Culegere de probleme de Analiz˘a numeric˘a
Culegere de probleme de Analiz˘a numeric˘a
Culegere de probleme de Analiz˘a numeric˘a
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90 Interpolare<br />
un<strong>de</strong><br />
Solut¸ie. a)<br />
Kn(x,t) = 1<br />
n!<br />
dar<br />
<br />
Pe <strong>de</strong> altă parte<br />
=<br />
En = (R−nf)(x) =<br />
(x−t) n + −<br />
n<br />
i=0<br />
b<br />
a<br />
li(x)(xi −t) n +<br />
Kn(x,t)f (n+1) (t)dt<br />
<br />
= 1<br />
n!<br />
n<br />
[(x−t) n +−(xi−t) n +li(x)<br />
b<br />
[(x−t)<br />
a<br />
n + −(xi −t) n + ]f(n+1) (t)dt =<br />
x<br />
[(x−t)<br />
c<br />
n −(xi −t) n ]f (n+1) x<br />
(t)dt+ (xi −t)<br />
xi<br />
n f (n+1) (t)dt<br />
n<br />
i=0<br />
i=0<br />
[(x−t) n −(xi −t) n ]li(x) = 0<br />
conform relat¸iei lui Cauchy.<br />
b)<br />
K1(x,t) = 0 dacă t ∈ (x0,x1)<br />
căci<br />
K1(t) = (x−t)+ −(x0 −t)+l0(x)+(x1 −t)+l1(x)<br />
l0(x) = x−x1<br />
x0 −x1<br />
= x1 −x<br />
x1 −x0<br />
⎧<br />
⎪⎨<br />
(x−x1)(t−x0)<br />
x1 −x0<br />
K1(x,t) =<br />
⎪⎩<br />
(t−x1)(x−x0)<br />
x1 −x0<br />
l1(x) = x−x0<br />
x1 −x0<br />
t ∈ [x0,x]<br />
t ∈ [x,x1]<br />
K1(x,t) ≤ 0 t.medie<br />
⇒ E1(x) = (x−x0)(x−x1)<br />
f<br />
2<br />
′′ (x)<br />
c) Scriind căp1 = 0 este polinomul <strong>de</strong> interpolare al luiucu nodurilex0 s¸ix1<br />
obt¸inem<br />
u(x)−p1(x) =<br />
x1<br />
x0<br />
k1(x,t)u ′′ (t)dt =<br />
x1<br />
p1(x0) = u(x0) = 0 = p1(x1) = u(x1)<br />
x0<br />
k1(x,t)g(t)dt<br />
Se verifică us¸or că problema la limită admite efectiv o solut¸ie. K1 se numes¸te<br />
funct¸ia lui Green a <strong>probleme</strong>i la limită.