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Evans-Entropy and partial differential equations. Evans 版偏微分方程补充内容
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Solutions to exercises from Chapter 2 of
Lawrence C. Evans’ book ‘Partial Differential
Equations’
S¨umeyye Yilmaz
Bergische Universit¨at Wuppertal
Wuppertal, Germany, 42119
February 21, 2016
1
Write down an explicit formula for a function u solving the initial
value problem
u
t
+ b · Du + cu = 0 in R
n
× (0, ∞)
u = g on R
n
× {t = 0}
)
Solution. We use the method of characteristics; consider a solution to the
PDE along the direction of the vector (b, 1): z(s) = u(x + bs, t + s). We have
˙z(s) = u
t
(x+bs, t+s)+b·Du(x+bs, t+s) = −cu(x+bs, t+s) = −cz(s), thus
the PDE reduces to an ODE. The characteristic curves can be parametrized
by (x
0
+bs, t
0
+s); and all the characteristic curves are parallel to each other.
Given any (x
0
, t
0
) in R
n
× (0, ∞), we have u(x
0
, t
0
) = u(x
0
− bt
0
, 0)e
−ct
0
=
g(x
0
−bt
0
)e
−ct
0
; and this is an explicit formula for the solutions to the PDE.
1
2
Prove that Laplace’s equation ∆u = 0 is rotation invariant; that is,
if O is an orthogonal n × n matrix and we define
v(x) := u(Ox) (x ∈ R
n
),
then ∆v = 0.
P roof. By chain rule we have
D
x
i
v(x) = Σ
n
k=1
D
x
k
u(Ox)o
ik
,
then
D
x
i
x
j
v(x) = Σ
n
l=1
Σ
n
k=1
D
x
k
x
l
u(Ox)o
ik
o
jl
.
Since O is orthogonal, OO
T
= I, that is,
o
ik
o
il
=
(
1 if k = l
0 if k 6= l.
Thus
∆v = Σ
n
i=1
Σ
n
k=1
Σ
n
l=1
D
x
k
x
l
u(Ox)o
ik
o
il
= ∆u = 0.
3
Modify the proof of the mean value formulas to show for n ≥ 3 that
u(0) =
∂B(0,r)
gdS +
1
n(n − 2)α(n)
ˆ
B(0,r)
(
1
|x|
n−2
−
1
r
n−2
)fdx,
provided
−∆u = f in B
0
(0, r)
u = g on ∂B(0, r)
)
P roof. Define
φ(s) :=
∂B(x,s)
u(y)dS(y) =
∂B(0,1)
u(x + sz)dS(z)
2
we shall have
φ
0
(s) =
∂B(0,1)
Du(x + sz) · zdS(z) =
∂B(x,s)
D(y)
y − z
s
dS(y)
=
∂B(x,s)
∂u
∂ν
dS(y) =
s
n
B(x,s)
∇
2
u(y)dy =
s
n
B(x,s)
∆u(y)dy
We have φ(r) − φ() =
ˆ
r
φ
0
(s)ds =
ˆ
r
s
n
B(0,s)
∆u(y)dy
ds
=
ˆ
r
s
n
B(0,s)
f(y)dy
ds =
ˆ
r
1
nα(n)s
n−1
ˆ
B(0,s)
f(y)dy
ds
=
1
n(2 − n)α(n)
1
s
n−2
ˆ
B(0,s)
f(y)dy
r
−
ˆ
r
1
s
n−2
ˆ
∂B(0,s)
f(y)dy
ds
=
1
n(n − 2)α(n)
−
1
r
n−2
ˆ
B(0,r)
f(y)dy +
1
n−2
ˆ
B(0,)
f(y)dy
+
ˆ
r
1
s
n−2
ˆ
∂B(0,s)
f(y)dy
ds
Now notice that
1
n−2
ˆ
B(0,)
f(y)dy ≤ C
2
;
and
ˆ
r
0
1
s
n−2
ˆ
∂B(0,s)
f(y)dy
ds =
ˆ
r
0
ˆ
∂B(0,s)
f(y)
s
n−2
dyds
=
ˆ
B(0,r)
f(x)
|x|
n−2
dx
As → 0 we have
1
n−2
ˆ
B(0,)
f(y)dy +
ˆ
r
1
s
n−2
ˆ
∂B(0,s)
f(y)dy
ds →
ˆ
B(0,r)
f(x)
|x|
n−2
dx
and φ() → u(0). We have thus demonstrated
3
u(0) =
∂B(0,r)
gdS +
1
n(n − 2)α(n)
ˆ
B(0,r)
(
1
|x|
n−2
−
1
r
n−2
)fdx.
4
Give a direct proof that if u ∈ C
2
(U) ∩ C
(
¯
U) is harmonic within a
bounded open set U, then max
¯
U
u = max
∂U
u.
P roof. Define u
:= u + |x|
2
. Suppose u
is achieveing maximum in
¯
U
at an interior point x
0
, then D(u
(x
0
)) = 0 and H = D
ij
(u
(x
0
)) is negative
definite. Yet, ∆(u
) = 2 ≥ 0, a contradiction, as the Laplacian is the trace
of the Hessian. Letting → 0, we can conclude that u cannot attain an
interior maximum.
5
We say v ∈ C
2
(
¯
U) is subharmonic if
−∆v ≤ 0 in U.
(a) Prove for subharmonic
v(x) ≤
B(x,r)
vdy for all B(x, r) ⊂ U.
(b) Prove that therefore max
¯
U
v = max
∂U
v.
(c) Let φ : R 7→ R be smooth and convex. Assume u is harmonic
and v := φ(u). Prove v is subharmonic.
(d) Prove v := |Du|
2
is subharmonic, whenever u is harmonic.
Solution. (a) We set
f(r) :=
∂B(x,r)
u(y)dS(y) =
∂B(0,1)
u(x + rz)dS(z).
4
Taking derivative we have
f
0
(r) =
∂B(0,1)
Du(x + rz) · zdS(z)
and consequently using Green’s formula we have
f
0
(r) =
∂B(0,1)
Du(y) ·
y − x
r
dS(z)
=
∂B(x,r)
∂u
∂ν
dS(y)
=
r
n
∂B(x,r)
∆u(y)dy ≥ 0
This means that f (r) is non-decreasing, therefore given r > 0
u(x) = lim
t→0
f(t) = lim
t→0
∂B(x,t)
u(y)dS(y)
≤
∂B(x,r)
u(y)dS(y)
And this implies
α(n)r
n
u(x) ≤
ˆ
r
0
ˆ
∂B(x,s)
u(y)dS(y)
ds
u(x) ≤
B(x,r)
u(y)dy
(b) Assume the subharmonic function v attains maximum at x
0
∈ U
0
, for
any ball B(x
0
, r) ⊂ U
0
we have by (a) v(x
0
) ≤
ffl
B(x
0
,r)
v(y)dy. Yet, v(x
0
) ≥
v(y) for any y ∈ B(x
0
, r), thus v(x
0
) =
ffl
B(x
0
,r)
v(y)dy, and v(x
0
) = v(y) for
any y ∈ B(x
0
, r). We can choose r so that ∂
¯
U ∩ ∂B(x
0
, r) = {x
1
}. We have
thus shown that max
x∈
¯
U
v = max
x∈∂U
v.
(c) As φ is convex and smooth, φ
00
≥ 0; and ∆u = 0. Hence
−∆φ(u) = −(
n
X
i=1
φ
00
· (u
x
i
)
2
+ φ
0
· u
x
i
x
i
) ≤ 0,
we can conclude that φ(u) is subharmonic.
(d) If u is harmonic then ∆u = 0, thus ∆u
x
i
= (∆u)
x
i
= 0, hence Du is
harmonic. Since |Du|
2
is convex with respect to Du, the result follows from
(c).
5
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