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# Multiple blowing-up and concentrating solutions for Liouville-type equations with singular sources under mixed boundary conditions

Yibin Chang* and Haitao Yang

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Department of Mathematics, Zhejiang University, Hangzhou 310027, China

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Boundary Value Problems 2012, 2012:33  doi:10.1186/1687-2770-2012-33

 Received: 17 October 2011 Accepted: 23 March 2012 Published: 23 March 2012

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

In this article, we mainly construct multiple blowing-up and concentrating solutions for a class of Liouville-type equations under mixed boundary conditions:

for ε small, where , Ω is a bounded, smooth domain in , Γ := {p1, ..., pN} ⊂ Ω is the set of singular sources, δp denotes the Dirac mass at p, ν denotes unit outward normal vector to Ω and b(x) > 0 is a smooth function on Ω.

2000 Mathematics Subject Classification: 35B25; 35J25; 35B38.

##### Keywords:
multiple blowing-up and concentrating solution; Liouville-type equation; singular source; mixed boundary conditions; finite dimensional reduction

### 1 Introduction

(1)

for ε small, where , Ω is a bounded, smooth domain in , Γ:= {p1, ..., pN} ⊂ is the set of singular sources, δp denotes the Dirac mass at p, ν denotes unit outward normal vector to Ω and b(x) > 0 is a smooth function on Ω.

Such problems occur in conformal geometry [1], statistical mechanics [2-4], Chern-Simons vortex theory [5-11] and several other fields of applied mathematics [12-16]. In all these contexts, an interesting point is how to construct solutions which exactly "blow-up" and "concentrate" at some given points, whose location carries relevant information about the potentially geometrical or physical properties of the problem. However, the authors mainly consider the Dirichlet boundary value problem, and little is known for the problem with singular sources satisfying αi ∈ (-1, 0) for some i = 1, ..., N. The main purpose of this article is to study how to construct multiple blowing-up and concentrating solutions of the Equation (1) with the mixed boundary conditions and singular sources.

Let Gt,ε denotes the Green's function of -∆ with mixed boundary conditions on Ω, namely for any y ∈ Ω,

(2)

and let Ht,ε(x, y) = Gt,ε(x, y) + log |x - y| be its regular part. Set G1 = G1,ε and H1 = H1,ε. Since ε exactly disappears in the Equation (2)|t = 1, G1 and H1 don't depend on ε. The Equation (1) is equivalent to solving for , the regular part of v, the equation

Thus, we consider the more general model problem:

(3)

where is a smooth function such that f(pi) > 0 for any i = 1, ..., N. Set Ω' = {x ∈ Ω: f(x) > 0}, and Δm = {p = (p1, ..., pm) ∈ m: pi = pj for some i ≠ j}.

It is known that for , or αi = 0 for any i = 1, ..., N, if uε is a family of solutions of the Equation (3)|t = 1 with inff > 0, which is not uniformly bounded from above for ε small, then uε blows up at different points with n + m ≥ 1, 0 ≤ n N, and , and satisfies the concentration property:

(4)

in the sense of measures in . Moreover, is a critical point of the function:

(5)

(see [7,17-23]). An obvious problem for the Equation (3) is the reciprocal, namely the existence of multiple blowing-up solutions with concentration points near critical points of φn,m.

The earlier result concerning the existence of multiple blowing-up and concentrating solutions of the Equation (3) is given by Baraket and Pacard in [24]. When t = 1 and αi = 0 for any i = 1,2, ..., N, they prove that any non-degenerate critical point of the function φn,m with n = 0 generates a family of the solutions uε which blow-up at , and concentrate in the sense that (4) holds. Esposito [20] performs a similar asymptotic analysis and extends the previous result by allowing the presence of singular sources in the Equation (3)|t = 1, that is, . However, the asymptotic analysis method depends on the non-degenerate assumption of critical point of the function φn,m so much that it pays in return at a price of the very complicated and accurate control on the asymptotics of the solutions.

In fact, the finite dimensional reduction method, used successfully in higher dimensional nonlinear elliptic equation involving critical Sobolev exponent (see [6,25]), can avoid the technical difficulty in carrying out the asymptotic analysis method for the Equation (3). It is necessary to point out that the key step of the finite dimensional reduction is the analysis of the bounded invertibility of the corresponding linearized operator L of the Equation (3) at the suitable approximate solution. In [26,27], the authors construct the approximate solution, carry out the finite dimensional reduction and use some stability assumptions of critical points of φ0,m to get the existence of multiple blowing-up and concentrating solutions for the Equation (3)|t = 1 with , namely αi = 0 for any i = 1,2, ..., N. When , a similar result for the Equation (3)|t = 1 under C0-stable assumption of critical point of φn,m (see Definition 4.1) is also established in [28].

Here in the spirit of the finite dimensional reduction, we try to extend the result of the Equation (1) in [20,28] by allowing the presence of singular sources with some αi ∈ (-1, 0) and Robin boundary conditions with t ∈ (0,1). When we carry out the finite dimensional reduction, we need to get the invertibility of the desired linearized operator L for the Equation (3) under some αi ∈ (-1, 0). Obviously, the linearized operator L easily produces the singularities at some singular sources with αi ∈ (-1, 0), which makes trouble for the analysis of the bounded invertibility of L. But we can successfully get rid of it by introducing a suitable L-weighted norm (see (30) below) related with a "gap interval" (-1, α0), where α0 = min{0, α1, ..., αN}. On the other hand, the presence of the term in the Equation (3)|0<t<1 brings some new technical difficulties. A flexible approach exactly helps us overcome the difficulties by making use of the maximum principle. In addition, a weaker stable assumption of critical points of the function φn,m also helps us construct multiple blowing-up and concentrating solutions of the Equation (3). As a consequence, we have the following result.

Theorem 1.1 Let 0 ≤ n ≤ N and such that n + m ≥ 1. Assume that and is a C0-stable critical point for φn,m in (Ω' \ Γ)m \ Δm with m ≥ 1 (see Definition 4.1). Then there exists a family of solutions uε for the Equation (3) with the concentration property (4), which blow up at n-different points in Γ, and m-points in (Ω' \ Γ)m \ Δm with φn,m(p*) = φn,m(p). Moreover, uε remains uniformly bounded on , and for any λ > 0.

Let us point out that from the proof of Theorem 1.1 Robin boundary condition can be considered as a perturbation of Dirichlet boundary condition for the problem (3) in using perturbation techniques to construct multiple blowing-up and concentrating solutions. Based on this point, we also consider the Dirichlet-Robin boundary value problem:

(6)

where T Ω is a relatively closed subset and . This together with other similar mixed boundary value problems can be founded in [29,30]. For the problem (6), we obtain the following result.

Theorem 1.2 Under the assumption of Theorem 1.1, then there exists a family of solutions uε for the Equation (6) with the concentration property (4), which blow up at n-different points in Γ, and m-points in (Ω' \ Γ)m \ Δm with φn,m(p*) = φn,m(p). Moreover, uε remains uniformly bounded on , and for any λ > 0.

Finally, it is very interesting to mention that to prove the above results we need to choose the classification solutions of the following Liouville-type equation to construct concentrating solutions of the Equation (1) or (3):

(7)

given by

(8)

with if , c = 0 if (see [5,11,31,32]). Using these classification solutions scaled up and projected to satisfy the mixed boundary conditions up to a right order, the initial approximate solutions can be built up. Then through the finite dimensional reduction procedure and the notions of stability of critical points of the asymptotic reductional functional φn,m, multiple blowing-up and concentrating solutions can be constructed as a small additive perturbation of the initial approximations.

This article is organized as follows. In Section 2, we will construct the approximate solution and rewrite the Equation (3) in terms of a linearized operator L. In Section 3, we give the invertibility of the linearized operator L, carry out the finite dimensional reduction and get the asymptotical expansion of the functional of the Equation (3) with respect to the suitable approximate solution. In Section 4, we give the proofs of Theorems 1.1 and 1.2.

### 2 Construction of the approximate solution

In this section, we will construct the approximate solution for the Equation (3). Let μi, i = 1, ..., N + m, be positive numbers and set

and

Obviously, Qi(x) = S(x) for any i = N + 1, ..., N + m. Then the function

(9)

satisfies

(10)

Set {k1, ..., kn} ⊂ {1, ..., N} and kn+i = N + i for any i = 1, ..., m.

We hope to take as an initial approximate solution of the problem (3). So we modify it to be

(11)

where (x) is the solution of

(12)

Then satisfies

(13)

Lemma 2.1 For t ∈ (0, 1] and ,

(14)

uniformly in and in for ε small.

Proof. Set . Then zt(x) satisfies

where

For any t ∈ (0, 1], it is easy to check . If t = 1, from the maximum principle and smooth function b(x) > 0, it follows

If 0 < t < 1, from the maximum principle with the Robin boundary condition (see [[33], Lemma 2.6]), it also follows

Thus using the interior estimate of derivative of harmonic function (see [[34], Theorem 2.10]), there holds

for any compact subset K of , any t ∈ (0, 1] and any multi-index α with |α| ≤ 2, which derives (14) uniformly in and in for ε small. □

From this lemma we can construct the approximate solution , which satisfies the mixed boundary conditions. On the other hand, we hope that the error is smaller near every . In fact, we can realize this point by further choosing positive number such that

(15)

Consider the scaling of the solution of the Equation (3)

then v(y) satisfies

(16)

where . We also set and define the new approximation in expanded variables as V(y) = U(εy) + 4 log ε. Furthermore, set

(17)

and

(18)

Obviously, for all i = 1, ..., m.

Here, we want to see how well -∆V(y) match with W(y) through V(y). A simple computation shows

Then given a small number δ > 0, if for all i = 1, ..., n + m,

(19)

and if for some i,

(20)

On the other hand, if for all i = 1, ..., n + m, obviously,

(21)

and if for some i,

Now from (14), (15) and (17), we have

(22)

In summary, we set

(23)

and if for all i = 1, ..., n + m,

(24)

while for some i,

(25)

In the rest of this article, we try to find a solution v of the form v = V + ϕ of the Equation (16). In terms of ϕ, the problem (3) becomes

(26)

where

(27)

### 3 The finite dimensional reduction

In this section, we will carry out the finite dimensional reduction to solve the Equation (26). First of all, we need to get the desired invertibility of linearized operation L. Set

A basic fact to get the needed invertibility is that the linearized operator L formally approaches to the operator Li under suitable dilations and translations, which have some well-known properties that any bounded solution of Ltϕ = 0 is

- a linear combination of zi0 and zij for i = n + 1, ..., n + m, j = 1, 2 (see [24,35]);

- proportional to zi0 for and i = 1, 2, ..., n (see [20,28,36]).

Remark 3.1 These properties of the operator Li have been discussed in the above papers only if for i = 1, ..., n, or for i = n + 1, ..., n + m. In fact, if for some i = 1, ..., n, the operator Li has also the corresponding properties.

Lemma 3.2 For , any bounded solution ϕ of

(28)

is proportional to .

Proof. If we express the bounded solution ϕ of the Equation (28) in Fourier expansion form as follow

un(r) is a bounded nontrivial solution of the equation

(29)

Since any solution of -∆u = eu in is given by the Liouville formula

for any meromorphic function F defined on , the function

with any and , is the solution of -∆u = |z|2αeu in . Moreover, its derivative with respect to a at a = 0

is a solution of the Equation (29) with r = |z|.

For |n| ≥ 1, since {ϕn(r), ϕ-n(r)} is a set of linearly independent solutions of the second order linear homogeneous ODE (29), any bounded solution is a linear combination of ϕn(r) and ϕ-n(r). However, ϕ|n|(r) ( resp. ϕ-n(r) ) tends to 0 ( resp. ∞ ) as r ↦ 0 and ϕ|n|(r) ( resp. ϕ-|n|(r) ) tends to ∞ ( resp. 0 ) as r ↦ + ∞, which implies that the Equation (29) ||n|≥1 has no bounded nontrivial solution.

For n = 0, is a bounded solution of the Equation (29)|n = 0, that is, of the Equation (28). We claim that there does not exist the second linearly independent bounded solution of the Equation (29)|n = 0. Otherwise, let ω be another linearly independent bounded solution of (29)|n = 0. Writing ω(r) = c(r)ϕ0(r), we get that

Then there exists a constant C > 0 such that

Hence, ω(r) ~ C log r for r small, which implies ω(r) is unbounded on (0, + ∞). It contradicts the assumption that ω is bounded. □

Let us denote

where χ(r) is a smooth, non-increasing cut-off function such that for a large but fixed number R0 > 0, χ(r) = 1 if r R0, and χ(r) = 0 if r R0 + 1. Additionally, set α0 = min{0, α1, ..., αN}. For any α ∈ (-1, α0), we introduce the Banach space

with the norm

(30)

Now to get the invertibility of the linearized operator L, we only need to solve the following linear problems: given h of class with β ∈ (0,1), for m ≥ 1 and 0 ≤ n N, we find a function ϕ and scalars cij, i = n + 1, ..., n + m, j = 1, 2, such that

(31)

and for m = 0 and 1 ≤ n ≤ N, we find a function ϕ such that

(32)

Proposition 3.1 (i) If m ≥ 1 and 0 ≤ n N, given a fixed number δ > 0, there exist positive numbers ε0 and C such that for any points , l = n + 1, ..., n + m, in ', with

(33)

there is a unique solution , of the Equation (31), which satisfies

(34)

for all ε < ε0 and t ∈ (0, 1]. Moreover, the map p' ϕ is C1 and

(35)

where .

(ii) If m = 0 and 1 ≤ n ≤ N, there exist positive numbers ε0 and C such that there is a unique solution ϕ L(Ωε) of the Equation (32), which satisfies

(36)

for all ε < ε0 and t ∈ (0, 1].

These results can be established through some technical lemmas. First for the linear Equation (32) under the additional orthogonality conditions with respect to Zi0, i = 1, ..., n + m, and Zij, i = n + 1, ..., n + m, j = 1, 2, we prove the following priori estimates.

Lemma 3.3 (i) If m ≥ 1 and 0 ≤ n ≤ N, given a fixed number δ > 0, there exist positive numbers ε0 and C such that for any points , l = n + 1, ..., n + m, in ', which satisfy the relation (33), and any solution ϕ of the Equation (32) with t ∈ (0, 1] under the orthogonality conditions

(37)

one has

(38)

for all ε < ε0.

(ii) If m = 0 and 1 ≤ n ≤ N, there exist positive numbers ε0 and C such that for any solution ϕ of the Equation (32) with t ∈ (0, 1] under the orthogonality conditions

(39)

one has

(40)

for all ε < ε0.

Remark 3.4 The idea behind these estimates partly comes from observing the linear Equation (32) with h = 0 on bounded set for ε small. After a translation and a rotation so that Ωε converges to the whole plan , the Equation (32) approaches Liϕ = 0 in . As a result, the solution of the Equation (32) under the additional orthogonality conditions (37) should be zero.

Proof. Case (i): First consider the "inner norm" and the "boundary norm" , we claim that there is a constant C > 0 such that if Lϕ = h in Ωε, then

(41)

We will establish it with the help of suitable barrier.

Consider that the function is a radial solution in of

we define a bounded comparison function

with a > 0. Set . While for all i = 1, ..., n + m,

(42)

Moreover, according to (21) and (22), on the same region,

(43)

So if a is small enough to satisfy (1 + α)2a-2(1+α) > C + 1, Ra is sufficiently large. As a result, by (42) and (43), for any R ≥ Ra, we have Z(y) > 0 and L(Z) < 0 in .

Let M be a large number such that for all i = 1, ..., n + m, . Consider now the solution of the problem

A direct computation shows

where

and

For the sake of the convenience, we choose R larger if necessary. Then it easily see that these functions ψi, i = 1, ..., n + m, have a uniform bound independent of ε.

Now we can construct the needed barrier:

It is easy to check that in , and on . Since Z(y) > 0 and LZ(y) < 0 in , from the maximum principle (see [[37], Theorem 10, Chap. 2 ]), it follows that in . Similarly, in , which derives the estimate (41).

We prove the priori estimate (38) by contradiction. Assume that there exist a sequence εk → 0, points , l = n + 1, ..., n + m, in Ω' which satisfy relation (33), functions hk with ║hkn,m → 0, solutions ϕk with ║ϕk= 1, such that

Then from the estimate (41), ║ϕkl κ or ║ϕko κ for some κ > 0. Briefly set ε:= εk, . If ║ϕkl κ, with no loss of generality, we assume that for some i Then if we set and satisfies

for z BR(0). Obviously, for any we easily get in Lq(BR(0)). Since is bounded in Lq(BR(0)) and , elliptic regularity theory readily implies that converges uniformly over compact subsets near the origin to a bounded nontrivial solution of the equation

From Lemma 3.2, this equation implies that is proportional to zi0 for i = 1, ..., n, or a linear combination of zi0 and zij for i = n + 1, ..., n + m, j = 1, 2. However, our assumed orthogonality conditions (37) on ϕk pass to limit and yield the corresponding conditions (37) on , which means . Hence, it is absurd because is nontrivial.

If ║ϕko ≥ κ and ║ϕkl → 0, there exists a point q Ω and a number R1 > 0 such that with . Consider and let us translate and rotate Ωε so that q' = 0 and Ωε approaches the upper half-plan . Since for all i = 1, ..., n + m, satisfies

with . Moreover, we easily get hk(y) → 0 in . While t = 1, it is obvious to see that on ε. So it is absurd because of . On the other hand, for any t ∈ (0,1), elliptic regularity theory with the Robin boundary condition (see [30,34,38] and the references therein) implies that converges uniformly on compact subsets near the origin to a bounded nontrivial solution of the equation

with . It follows that its bounded solution is zero. Hence, it is also absurd because is nontrivial, which derives the priori estimate (38) of the case (i). Since the proof of the case (ii) is similar to that of the case (i), we omit it.□

We will give next the priori estimate for the solution of the Equation (32) that satisfies orthogonality conditions with respect to Zij, i = n + 1, ..., n + m, j = 1, 2, only.

Lemma 3.5. (i) If m ≥ 1 and 0 ≤ n ≤ N, given a fixed number δ > 0, there exist positive numbers ε0 and C such that for any points , l = n + 1, ..., n + m, in ', which satisfy the relation (33), and any solution ϕ of the Equation (32) with t ∈ (0, 1] under the orthogonality conditions

(44)

one has

(45)

for all ε < ε0.

(ii) If m = 0 and 1 ≤ n ≤ N, there exist positive numbers ε0 and C such that for any solution ϕ of the Equation (32) with t ∈ (0, 1], one has

(46)

for all ε < ε0.

Proof. Case (i): Let ϕ satisfy the Equation (32) under the orthogonality conditions (44). We will modify ϕ to satisfy the orthogonality conditions (37). To realize this point, we consider some related modifications with compact support of the functions Zi0, i = 1, ..., n + m.

Let R > R0 + 1 be large and fixed, and let be the solution of the equation

A simple computation shows that this solution is explicitly given by

Set

where η1(r) and η2(r) are smooth cut-off functions with the properties: η1(r) = 1 for r < R, η1(r) = 0 for ; η2(r) = 1 for r < δ, η2(r) = 0 for . We define a test function

Obviously, if , and if , in particular, near ∂Ωɛ.

Now we modify ϕ to satisfy the orthogonality conditions with respect to Zi0, i = 1, ..., n+ m, and set

where the numbers di are chosen such that

Thus

(47)

and satisfies all the orthogonality conditions in (37). From Lemma 3.3 (i), we have

(48)

In order to get the estimate (45) of ϕ, we need to give the sizes of di and for any t ∈ (0, 1]. Multiplying the first equation of (47) by , integrating by parts and using the mixed boundary conditions of (47), we get

(49)

where . A simple computation shows that , which in combination with (48) and (49) yields

(50)

From some similar computations (see [[26], Lemma 3.2] and [[28], Lemma 3.3]), there exists a constant C > 0 independent of ε such that

(51)

and

(52)

which combined with (50) yields

(53)

Furthermore, from (48), (51), (53), and the definitions of and , we have

(54)

Similarly, the proof of the case (ii) can also be done, we omit it. □

Proof of Proposition 3.1. Case (i): From Lemma 3.5 (i), and the Fredholm's alternative theory with Robin boundary condition instead of Dirichlet boundary condition if necessary (see [34,38] and the references therein), the proof can be similarly given through those in [[26], pp. 61-63].

Case (ii): Since the priori estimate (36) of the solution of the Equation (32) has been established in Lemma 3.5 (ii), we can use the Fredholm's alternative and obtain the unique solution of the Equation (32). □

Let us now introduce the auxiliary nonlinear problems: for m ≥ 1 and 0 ≤ n < N, we find the function ϕ and scalars cij, i = n + 1, ..., n + m, j = 1, 2, such that

(55)

and for m = 0 and 1 ≤ n ≤ N, we find the solution ϕ of the nonlinear Equation (26).

The following result can be proved using standard arguments as in [26,28].

Proposition 3.2 (i) If m ≥ 1 and 0 n ≤ N, given a fixed number δ > 0, there exist positive numbers ε0 and C such that for any points , l = n + 1, ..., n + m, in ' satisfying the relation (33), there is a unique solution for the Equation (55) which satisfies

(56)

for all ε < ε0 and t ∈ (0, 1]. Moreover, the map p' ϕ is C1 and

(57)

where and .

(ii) If m = 0 and 1≤n≤N, there exist positive numbers ε0 and C such that there is a unique solution for the Equation (26) which also satisfies the estimate (56) for all ε < ε0 and t ∈ (0, 1].

Now we only need to find a solution to the Equation (26) with m ≥ 1 and 0 ≤ n ≤ N, and hence to the Equation (55) if is such that

(58)

Let us introduce the energy functional of the Equation (3), namely for t = 1,

and for t ∈ (0,1),

Furthermore, we define

(59)

where and with the solution ϕ of the Equation (55).

The finite dimensional variational reduction is meaningful in view of the following property.

Proposition 3.3 If satisfying the relation (33) is a critical point of Fε,t with t ∈ (0, 1], then is a critical point of Jε,t, namely a solution of the Equation (3). Besides, on any compact subsets S of (Ω' \ Γ)m \ Δm the following expansion holds

(60)

where

(61)

for ε small.

Proof. Step 1: Let us define for t = 1,

and for t ∈ (0,1),

Then , where . Moreover, for k = n + 1, ..., n + m, l = 1, 2, it holds

(62)

Since ϕ(p') is a solution of the Equation (55), v = V(p') + ϕ(p') satisfies

(63)

By (62), (63), DpFε,t(p) = 0 implies for t ∈ (0, 1],

From the definition of V, it can be directly checked , where o(1) is in the sense of the L-norm for ε small. Since . Hence it follows

which is a strictly diagonal dominant system. This implies that cij = 0, ∀ i = n + 1, ..., n + m, j = 1, 2. By (63), is a critical point of Jε,t, that is, a solution of the Equation (3).

Step 2: Set . Using DIε,t(V + ϕ)[ϕ] = 0, a Taylor expansion and an integration by parts, it follows that for t ∈ (0, 1),

(64)

and similarly, for t = 1,

(65)

Note that ║ϕ= O(ρ|log ε|), ║N (ϕ)n,m = O(ρ2| log ε|2), ║R║n,m = O(ρ), and ║W║n,m = O(1). Then from (64), (65), it is easy to deduce for t ∈ (0, 1],

(66)

Hence, from (66), the expansion (60) satisfies the property (61).□

Finally, we need to write the precisely asymptotical expansion of Jε,t(U). To realize it, we first establish the following result:

Lemma 3.6 Assume that points , satisfy the relation (33), then fort ∈ (0,1) and i = 1, ..., n + m, there hold

(67)

and

(68)

uniformly in and in for ε small.

Proof. Set . Then satisfies

where . Since has the gradient estimate on Ω (see [[26], p. 76])

we can easily get . Using the same technique with the proof of Lemma 2.1, we can also get uniformly in and in for ε small, which means (67). From the definition of the regular part of Green function, we can also derive (68). □

Proposition 3.4 The following asymptotical expansions hold for t = 1,

(69)

and for t ∈ (0,1),

(70)

where φn,m(p) is defined by (5), and for ε small, Θε is a bounded, smooth function of p = , uniformly on points in ' satisfying the relation (33).

Proof. According to [[26,28], Lemma 6.1], it only remains to discuss the asymptotical expansion of the energy Jε,t(U) with respect to t ∈ (0,1). By (11), it follows

and

which together with the Equation (13) yields

Furthermore

(71)

From (9), (13), and (14), it implies

Then if i ≠ j for any i and j,

(72)

and if i = j for any i,

(73)

On the other hand, from (21) and (22), it follows

As a result, it derives

(74)

Now using the choice for 's by (15), together with (71)-(74), it holds

which derives the asymptotical expansion (70) by (5), (67), and (68). □

### 4 Proofs of theorems

In this section, we carry out the proofs of Theorems 1.1 and 1.2 basing on the finite dimensional reduction. Now we introduce the definition of C0-stable critical point of the function φn,m just like in [28,36,39].

Definition 4.1. We say that p is a C0-stable critical point of φn,m in (Ω' \ Γ)m \ Δm, which says that if for any sequence of the functions ψj such that ψj φn,m uniformly on the compact subsets of (Ω' \ Γ)m \ Δm, ψj has a critical point ξj such that ψj(ξj) → + φn,m.

In particular, if p is a strict local maximum or minimum point of φn,m, p is a C0-stable critical point of φn,m.

Proof of Theorem 1.1. Case (i): m ≥ 1 and 0 ≤ n ≤ N. Let

where ϕ is the unique solution of the problem (55), which is established in Proposition 3.2. From Proposition 3.3, v(y) is a solution of the Equation (16), namely is a solution of the Equation (3) if satisfying the relation (33) is a critical point of the function Fε,t(p) with t ∈ (0, 1]. This implies that we only need to find a critical point pε of the following function in (Ω' \ Γ)m \ Δm

(75)

where

From Propositions 3.3 and 3.4, it follows that

(76)

where θε,t(p) = O(ρ2| log ε|) for any t ∈ (0, 1], and Θε(p) is uniformly bounded on any compact subset S of (Ω' \ Γ)m\ Δm for ε small. Then , and for any t ∈ (0,1), uniformly on for ε small. By Definition 4.1, there exists a critical point of the function such that , and for any t ∈ (0,1). Moreover, up to a subsequence, there exists such that pε p for ε small, and φn,m(p*) = φn,m(p). Hence, is a family of solutions of the Equation (3). As a consequence, from the related properties of U(pε) and , we easily know that for any λ > 0, uε is uniformly bounded on , and for ε small.

Finally, we show that uε satisfies the concentration property:

(77)

for ε small. In fact, using the inequality |es - 1| e|s||s| for any and the estimate (56), we obtain

Then from the asymptotical expression (21) and (22) of W(pε)(y), we can easily get

which implies (77).

Case (ii): m = 0 and 1 ≤ n ≤ N. From Proposition 3.2 (ii), we find that is a family of solutions of the Equation (3) with . Then we can get the needed multiple blowing-up and concentrating properties of uε through the similar proof of Case (i). □

In order to give the proof of Theorem 1.2, we need a version of the maximum principle under Dirichlet-Robin boundary conditions, which is the extension of the corresponding one with respect to Dirichlet or Robin boundary condition only.

Lemma 4.2 Assume that T is a relatively closed subset, b > 0 is a smooth function on is a smooth function. If u is a solution of the equation

where λ > 0, there exists a constant C(b) > 0 only depending on b(x) such that

Proof. The proof is similar to that of Lemma 2.6 in [33]. □

Proof of Theorem 1.2. Using the maximum principle with Dirichlet-Robin boundary conditions instead of Robin boundary condition if necessary (see Lemma 4.2), the proof can be similarly given out through that of Theorem 1.1. □

### Competing interests

The authors declare that they have no competing interests.

### Authors' contributions

All authors typed, read and approved the final manuscript.

### Acknowledgements

This study was supported by N. N. S. F. C. (11171214).

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