Research Article

# Positive blow-up solutions of nonlinear models from real world dynamics

Jürgen Gschwindl1, Irena Rachůnková2*, Svatoslav Staněk2 and Ewa B Weinmüller1

Author Affiliations

1 Department for Analysis and Scientific Computing, Vienna University of Technology, Wiedner Hauptstraße 8-10, Wien, A-1040, Austria

2 Department of Mathematics, Faculty of Science, Palacký University, 17. listopadu 12, Olomouc, 77146, Czech Republic

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Boundary Value Problems 2014, 2014:121  doi:10.1186/1687-2770-2014-121

Dedicated to Professor Ivan Kiguradze for his merits in mathematical sciences

 Received: 13 December 2013 Accepted: 5 May 2014 Published: 16 May 2014

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 credited.

### Abstract

In this paper, we investigate the structure and properties of the set of positive blow-up solutions of the differential equation , , where . The differential equation is studied together with the boundary conditions , . We specify conditions for the data function h which guarantee that the set of all positive solutions to the above boundary value problem is nonempty. Further properties of the solutions are discussed and results of numerical simulations are presented.

MSC: 34B18, 34B16, 34A12.

##### Keywords:
singular ordinary differential equation of the second order; time singularities; blow-up, positive solutions; existence of solutions; polynomial collocation

### 1 Introduction

In this paper, we investigate the structure and properties of the set of positive blow-up solutions of the differential equation

(1)

where .

Models in the form of (1) arise in many applications. Among others, they occur in the study of phase transitions of Van der Waals fluids [1-3], in population genetics, where they characterize the spatial distribution of the genetic composition of a population [4,5], in the homogeneous nucleation theory [6], in relativistic cosmology for particles which can be treated as domains in the universe [7], and in the nonlinear field theory, in particular, in context of bubbles generated by scalar fields of the Higgs type in Minkowski spaces [8].

Here, we assume that h is positive and satisfies the Carathéodory conditions on . We define a positive solution of (1) as a function v which satisfies (1) for a.e. , is positive on , and has absolutely continuous first derivative on each compact subinterval in .

According to Lemma 4, if v is a positive solution of (1), then either

or

In the literature, bounded solutions of (1) have been widely investigated; see e.g.[9-13]. Such solutions are characterized by the initial condition . In contrast to this, some real problems lead to the investigation of unbounded solutions which are characterized by the condition for some and which are called blow-up solutions. We refer to [14-16]. Here, we are interested in blow-up solutions of (1), where . In particular, (1) will be considered together with the boundary conditions

(2)

In this case, we speak about a positive solution of problem (1), (2). Let us denote by the set of all positive solutions to (1), (2). Moreover, let and for . Since for each , it is obvious that .

Our main goal is to find conditions for the data function h in (1), which guarantee that the set is nonempty for each and then to investigate the properties of this set. For example, we prove that the difference of any two functions in , , retains its sign on , and that there exist minimal and maximal solutions for each , cf. Theorem 5. If the interior of the set is nonempty, we show that this interior is fully covered by ordered graphs of other functions belonging to for each ; see Theorem 6. Finally, in Theorem 7, the existence of a positive constant such that is shown and all functions v from ℛ are uniquely characterized by the condition

If we denote such v by and define for , and , then we find that the graphs of these functions do not intersect, cf. Theorem 8, and that for each , the set is compact in ; see Theorem 9.

The study of a structure of positive solutions to other types of ordinary differential equations can be found for example in [17-19].

#### Notation

Let us denote by the Banach space of functions continuous on equipped with the maximum norm

Similarly, means the Banach space of functions having a continuous first derivative on with the corresponding maximum norm . By we denote the set of functions which are Lebesgue integrable on . Moreover, is the set of functions whose first derivative is absolutely continuous on , while is the set of functions having absolutely continuous first derivative on each compact subinterval of . We say that satisfies the Carathéodory conditions on , if the following three conditions hold:

(i) The function is measurable for all .

(ii) The function is continuous for a.e. .

(iii) For each compact set there exists a function such that

For functions satisfying above conditions, we use the notation .

#### Structure of the paper

The paper is organized as follows. In Section 2 we discuss properties of the solutions of the auxiliary Dirichlet problem (3), (4). We recapitulate previous results from [20] and also present new results in Theorems 1, 2, and 3. The main results of the paper can be found in Section 3, where we describe a relation between solutions of problem (3), (4) and blow-up solutions of problem (1), (2); see Theorem 4. Using this relation and the results of Section 2, we obtain various interesting properties of blow-up solutions; see Theorems 5 to 9. Section 4 contains three examples illustrating the theoretical findings. Final remarks and open problems are formulated in Section 5.

### 2 Auxiliary results

In this section we consider the auxiliary singular differential equation,

(3)

where . For and , (3) becomes a special case of (1) and therefore, results obtained for (3) apply for (1). For the further analysis, we assume that f satisfies the following conditions:

(H1) ,

(H2) for a.e. and all ,

(H3) is increasing in x for a.e. and

We now study (3) subject to the boundary conditions

(4)

and require that

(5)

holds. We call a function a positive solution of the Dirichlet problem (3), (4) if , on , u satisfies the boundary conditions (4), and (3) holds for a.e. . Clearly, for each positive solution u of (3), (4), there exists such that (5) is satisfied.

We now denote by the set of all positive solutions of problem (3), (4), and , where .

In the following lemma we cite those results from [20] which will be used in the analysis of problem (1), (2).

Lemma 1Let (H1)-(H3) hold. Then the following statements hold:

(a) For eachthe setis nonempty and there exist functionssuch thatforand.

(b) If, , , thenfor.

(c) If, , , andfor some, then eitherforor there existssuch thatforandfor.

(d) For eachand eachthere existssatisfying.

(e) is a one-point set for each, whereis at most countable.

(f) For each, the setis compact in.

(g) If, thenif and only if it is a solution of the equation

(6)

in the set.

We now formulate new results which complete those from [20]. We first establish a relation between and the set if its interior is nonempty. This question was a short time ago an open problem [[20], Remark 4.4]. We note that the relation between and the set with having nonempty interior is described in Lemma 1(d).

Theorem 1Let (H1)-(H3) hold. Let us assume that there existssuch that. Then for anythere existssatisfying.

ProofStep 1. Auxiliary Dirichlet problem.

Choose . Consider (3) subject to the Dirichlet conditions

(7)

We claim that there exists a solution v to problem (3), (7) such that

(8)

We show this result utilizing the method of lower and upper functions. It follows from (H1) that there exists such that

(9)

Let

and let be given as

We now consider the auxiliary differential equation

(10)

It is not difficult to verify that for . Hence, cf. (9),

(11)

for a.e. and all , . Since and for , , , and , solve (3) on , we conclude that and are lower and upper functions of problem (3), (7) (see e.g.[21] or [22]). This fact together with (11) implies the existence of a solution v to problem (3), (7) satisfying (8), cf. [[21], Lemma 3.7]. Moreover,

and taking the limit we obtaina, which together with gives .

We now prove that on . The proof is indirect. Let us assume that there exists such that and for . Then on this interval, and therefore

From (8) and (9) we conclude that

Since

we have

In particular,

By the Gronwall lemma,

Therefore

Consequently, for , and so = 1 on this interval. Thus, v is a solution of problem (3), (7).

Step 2. Continuation of the solutionv.

It follows from the arguments given in Step 1 that v is a solution of problem (3), (7) on , and because . It is easy to verify that the equality

(12)

is satisfied for a.e. . We now integrate the last equality two times over and have (note that )

Let u be a solution of problem (3), (7) on an interval J which is a left-continuation of v. Let us assume that u is not continuable. Let . Then and (12) with v replaced by u holds for a.e. . The integration now yields

(13)

and we claim that

(14)

We show inequality (14) indirectly. Let us assume that (14) does not hold. Then there exists such that

and either or . Assume that .

Since, by (H3), for a.e. and for a.e. , we have

which is not possible. The case can be discussed analogously. Hence, (14) holds.

Suppose that . Then and since u is bounded on , it follows that . The integration of the equality

over gives

Since for a.e. , where , we have

Using the Gronwall lemma, we deduce that for ,

Therefore , and then (14) yields . Consequently , and the assertion of the theorem follows from (13). □

In the next corollary we extend the statement (d) from Lemma 1 to a large set of A values.

Corollary 1For eachand eachthere existssatisfying.

Proof The result follows immediately from Lemma 1(d) and Theorem 1. □

Remark 1 Corollary 1 says that the set is covered by the graphs of functions from , that is,

By Lemma 1(b), (c) we know that functions from are uniquely determined by the values −c of their derivatives at the right end point only in the case that is a singleton set for each . Since we cannot uniquely determine all functions from via their derivatives at , see Lemma 1(e), we discuss their derivatives at the singular point .

Lemma 2Let (H1)-(H3) hold. Let, . Then

(15)

Proof It follows from Lemma 1(g) that (6) holds for . Since , we have (note that )

and (15) follows. □

Corollary 2Let. Then

Proof The result follows from Lemma 2, since . □

Corollary 3Let, . Then.

Proof Since , there exists such that . We can assume that for instance . Then on and on by Lemma 1(b)(c). Hence, and

which together with Corollary 2 gives . □

Corollary 4Let, . Then

(16)

In particular, .

Proof Inequality (16) follows from Lemma 2 and the fact that for a.e. by (H2). □

Corollary 5Let. Then

Proof The integration of (3) over gives

Using integration by parts we obtain

and, therefore,

Taking the limit , we have

On the other hand, it follows from Corollary 2 that

Combining the above two equalities yields the result. □

Lemma 3Let (H1)-(H3) hold. Letandbe not a singleton set. Then for eachthere exists a uniquesuch that. Consequently, .

Proof Let , . Then it follows from Lemma 1(a) and (H3) that

Hence, by Lemma 2,

Since is a compact set in by Lemma 1(f), the set is closed. In fact, let , , where , and let . Then there exist a subsequence of and such that in . In particular, as . Therefore, .

It remains to prove that . Assume that the equality does not hold. Then, from the structure of bounded and closed subsets of ℝ the existence of an open interval , , follows. Let and , where . Due to Lemma 1(c), there exists such that on , on . Choose . By Corollary 1, there exists such that . Then on and on . Consequently, on , that is, , which is not possible. □

Since functions from belong to , we have for each . Corollary 4 yields . Let us denote

(17)

Lemma 3 implies that functions from can be uniquely determined by the values of their derivatives at the singular point ; see Theorem 2.

Theorem 2Let (H1)-(H3) hold. Then there exists a uniquesatisfyingif and only if.

Proof We first show

(18)

It follows from Lemma 3 that

for each , where we set if is a singleton set and . In view of Corollary 4, for . Consequently, (18) follows.

Let us now choose . Then there exists satisfying . The uniqueness of u follows from Corollary 3. □

For , we denote by the unique element of satisfying .

Theorem 3Let (H1)-(H3) hold. Assume thatis a convergent sequence and. Thenin.

Proof Let . Then because for by Corollary 4. Since and is compact in by Lemma 1(f), the sequence is relatively compact in . Let be a subsequence of which is convergent in , and let . Then for a and . Therefore, and hence any subsequence of converging in has the same limit . Consequently, is convergent in and is its limit. □

### 3 Blow-up solutions and their properties

This section contains the main results of the paper. First, we present a lemma which describes how positive solutions of (1) may behave at the singular point .

Lemma 4Letand let the functionbe positive. Letvbe a positive solution of (1). Then either

(19)

or

(20)

Proof By Corollary 3.5 in [12], if , then v satisfies (19). Now, assume that

(21)

Then v has to satisfy

(22)

because otherwise, if (22) was not true, then , contradicting (21). Since h is positive, (1) indicates that the function is increasing on and hence there exists . The next part of the proof is divided into three cases.

(i) Let us assume that

(23)

Since is increasing on , we deduce and for . Consequently, v is increasing on . This together with and yields . Therefore, . Integrating (1) and using (23), we obtain

which is a contradiction to (22).

(ii) Assume that

Then there exists such that

(24)

Hence, v is increasing on and there exists .

By integration, we obtain from (24)

and, by virtue of and , we arrive at

(iii) Assume that

(25)

Then there exists such that

(26)

Hence, v is decreasing on and there exists . By integration, we obtain from (26)

and, since and , we have

In addition (25) yields and (20) follows. □

Now, we investigate the existence and properties of blow-up solutions of (1), for the case that the function h has the form

In particular, we study the equation

(27)

where and ψ, g satisfy the conditions

() and for a.e. ,

() and is increasing in x for a.e. ,

() for a.e. and all , where , ϕ is increasing and

Together with (27) we consider the conditions

(28)

and

(29)

We define a positive solution to problem (27), (28) as a function such that on , v satisfies the boundary conditions (28), and (27) holds for a.e. .

Define a set by

(30)

Clearly, for each there exists such that (29) holds.

Lemma 5Let ()-() hold. Let. Thenon.

Proof Since , the integration of (27) over the subinterval gives

Assume that for some . Then

which is not possible, since and . □

Moreover, let us define sets

(31)

and

(32)

It is obvious that . Using the substitution

(33)

we can rewrite (27) and obtain the form

(34)

We now introduce a function f,

Under conditions ()-(), the function f satisfies (H1)-(H3); see the proof of [[20], Theorem 5.1]. Therefore, the results of Section 2 hold for problem (34), (4). As in Section 2, we define the sets and for solutions of (34).

The following result is the key-stone to the analysis of the structure of the set ℛ and describes the relation between the sets and ℛ.

Theorem 4Let ()-() hold and. Let us assume that (33) holds. Thenif and only ifand.

Proof (⇒) Let . Then (28), (32), and (33) provide the following properties: , , and u is bounded on . It follows from the equality

that

Hence,

We now argue as in the proof of [[20], Lemma 3.3] and arrive at

(35)

for . Since u is bounded on , we have . Then, by [[20], Lemma 2.1],

for , and hence, . Now, [[23], Corollary 1] guarantees that u can be extended on with such that the equality (35) holds for . Consequently, .

(⇐) Let and . Then it follows from (4), (5), and (33) that and , . Hence, is bounded on and

yields , that is,

Since , the equality (35) holds on , and consequently,

Hence, note that , , and so . As a result, we have . □

Now, we are in the position to provide results on the solvability of problem (27), (28) and formulate the properties of its solutions.

Theorem 5Let ()-() hold. Then the following statements hold:

(a) For eachthe setis nonempty and there existsuch thatforand.

(b) If, , , thenfor.

(c) If, , , andfor some, then eitherforor there existssuch thatforandfor.

(d) is a singleton set for each, whereis at most countable.

Proof The result follows by combining results from Lemma 1(a), (b), (c), and (e) with those from Theorem 4. □

If , , and if (33) holds, then Theorems 4 and 5 yield

(36)

The next theorem shows that the set

is covered by graphs of the functions from ℛ.

Theorem 6Let ()-() hold. Then, for eachand each, there existssatisfying. In particular, if for someandthe inequalityholds, then for eachthere existsatisfying.

Proof Choose and . Define . Then, using (33) and (36), we deduce . Therefore, by Corollary 1, there exists such that . Consequently, (33) and Theorem 4 give and . The last statement follows from Theorem 5(c). □

By Corollary 4, there exists such that

(37)

and hence, by (33) and (36) with ,

(38)

Using constants from the interval , we can uniquely determine all functions in ℛ.

Theorem 7Let ()-() hold and letbe as in (38). Then the following statements hold:

(a) For eachthere existssuch that

(39)

(b) For eachthere exists a uniquesatisfying (39).

Proof (a) Choose . Using (33), (37), and Theorem 4, we have and

(40)

By Lemma 1(c), . Hence, if we denote , the first condition in (39) follows. Moreover,

(41)

and therefore the second condition in (39) holds.

(b) Choose . Theorem 2 guarantees that there exists a unique satisfying . Using (33) and Theorem 4, we conclude . Then v satisfies (40) and (41) which results in (39). It remains to prove that v is unique. Assume that there exists a function , , such that w satisfies (39). Let . Then and . Consequently, by Theorem 2, we arrive at and , which is a contradiction. □

Let be given in (38) and choose . Keeping Theorem 7(b) in mind, there exists a unique satisfying (39). We denote such v by and define a function as

(42)

Then . We now specify further properties of functions .

Theorem 8Let ()-() hold and letbe from (38). Letand. Then eitherforor there existssuch thatforandfor.

Proof According to (42), there exist , , and such that , for . Also, and, since , it follows that in a right neighborhood of . Therefore, there exists such that and hence, . (Note that if , then Theorem 5(b) yields for , which is not possible.) The result now follows from Theorem 5(b) for and from Theorem 5(c) for . □

Corollary 6Letbe a convergent sequence and let us denote. Thenin.

Proof The proof is indirect. Assume that the statement of the corollary does not hold. Then there exist ε, and a subsequence of , we denote it again by , such that

(43)

Let and . Let be countable. Then there exists a decreasing subsequence of . By Theorem 8, the sequence is not increasing on and on this interval. Using (33) and (42), we obtain , . Since in it follows from Theorem 3, note that , that and that for . This fact together with the monotonicity of and gives in , which contradicts (43). Hence in . □

For , we now define a set ,

(44)

Theorem 9Let ()-() hold and letbe from (38). For eachthe setdefined by (44) is compact in.

Proof Consider . By Theorem 8 we have

According to (33), (42), and Theorem 4 we have

(45)

(46)

Let us denote

Conditions (4) and (46) imply . Consequently, using Theorem 4, we obtain

(47)

Consider a sequence . Then there exists a sequence with . By Lemma 1(f), the set is compact in . Therefore, there exists a subsequence which converges in to . In particular, . Let us denote , then we have . This yields, by Corollary 6, in and . Thus we showed that for any sequence in , there exists a uniformly converging subsequence on with a limit in . □

### 4 Examples

Using the open domain MATLAB Code bvpsuite, we numerically simulate three model problems in order to illustrate theoretical statements made above.

#### 4.1 MATLAB Code bvpsuite

The MATLABTM software package bvpsuite is designed to solve BVPs in ODEs and differential algebraic equations. The solver routine is based on a class of collocation methods whose orders may vary from 2 to 8. Collocation has been investigated in context of singular differential equations of first and second order in [24,25], respectively. This method could be shown to be robust with respect to singularities in time and retains its high convergence order in case that the analytical solution is appropriately smooth. The code also provides an asymptotically correct estimate for the global error of the numerical approximation. To enhance the efficiency of the method, a mesh adaptation strategy is implemented, which attempts to choose grids related to the solution behavior, in such a way that the tolerance is satisfied with the least possible effort. The error estimate procedure and the mesh adaptation work dependably provided that the solution of the problem and its global error are appropriately smooth.b The code and the manual can be downloaded from http://www.math.tuwien.ac.at/~ewa . For further information see [26]. This software proved useful for the approximation of numerous singular BVPs important for applications; see e.g.[9,27-29].

Example 1 The first example is used to comment on Theorem 5 and to show that if a function in (27) is increasing in x, the graphs of positive solutions of problem (27), (28) cannot intersect. Here, we choose

and thus problem (27), (28) becomes

(48)

Using substitution (33), which has the form

(49)

we transform (48) onto the form, cf. (34),

and we solve the terminal value problem given by

(50)

where , and 4. Then, using (49), we recalculate these solution to their blow-up form and obtain different positive solutions v of problem (48). Note that their graphs are ordered after c, cf. Figure 1.

Figure 1. Example 1. Solutions of (50) (first) and of (48) (second) for different values of c, orange, green, red, dark blue, magenta. We used a collocation method of order 8 based on Gaussian points and grid adaptation strategy to satisfy the tolerances .

Example 2 We designed this example to see if the condition () is essential for statement (b) of Theorem 5. To this aim, we choose a function which is decreasing in x in (27). Let

Then problem (27), (28) has the form

(51)

Using substitution (49), we rewrite (51) and have

The resulting terminal value problem reads

(52)

where , and 4. Then, using (49), we obtain different positive blow-up solutions of problem (51). However, their graphs are again ordered; see Figure 2. Hence, condition () seems not to be necessary for statement (b) of Theorem 5 to hold. On the other hand, the question, if another model with a decreasing nonlinearity g exists, where solutions cross each other and are not ordered after c, remains open.

Figure 2. Example 2. Solutions of (52) (first) and of (51) (second) for different values of c, orange, green, red, dark blue, magenta. We used a collocation method of order 8 based on Gaussian points and grid adaptation strategy to satisfy the tolerances .

Example 3 Since, Example 2 did not provide the solution crossings, we constructed another model problem, where in (27) the function oscillates in x. Let

then problem (27), (28) has the form

(53)

Using (49), we again transform (53) and solve the terminal value problem

(54)

where , and 4. Then, using (49), we calculate the related positive blow-up solutions of problem (53). As can be seen in Figure 3, their graphs are again ordered after c and the question of solution crossings remains open even if we consider oscillating nonlinearities.

Figure 3. Example 3. Solutions of (54) (first) and of (53) (second) for different values of c, orange, green, red, dark blue, magenta. We used a collocation method of order 8 based on Gaussian points and grid adaptation strategy to satisfy the tolerances .

To show the advantage of the grid adaptation, we carried out the following additional test. For , we evaluated the solution u at the point and solved the boundary value problem

(55)

In Figure 4, solutions of (54) and of (55), together with the final grids are shown on the interval (left) and (right). Due to the different smoothness of u and v, their accuracy strongly varies. While the absolute and relative errors of u (or rather their estimates) on the grid with 52 grid points is 10−13, the absolute and relative errors of v on the grid with exactly the same number of points is only 10−9. Note that the code has automatically adapted the location of the grid points to correctly reflect the solution behavior.

Figure 4. Example 3,. Solutions of (54) and of (55), shown on the interval (first) and (second). We used a collocation method of order 8 based on Gaussian points and grid adaptation strategy.

### 5 Conclusions

We have described the set ℛ of all positive solutions of problem (1), (2), where h has the form and assumptions ()-() hold. By Theorem 5 we know that ℛ is nonempty and that for each there exists at least one function fulfilling . In addition, graphs of functions from ℛ do not intersect and ℛ has a minimal element .

If we choose an arbitrary and denote , then there exists a maximal element in . Clearly and . According to Theorem 6, the interior of the set is fully covered by graphs of functions from ℛ. Finally, we deduce from Theorems 7-9 that the set is compact in .

Example 1 illustrates the results of Section 3 and hence, according to (), we have chosen the increasing function in (48). Figure 1 shows ordered graphs of solutions.

In contrast to this, in Example 2 and Example 3, we have chosen the decreasing function in (51) and the non-monotonous function in (53), respectively. We can see on Figure 2 and Figure 3 that the graphs of solutions are ordered in both cases. But to prove such order of solutions for non-increasing g is an open problem. On the other hand, the question of the construction of problem (27), (28) whose positive solutions cross each other remains open, as well.

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

The authors read and approved the final draft. IR and SS contributed to the analytical part of the paper and JG and EW contributed to the numerical part of the paper.

### Acknowledgements

This research was supported by the grants IGA PrF_2013_013 and IGA-PrF_2014028. The authors are grateful to the referees for useful comments and suggestions which improved the paper.

### End notes

1. Note that .

2. The required smoothness of higher derivatives is related to the order of the used collocation method.

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