<br />

5 5 5<br />

n<br />

j=i+1<br />

⎞<br />

⎟<br />

⎠<br />

aijx (k−1)<br />

j<br />

<br />

(4.7)<br />

(4.8)

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