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## value problem in a new space

1*

### and Young Ja Park

2

*Correspondence:

hpak@dankook.ac.kr 1Department of Mathematics,

Dankook University, Anseo-Dong 29, Cheonan, Chungnam 330-714, Republic of Korea

Full list of author information is available at the end of the article

Abstract

We develop a new function space and discuss trace operator on the same

genealogical spaces. We also prove that the nonlinear boundary value problem with Dirichlet condition: –u=f(|u|) sgnuin the given domain,u= 0 on the boundary, possesses only a trivial solution iffobeys the slope condition:

### α

(x) >n2–2n αx(x), where

### α

is the anti-derivative offwith

### α(0) = 0.

MSC: Primary 30E25; 35A01; secondary 46E35; 42B35

Keywords: trace operator; boundary value problem; function space; non-existence

1 Introduction

We are interested in the nonlinear boundary value problem with Dirichlet condition: ⎧

⎨ ⎩

u=f(|u|)sgnu in,

u=  on, ()

whereis an open subset of then-dimensional Euclidean spaceRn. Questions of this kind

occur in many problems of mathematical physics, in the theory of traveling waves, homog-enization, stationary states, boundary layer theory, biology, ﬂame propagation, probability theory, and so on. This problem has been one of the most important and most discussed topics in the theory of partial diﬀerential equations during the past several decades.

Many physicists and mathematicians have studied the simplest form

u=|u|psgnu=|u|p–u, p> ,

which is a special case of () withf(|x|) =|x|p. A critical reason for studying this special

case is that the function spaces that have been used to deal with these problems are just the Lebesgue spacesLp() (especially for the existence theory). However, it is too good to

be true in reality!

In this paper we build up a new function space which has been designed to handle so-lutions of the general nonlinear boundary value problem () without imposing too much assumption on the functionf. As a matter of fact, in [], one can ﬁnd series of attempts to construct new function spaces which generalize classical Lebesgue spaces. We extend those ideas to obtain a better space. The motivation of these research comes from taking a close look at theLp-norm:f

Lp= (

X|f(x)|pdμ)/pof the classical Lebesgue spacesLp(X),

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≤p<∞. It can be rewritten as

fLp:=α–

X

α f(x)

, α(x) :=xp.

Even though the positive real-variable functionα(x) :=xp has very beautiful and conve-nient algebraic and geometric properties, it also has some practical limitations for the the-ory of diﬀerential equations, as pointed out above. The new space is devised to overcome these limitations without hurting the beauty ofLp-norm too much.

There are two diﬀerent attempts to generalize the classical Lebesgue spaces - Orlicz spacesLA(X) and Lorentz spacesLp,r(X,μ). The theory of Orlicz spaces has been well

de-veloped which is similar to our new spaces. The Orlicz spaceLA(X) requires the convexity

of theN-functionAfor the triangle inequality of the norm, whereas the norm for the space (X) does not require the convexity of the Hölder function and it has indeed inherited

the beautiful and convenient properties from the classical Lebesgue norm.

Based on the new function spaces(X), we present two main results. One of them is the

trace theorem in the space(X). The trace theorem is one of the basic requirements to

deal with the boundary value problems. We state and prove it in Section . We also discuss the non-existence of non-trivial solutions for the problem () if the given functionf is of fast growth. To be more precise, we prove that the boundary value problem () possesses only a trivial solution iff obeys the following slope condition:

α(x) > n n– 

α(x) x ,

whereαis the anti-derivative off withα() =  andn>  (see Section  for details). Throughout this paper,represents an open subset ofRnand (X,M,μ) is an abstract

measure space (Section ). Also,Cdenotes various real positive constants.

2 The spaceLα(X)

We introduce some terminologies to deﬁne the Lebesgue type function spaces (X)

which improve the original version introduced in []. In this section,R¯+={x∈R:x≥}. Apre-Hölder functionα:R¯+→ ¯R+is anabsolutely continuous bijectivefunction satis-fyingα() = . If there exists a pre-Hölder functionβsatisfying

α–(x)β–(x) =x ()

for allx∈ ¯R+, thenβis called theconjugate(pre-Hölder)functionofα. In the relation (), the notationsα–,β–are meant to be the inverse functions ofα,β, respectively. Examples of pre-Hölder pairs are (α(x),β(x)) = (xp,xq) forp> ,

p+

q=  and (α(x),β(x)) = (¯–◦

¯

α(x),¯–◦ ¯β(x)) forα¯(x) = ex– x– ,β¯(x) = ( +x)log( +x) – x. In fact, for any Orlicz

N-functionAtogether with complementaryN-functionA˜, (λA,λ◦ ˜A) is a pre-Hölder pair withλ(x) =A–(x)A˜–(x).

Some basic identities for a pre-Hölder pair (α,β) are listed:

x=β

x

α–(x)

or α(x) =β

α(x) x

, ()

x= α(x)

β–(α(x)) or

α(x)

x =β

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β–(x)

α(α–(x))+

α–(x)

β(β–(x))= , ()

y

α(x)+ x

β(y)=  fory:=

α(x)

x , ()

α(x) =α(x)

x +

α(x)

β(α(xx)) –x. ()

In the following discussion, a functionrepresents the two-variable function onR¯+× ¯

R+deﬁned by

(x,y) :=α–(x)β–(y),

provided that a pre-Hölder pair (α,β) exists.

Deﬁnition . Let> . A pre-Hölder functionαwith the conjugate functionβis said to be a Hölder function if for any positive constantsa,b> , there exist constantsθ,θ (depending ona,b) such that

θ+θ≤

and that acomparable condition

(x,y)≤θab

α(a)x+θab

β(b)y ()

holds for all (x,y)∈ ¯R+× ¯R+.

The following proposition and the proof may illustrate that the comparable condition () is not far-fetched.

Proposition . Letαbe a convex pre-Hölder function together with the convex conjugate functionβ.Suppose that for any a,b≥,there are constants p,p,q,q(depending on a, b)with p

+ 

p ≤and

q+ 

q ≥satisfying the slope conditions;

p

α(a)

aα

(a)q

α(a)

a , ()

p

β(b)

bβ

(b)q

β(b)

b . ()

Thenαis a Hölder function. (So isβ.)

Proof The equation of the tangent plane of the graph ofat (¯a,b¯) reads

z=xa,b¯)(xa¯) +ya,b¯)(yb¯) +(a¯,b¯)

= β –(b¯)

α(α–(¯a))(xa¯) +

α–a)

β(β–(b¯))(yb¯) +α

–(a¯)β–(b¯)

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Then forα–a) :=aandβ–(b¯) :=b,T ¯

a,b¯(x,y) can be rewritten as

Ta¯,b¯(x,y) = b

α(a)x+ a

β(b)y+ab(a)

α(a) – (b)

β(b). ()

From the slope conditions (), () together with the observation that

(a)

α(a) + (b)

β(b) ≥  q

ab+  q

abab,

we have

Ta¯,b¯(x,y)≤  p

ab

α(a)x+  p

ab

β(b)y. ()

On the other hand, we observe that every point on the surface z=(x,y) is an elliptic point since the Gaussian curvature of a point on the surfacez=(x,y) is positive from the convexity hypotheses onαandβ(we refer to p. in []). Hence the tangent planes z=Ta¯,b¯(x,y) touch the graph at (¯a,b¯) and nowhere lie below the graphz=(x,y), that is, for anya¯,b¯,

(x,y)≤Ta¯,b¯(x,y). ()

In fact, since the restrictionz=Ta¯,b¯(x,b¯) of the tangent planez=Ta¯,b¯(x,y) is a tangent line

to the graph(x,b¯) =–(x) (β–(b¯) =b) in thex-zplane andα–is concave up onR¯+, we get(x,b¯)≤Ta¯,b¯(x,b¯). Furthermore, since (a¯,b¯) is an elliptic point, a local neighborhood of (a¯,b¯) in the surfacez=(x,y) belongs to the same side ofz=Ta¯,b¯(x,y) (p. in []). So on a local neighborhood of (a¯,b¯), the graphz=(x,y) lies below the tangent plane z=Ta¯,¯b(x,y). This holds for all (a¯,b¯). Hence(x,y) is concave up onR¯+× ¯R+, which, in turn, illustrates (). Combining () and (), we conclude that

(x,y)≤θab

α(a)x+θab

β(b)y,

where we setθ:=p andθ:=p.

We now deﬁne theLebesgue-Orlicz type function spaces Lα(X): for a Hölder functionα,

(X) :=

f |f is a measurable function onXsatisfyingfLα<∞

,

where we set

fLα:=α

–

X

α f(x)

. ()

A Hölder type inequality and a Minkowski inequality on the new space(X) are

pre-sented as follows:

Letαbe a Hölder function andβbe the corresponding Hölder conjugate function. Then for anyf(X) andg(X), we have

X

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The name of Hölder functions originates from the Hölder inequality (). So we brieﬂy sketch the idea. Leta:=gLβ (= ),b:=fLα (= ), and then there existθ,θsuch that θ+θ≤and

f(x)g(x) =α–α f(x) β–β g(x)

θab

α(b)α f(x) +θab

β(a)β g(x) .

Integration of both sides yields

X

f(x)g(x) θab

α(b)

X

α f(x) +θab

β(a)

X

β g(x)

fLαgLβ.

As an important application of a Hölder inequality, we have the Minkowski inequality on(X). We omit the proof.

Remark . (Generalized Minkowski inequality) Let (,M,ν) be a σ-ﬁnite measure space. Suppose thatαis a Hölder function andf is a nonnegative measurable function onX×satisfyingf(·,y)∈(X) for almost everyy. Then

f(·,y)(y)

(X)

f(·,y)L

α(X)(y).

In particular, for anyf,f∈(X), we have

f+f

f+f

.

We can also show that the metric space(X) is complete with respect to the metric:

d(f,g) :=fgLα forf,g(X).

We now present some remarks on the dual space of(X). To eachg(X) is

associ-ated a bounded linear functionalFgon(X) by

Fg(f) :=

X

f(x)g(x),

and the operator (inhomogeneous) norm ofFgis at mostgLβ:

FgLα:=sup

|

Xfg dμ|

fLα

:f(X),f = 

gLβ. ()

For =g(X), if we putf(x) :=

β(|g(x)|)sgn(g(x))

|g(x)| , we havef(X) and

FgLα=sup

|

Xfg dμ|

fLα

:f(X),f = 

≥|

Xfg dμ|

fLα

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This implies that the mappinggFgis isomorphic from(X) into the space of

continu-ous linear functionals(X). Furthermore, it can be shown that the linear transformation

F:(X)→(X)isonto.

Remark .(Dual space of(X)) Letβ be the conjugate Hölder function of a Hölder

functionα. Then the dual space(X)is isomorphic to(X).

The proof is quite parallel to the classical Riesz representation theorem, so we omit the proof.

The two inequalities () and () explain the quasi-homogeneity of · . That is, we

have the following.

Proposition . For all k≥and f(X),

k

fLαkfLαkfLα.

In particular,when= ,we have homogeneity:

kfLα=kfLα.

The metric space(X) and the classical Orlicz spaceLA(X) diﬀer by the choice of the

conjugate function. In fact, for the Orlicz spaceLA(X), the complementaryN-functionA˜

ofAis designed to satisfy the relation

˜

A=A–,

which implies, in turn,

cxA–(x)A˜–(x)≤cx

for some constantsc,c> . Also, theLuxemburg norm

uA=inf

k>  :

A |

u(x)| k

dx≤

for the Orlicz spaceLA(X) requires the convexity of theN-functionAfor the triangle

in-equality of the norm. On the other hand, the (inhomogeneous) norm for the space(X)

does not require the convexity of the Hölder function and it has indeed inherited the beau-tiful and convenient properties from the classical Lebesgue norm.

3 Trace operator on Sobolev type spaceWα1

Letbe an open subset ofRn. The Sobolev type spaceW

α() is employed in

() :=

u()|∂xju(),j= , , . . . ,n

together with the norm

uW

α:=uLα+

n

j=

(7)

where∂xj:=

∂xj. Then it can be shown that the function spaceW

α() is a separable

com-plete metric space, and thatC∞()∩() is dense in().

The completion of the space Cc∞() with respect to the norm · W

α is denoted by

W

α,(), whereCc () is the space of smooth functions onwith compact support.

We introduce the trace operator onW

α(), which is important by itself and also useful in

Section . We want to point out that the trace operator we present here is an improvement and a completion of the one brieﬂy introduced in [].

We say that a pre-Hölder functionβsatisﬁes aslope conditionif there exists a positive constantc>  for which

β(x)≥(x)

x ()

holds for almost everyx> . The slope condition (), in fact, corresponds to the -con-dition for Orlicz spaces.

We prove that the boundary trace on C∞(¯)∩() can be extended to the space

W

α() as follows.

Theorem . (Trace map onW

α) Let(α,β)be a Hölder pair obeying the slope

condi-tion()and be a bounded open subset ofRnwith smooth boundary.Then the trace

operator:W

α()→()is continuous and uniquely determined by(u) =u|∂for

uC∞(¯)∩W

α().

Proof We ﬁrst prove the theorem for the special case of ﬂat boundary, and by using it, we take care of the general cases.

A special case- =Rn

+. For the case=Rn+:={(x,xn)|x∈Rn–,xn> } and for a

smooth functionfC∞(Rn

+)∩(Rn+), we observe that

α fx,  = –

∂xnα f

x,xn dxn

α fx,xn ∂xnf

x,xn dxn

∂xnf

xL

α(,∞)α

fx,·

(,∞). ()

Owing to the identity (), we have

α(t) =s+ (t)

β(s), s=

α(t)

t . ()

On the other hand, using the identity (), we can notice that the slope condition () is equivalent to

β

α(t) t

ct. ()

Reﬂecting () to the identity (), we have

α f(x) ≤ c c– 

α(|f(x)|) |f(x)| , x=

x,xn

(8)

Therefore we have

β–

βα fx,xn dxn

β–

β

c c– 

α(|f(x,xn)|)

|f(x,xn)|

dxnc c– 

β– ∞  β

α(|f(x,xn)|)

|f(x,xn)|

dxn = c c– 

β–

α fx,xn dxn

.

Inserting this into the right side of (), we obtain

α fx,  ≤C∂xnf

xL

α(,∞)β

–αfx,·

(,∞)

Cα∂xnf

xL

α(,∞)

+αfxL

α(,∞)

for some positive constantsC(twoC’s may be diﬀerent). The comparable condition () has been used in the second inequality. Taking integrations on both sides overRn–, we obtain

αfLα(Rn–)

Cα∂xnfLα(Rn+)

+αfLα(Rn+)

. ()

This inequality says that the trace onC∞(Rn

+)∩(Rn+) can be uniquely extended to the space(Rn+).

The general case-being bounded open inRn. In this section we restrict our attention

to the case ofbeing a bounded open subset. However,can be more general, such as unbounded domains satisfying the uniformCm-regularity condition (p. in []).

Assume thatis ann–  dimensionalCm-manifold. Letting

Q:=y∈Rn| |yi| ≤

,

Q:={yQ|yn= },

Q+:={yQ|yn> },

the last condition can be stated as follows. There is a ﬁnite collection of open bounded sets inRn;,, . . . ,

N with

{j|≤iN} ⊃and correspondingıjCm(Q;j)

which are bijections satisfyingQ, Q+, and Q mapping ontoj,j, and j,

respectively, and each JacobianJ(ıj) is positive. Each pair (ıj,i) is acoordinate patch.

Let:=. We can construct℘jC∞(j), ≤jN, with℘j(x)≥, and

N

j=

℘j(x) =  forx∈ ¯.

Thus,{℘j|≤jN}is apartition-of-unitysubordinate to the open cover{j|≤jN}

of, and{℘j|≤jN}is a partition-of-unity subordinate to the open cover{j|≤

jN}of¯.

Iff is a function deﬁned on, then we have

f dS

N

j=

j

℘jf dS= n

j=

Q

(℘jf)◦ıj

y, Jj

(9)

wheres= (s,s, . . . ,sn) =ıj(y, ) andJj(y) is the magnitude of the vector

det

⎛ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎜ ⎝

ee · · · en

(s)y (s)y · · · (sn)y (s)y (s)y · · · (sn)y

. ..

(s)yn– (s)yn– · · · (sn)yn– ⎞ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎟ ⎠

()

atyn= . Here{e,e, . . . ,en}represents the standard basis forRn. From the fact that

() = (s, . . . ,sn)

(y, . . . ,yn–)

e+

(s,s, . . . ,sn)

(y, . . . ,yn–)

e+· · ·+

(s, . . . ,sn–)

(y, . . . ,yn–)

en

=

(s, . . . ,sn)

(y, . . . ,yn–)

, . . . ,(s, . . . ,sn–)

(y, . . . ,yn–)

,

we notice that

Jj

y=

N

k=

(s, . . . ,ˆsk, . . . ,sn)

(y, . . . ,yn–)

yn= /

.

Then by the smoothness property,

Jj

yK, ≤jN,yQ,

sincem≥. Finally, we construct the trace onas indicated. First, we represent

()→,((Q+)N,

u=

N

j=

℘ju

u, (u)◦ı, . . . , (℘Nu)◦ıN

.

We take the trace operator on each component except for the ﬁrst componentu, to say j, ≤jN:

,((Q+)N(Q)N,

u, (u)◦ı, . . . , (℘Nu)◦ın

→(uı), . . . ,N(℘NuıN)

.

We note thatj(℘juıj) = (℘jj(u))◦ıj, ≤jN. Finally, summing up all components,

we obtain

(Q)N(),

(u)◦ı, . . . ,

℘NN(u)

ıN

→ (u)≡

N

j=

(10)

The fact(u)∈() follows from the estimates

α (u) dS

N

j=

j

α|ju|

dSK

N

j=

Q

α j(u)◦ıj dy

KCα

N

j=

αuıjWα(Q+)

KCα

N

j=

αkıuW

α(j)

KCαNα

kıuWα()

,

whereKis the maximum of all Jacobians,is the norm of the trace from half-space as in

(), and is the largest norm inunder a change of variablesıj:(Q+)→(j). Clearly, ifuC∞(¯)∩() then(u) =u|. The proof is now completed.

4 Ill-posedness of boundary value problems

In this section we investigate a nonlinear elliptic partial diﬀerential equation, namely the nonlinear boundary value problem:

⎧ ⎨ ⎩

u=f(|u|)sgnu in,

u=  on. ()

Here we assume thatf is the derivative of a Hölder functionαsatisfying a slope condition; f :=α. Hence there exists a positive constantc>  for which

α(x)≥(x)

x ()

holds for almost every x> . Our goal is to demonstrate that the slope condition () with large constantcimplies thatu≡ is the only strong solution of () under a certain geometric condition on. We are going to ﬁnd such a constantcexplicitly.n–n turns out to be exact forn> .

In the following discussion,is assumed to be a bounded open set with smooth bound-ary. We multiply the PDE () byuand integrate overto ﬁnd

uu dx=

f|u|u dx=

α|u||u|dx.

The left side can be rewritten as

uu dx= –

div(∇u)u dx

= –

(∇u)u· n dS+

u· ∇u dx

=

(11)

sinceu=  on. In the above,nrepresents the unit outward normal vector. Therefore we get

|∇u|dx=

α|u||u|dxc

α|u|dx. ()

On the other hand, multiplying the PDE () byx· ∇uand integrating over, we get

(–u)(x· ∇u) =

α|u|x· ∇|u|dx. ()

We take a close look at the left side:

div(∇u)(x· ∇u)dx= –

(∇u)(x· ∇udS+

(∇u)· ∇(x· ∇u)dx,

and we considerIandIIseparately which are deﬁned as

I= –

(∇u)(x· ∇udS and II=

(∇u)· ∇(x· ∇u)dx.

We ﬁrst take care of the second termII. For it, we observe that

(∇u)· ∇(x· ∇u) = (∇u)·(∇u) + (∇ux∇u, ()

and the second term on the right side of () becomes

(∇ux∇u=x·∇u·(∇u) =x· ∇

(∇u)·(∇u)=x·  ∇

|∇u|.

This says thatIIcan be rewritten as

II=

|∇u|dx+ 

x· ∇|∇u|dx. ()

The second term of () is, in turn,

 

x· ∇|∇u|dx= 

|∇u|(x· n)dS– 

(divx)|∇u|dx

= 

|∇u|(x· n)dSn

|∇u|dx.

Therefore we obtain

II=

 –n

|∇u|dx+ 

|∇u|(x· n)dS.

We now take care ofI. We rewrite it as

I= –

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Sinceu=  on,∇u(x) is parallel to the normal vectorn(x) at each pointx. Thus we get∇u(x) =±|∇u|n. This identity makes it possible to rewriteIas

I= –

±|∇u|n· nx·±|∇u|ndS

= –

|∇u|(x· n)dS,

where we count on the fact that|n|= . In all, the left side of () becomes

div(∇u)(x· ∇u)dx

= –

|∇u|(x· n)dS+

 –n

|∇u|dx+ 

|∇u|(x· n)dS

=

 –n

|∇u|dx–  

|∇u|(x· n)dS. ()

Now we consider the right side of ():

α u(x) x· ∇ u(x) dx=

α u(x) ·x dx

=

α u(x) (x· n)dSn

α u(x) dx

= –n

α u(x) dx. ()

The last equality follows from the fact thatα(|u|) =  on. In view of () together with () and (), we get

n– 

|∇u|dx+ 

|∇u|(x· n)dS=n

α u(x) dx

n

c

|∇u|dx,

which can be written as

n–   –

n

c

|∇u|dx+ 

|∇u|(x· n)dS≤. ()

Hence if we suppose thatis a connected convex domain containing the origin, for ex-ample, an open ball={x:|x|<r}, thenx· n(x)≥ for allx. From this, we see that () implies

n– 

 – n

c

|∇u|dx≤,

which says, in turn, that the constantcshould be less than or equal ton–n. We summarize.

Theorem . Letαbe a Hölder function with the slope condition()and f :=αand let

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smooth boundary.Then the nonlinear boundary value problem: ⎧

⎨ ⎩

u=f(|u|)sgnu in,

u=  on∂(in the sense of a trace map),

has only a trivial solution in W

α()if c>n–n,where c is the constant appearing in().

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

HCP organized and wrote this paper. YJP contributed to all the steps of the proofs in this research. All authors read and approved the ﬁnal manuscript.

Author details

1Department of Mathematics, Dankook University, Anseo-Dong 29, Cheonan, Chungnam 330-714, Republic of Korea.

2Department of Mathematics, Hoseo University, Asan, Chungnam 336-795, Republic of Korea.

Acknowledgements

The ﬁrst author was supported by the research fund of Dankook University in 2012.

Received: 1 February 2014 Accepted: 5 June 2014

References

1. Pak, H-C: Existence of solutions for a nonlinear elliptic equation with general ﬂux term. Fixed Point Theory Appl.2011, Article ID 496417 (2011)

2. do Carmo, M: Diﬀerential Geometry of Curves and Surfaces. Prentice Hall, Englewood Cliﬀs (1976) 3. Adams, R: Sobolev Spaces, 2nd edn. Academic Press, Amsterdam (2003)

doi:10.1186/s13661-014-0153-z

References

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