Uniqueness of Positive Radial Solutions for a Class of Semipositone Systems on the Exterior of a Ball ()
1. Introduction
In reaction diffusion processes, steady states define the long term dynamics. Here we consider a steady state reaction diffusion equation on an exterior domain with a nonlinear boundary condition on the interior boundary. Namely, we study positive radial solutions to:
(1.1)
where
and
are the Laplacian of u,
is a positive parameter,
, let
then
is a continuous function such that
and
is the outward normal derivative, and
is a is an non
decreasing (increasing) function. Here the reaction term
is a
function. The case when
(see [1] [2], that the study of positive solutions to such problems is considerably more challenging than in the case
(positone problems). For a rich history on semipositone problems with Dirichlet boundary conditions on bounded domains, (see [3] - [8], and on domains exterior to a ball, see [9] [10] [11]. Such nonlinear boundary conditions occur very naturally in applications see [12] for a detailed description in a model arising in combustion theory. Recently, the existence of a radial positive solution for (1.1) when
has been established in [13], via the method of subsuper solutions. Here we discuss the uniqueness of this radial solution when some additional assumptions hold. In [14], the authors study such a uniqueness result for the case of Dirichlet boundary condition on
. Our focus in this paper is to consider the uniqueness result for semipositone problem when a class of nonlinear boundary condition is satisfied at
. The fact that we have no longer a fixed value of u on
results in quite a challenge in extending the results in [15].
Namely, we need to establish a detailed behavior of u at
to achieve our goal. Instead of working directly with (1), we note that the change of
variables
and
transforms (1) into the following boundary value problem:
(1.2)
where
. We will only assume
for
and for some
. then
could be singular at 0.if
,
will be nonsingular at 0 and it will be an easier case to study. Note that
and there exists a constant
such
that
for all
where
. Motivated by the above discussion, in this paper, we will study positive solutions in
to the following boundary value problems:
(1.3)
where
is a continuous function and
is such that:
(H1)
;
(H2) there exists a constant
such that
for all
where
and
(H3)
is decreasing. We consider various
classes of the reaction term
satisfying the following:
(F1)
and
;
(F2)
is increasing and
;
(F3)
is concave on
.
Theorem 1.1. Assume (H1) - (H3) and (F1) - (F3). Then (1.3) has a unique positive solution for all
sufficiently large.
In Section two we will establish important a priori estimates. We will first recall some important results from [8] where the authors studied the case of Dirichlet boundary condition, or equivalently (1.3) with the boundary condition
replaced by
. These results do not depend on the boundary condition at
and hence it is also true for solutions of (1.3). In view of the readers convenience we include the proofs of these results. In Section three, we prove Theorem 1.1.
2. Advance Estimates
Let
. Note that there exist unique positive numbers
such that
and
and
Theorem 2.1. (See [8].) Let
are a positive solution of (1.3). Then u and v has only one interior maximum in
, say at
, depending on
, and
,
.
Proof. Let
(1.4)
then
(1.5)
Note that by (H3),
and
for all
. Hence,
and
are increases when
,
and decreases when
,
.
Let
be the first point at which u has a local maximum and assume that
and
for all
. Then
and
are increases in
. Now integrating (1.3) from t to
, for
(1.6)
where
are such that
for all
using (H2). Integrating again (1.6) from 0 to t,
(1.7)
Since
are continuous, there exists
such that
and
for all
. Hence
Hence
. Since
increases on
,
and
and which is a contra-diction if
and
. Suppose that u and v has two interior maxima. Then there exists
such that
,
and
,
. Since
, and
we have
and
which implies that
and
. Thus
. Let
such that
and
. Then
,
and
increases in
since
and since
in
. Hence
, which is a contradiction. Therefore, we have only one interior maximum and that maximum value is larger than
.
Theorem 2.2. (See [8].) Let u and v be a positive solution of (1.3) and let
such that
and
. Then
as
.
Proof. Let
be the point such that
and
from Integrating (1.3) from 0 to t for some
Integrating the above again from 0 to
(1.8)
By the mean value theorem, there exists a
such that
,
and by (2.5)
and
. Since
and
are increases in
,
(1.9)
Integrating (2.6) from
to
, we have that
and
. This implies
by (2.5).
Lemma 2.3. Let u and v be a positive solution of (1.3). Then
and
as
Proof. We first claim that
and
for
. Assume that
and
Then there exists a
such that
and
where
is the point at which
achieves are maximum, and
,
by Lemma 2.1.
From (2.1) and (2.2),
,
and
on
since
and
in
. Hence we obtain that
,
and from (1.3), we have
(1.10)
This cannot hold unless
and
as
. However, rewriting (1.10) as
(1.11)
we obtain a contradiction when
since
,
if
and
as
. Hence,
(1.12)
Next, we claim that
and
as
. Let
and
then
,
in
and satisfies
(1.13)
Let
. Then
satisfies:
(1.14)
Multiplying (1.13) by
and (1.14) by h,and w integrating both from
to 1 and subtracting, we have
Since
and
by integration by parts, we can see that
(1.15)
Note that
,
and from Lemma 2.2 we can assume
. Thus (2.11) is only true if
and
when
. By (F1) (F2), we conclude that
and
as
. Notice that since
and
in
, we have
Since
,
and
as
, it is all true that
(1.16)
Now we show that
and
as
. Since
is a solution of (1.3), u and v can be written as: (see Appendix 8.2 in [5] for details)
(1.17)
(1.18)
where
Let
. Then from (1.17) and (1.18), we have
(1.19)
(1.20)
Then using the fact
and
as
, for
large we obtain
where the last inequality is obtained by (1.16). Hence, we have
(1.21)
where
and
.
Now, from (1.21), clearly
,
as
.
Lemma 2.4. Let u and v be a positive solution of (1.3). Then there exists
,
, both independent of
, such that
and
as
Proof. As
,
may converge to 1 or to any other point in
. First we consider the case
as
. Since
and
clearly there exists
such that
and
as
by Lemma 2.3.
Now, let us consider the case when
as
. By differentiating (1.17) and (1.18) (or integrating (1.3)), we obtain
which gives us that
(1.22)
(1.23)
Now we rewrite (1.17) and (1.18) by using (1.22), (1.23) as:
Note that if
, then
and
since
implies
. Now
and
as
. Hence, for
and
large we obtain
Thus,
and
on
, which means that
and
for all
as
.
Lemma 2.5. Let u and v be a positive solution of (1.3). Then there exists
such that if
, then
(1.24)
for some positive constant C, independent of
. Here
.
Proof. Let
be the unique solution of the problems
(1.25)
where
is the characteristic function. By the Hopf maximum principle there exists
such that
for all
, where
are a solution of
(1.26)
Let
be such that
, and this is possible by (F2). Let
and
satisfy
and
Now by Lemma 2.4, there exists
such that if
, then
(1.27)
Hence, by (1.27), for
we have that for
,
and
,
. By the maximum principle,
and
in
. Hence
and
for all
.
Note that there exists
such that
and
for all
. Hence, for
large
and
for all
, where
.
Lemma 2.6. Let u and v be a positive solution of (1.3). Then there exists
such that
and
.
Proof.
Let
. Then
since
for all
for some
. Now for each given
, there exists
such that if
, then
due to the hypothesis (F1). Also since
, there exists
such that
on
. Hence,
(1.28)
Now by Lemma 2.1 and (1.28), we have
Hence, for each
,
and
, where
.
3. Proof of Theorem 1.1
We first claim that (1.3) has a maximal positive solutions
for
. Let
be the solutions of the problems
(1.29)
Note that (1.29) has the unique solution since
,
are sub solutions and super solutions of (1.29), respectively, where
is defined in (1.26) and
is the solutions of the linear boundary condition problems
(1.30)
Since
satisfies (F1), given
, we can choose
such that
and
where
is as in Lemma 2.6. Then,
are a super solutions of (1.3) since
Next, we show that this super solution
is larger than any positive solutions of (1.3). Let
be any positive solutions of (1.3). From Lemma 2.6, we have
by the choice of
. Note that
. Now we show that
. Indeed, since
, we have
(1.31)
If we assume that
, then
since
increases. Hence from (3.3) we obtain
. However,
in
,
and
implies that
, which is a contradiction. Hence
. Therefore, by the maximum principle,
in
. Therefore, (1.3) has a maximal positive solutions
. Now, let u and v be any other positive solutions of (1.3). To establish our theorem, we will show that
and
for
. Since
and
are solutions of (1.3), we obtain
By the mean value theorem, there exists
such that
and
quadin
and
(1.32)
By multiplying (1.3) by
and (1.32) by
and integrating, we first obtain, using integration by parts,
since
are increasing. Using that
is concave, we also have
(1.33)
(1.34)
From (F1), there exist
,
such that
whenever
. From (1.20), for
,
and
if
. Let
and
. Then from (1.33), we have
(1.35)
since when
is concave
for all
.
Next let m and h satisfy
and
respectively. Now multiplying (1.32) by
and integrating, we obtain
(1.36)
Note that as
,
and
in
. Hence, for
large, we obtain
, in
and
(1.37)
and
(1.38)
Hence for
, (1.37) implies
,
and combining with (1.34) (which implies
and
) we have
and
. However, by (1.32), we also have
Now for
, using (1.36),
, and
we get
and
. Hence, we conclude that
and
for
, which implies that
and
in
. This proves that (1.3) has a unique positive solution for all
large.
4. Conclusion
In the paper, were studied the positive radial solutions for elliptic systems to the nonlinear Boundary Value problems. And then, we presented that by the Theorem 1.1, and Theorem 2.2, we can obtain a solution of the problem (1.3). Moreover, for all
, then (1.3) has a unique positive solution.
Acknowledgements
The authors would like to thank the anonymous referees for their helpful comments.