Following Morrey [14] we associate to any measurable symmetric
$2 \times 2$
2
×
2
matrix valued function
$A(x)$
A
x
such that
$$\frac{|\xi|^{2}}{K} \le (A(x) \xi,\xi) \le K |\xi|^{2} \quad \text{a.e.} \quad x \in \Omega, \, \forall \xi \in \mathbb{R}^{2},$$
ξ
2
K
≤
A
x
ξ
,
ξ
≤
K
ξ
2
a.e.
x
∈
Ω
,
∀
ξ
∈
R
2
,
$\Omega \in \mathbb{R}^{2}$
Ω
∈
R
2
and to any
$u \in W^{1,2}(\Omega)$
u
∈
W
1
,
2
Ω
another symmetric
$2 \times 2$
2
×
2
matrix valued function
$\mathcal{A} = \mathcal{A}(A,u)$
A
=
A
A
,
u
with
$det \, \mathcal{A} = 1$
d
e
t
A
=
1
and satisfying
$$\frac{|\xi|^{2}}{K} \le (\mathcal{A}(x) \xi,\xi) \le K |\xi|^{2} \quad \text{a.e.} \quad x \in \Omega, \, \forall \xi \in \mathbb{R}^{2},$$
ξ
2
K
≤
A
x
ξ
,
ξ
≤
K
ξ
2
a.e.
x
∈
Ω
,
∀
ξ
∈
R
2
,
The crucial property of
$\mathcal{A}$
A
is that
$\mathcal{A} \nabla u = A \nabla u$
A
∇
u
=
A
∇
u
, if
$\nabla u \neq 0$
∇
u
≠
0
. We study the properties of
$\mathcal{A}$
A
as a function of
$A$
A
and
$u$
u
. In particular, we show that, if
$A_{b} \rightarrow^{G} A$
A
b
→
G
A
,
$u_{b} \rightharpoonup u$
u
b
⇀
u
,
$\nabla u \neq 0$
∇
u
≠
0
and
$div \, A_{b} \nabla u_{b} = 0$
d
i
v
A
b
∇
u
b
=
0
then
$\mathcal{A} (A_{b},u_{b}) \rightarrow^{G} \mathcal{A} (A, u)$
A
(
A
b
,
u
b
)
→
G
A
(
A
,
u
)
.
Seguendo Morrey [14], ad ogni matrice simmetrica
$A(x)$
A
x
a coefficienti misurabili, tale che
$$\frac{|\xi|^{2}}{K} \le (A(x) \xi,\xi) \le K |\xi|^{2} \quad \text{a.e.} \quad x \in \Omega, \, \forall \xi \in \mathbb{R}^{2},$$
ξ
2
K
≤
A
x
ξ
,
ξ
≤
K
ξ
2
a.e.
x
∈
Ω
,
∀
ξ
∈
R
2
,
$\Omega \in \mathbb{R}^{2}$
Ω
∈
R
2
e ad ogni
$u \in W^{1,2}(\Omega)$
u
∈
W
1
,
2
Ω
si può associare un'altra matrice simmetrica
$\mathcal{A} = \mathcal{A}(A,u)$
A
=
A
A
,
u
con
$det \, \mathcal{A} = 1$
d
e
t
A
=
1
e soddisfacente
$$\frac{|\xi|^{2}}{K} \le (\mathcal{A}(x) \xi,\xi) \le K |\xi|^{2} \quad \text{a.e.} \quad x \in \Omega, \, \forall \xi \in \mathbb{R}^{2},$$
ξ
2
K
≤
A
x
ξ
,
ξ
≤
K
ξ
2
a.e.
x
∈
Ω
,
∀
ξ
∈
R
2
,
La principale proprietà di
$\mathcal{A}$
A
è che
$\mathcal{A} \nabla u = A \nabla u$
A
∇
u
=
A
∇
u
, se
$\nabla u \neq 0$
∇
u
≠
0
. Si studiano le proprietà di
$\mathcal{A}$
A
come funzione di
$A$
A
e di
$u$
u
. In particolare, si dimostra che, se
$A_{b} \rightarrow^{G} A$
A
b
→
G
A
,
$u_{b} \rightharpoonup u$
u
b
⇀
u
,
$\nabla u \neq 0$
∇
u
≠
0
and
$div \, A_{b} \nabla u_{b} = 0$
d
i
v
A
b
∇
u
b
=
0
then
$\mathcal{A} (A_{b},u_{b}) \rightarrow^{G} \mathcal{A} (A, u)$
A
(
A
b
,
u
b
)
→
G
A
(
A
,
u
)
.