Discrete Fučík spectrum - anchoring rather than pasting
© Stehlík; licensee Springer. 2013
Received: 23 November 2012
Accepted: 8 March 2013
Published: 29 March 2013
In this short note we study a simple discrete Fučík spectrum. Trying to imitate standard continuous pasting procedures, we derive a more complicated discrete analogue - anchoring. Using this technique, we show that the problem of finding the parametrization of the second discrete Fučík branch is equivalent to solving a transcendent equation . Based on this equivalence, we state a conjecture that already the second branch has no elementary parametrization, i.e., it cannot be described by a finite number of elementary functions.
MSC: 39A12, 34B15.
KeywordsFučík spectrum nonlinear difference equation resonance boundary value problem elementary functions second branch
In 1979, deep in the dark days of the Soviet occupation of Central and Eastern Europe, a car with a Belgian couple is crossing the heavily guarded border between West Germany and Czechoslovakia. In their luggage they hide a package with a big amount of cash. If revealed, Jean Mahwin and his wife would end up in a serious trouble. Custom officers wouldn’t believe the true story about a donation collection of West European mathematicians for the widow of the recently deceased young mathematician Svatopluk Fučík. The organizer of the collection is none other than Jean Mawhin himself. The driver not only brings dollars, but also brightens faces of hundreds of decent people who learn about this story. Even today. Mathematicians are commonly depicted as out-of-touch and asocial beings. Few people would connect them to acts of courage and compassion. Jean Mawhin defies this stereotype more than anyone else. I am very happy that I can offer my wishes to his 70th birthday. Happy birthday!
Comparison of related continuous and discrete nonlinear problems reveals a very interesting relationship between these two worlds. In some cases, the finite dimension of discrete function spaces could significantly simplify analysis and provide general results (see [1–3]). In other situations, the broken discrete topology, in which sequences or vectors appear instead of continuous curves, causes difficulties without analogies in the continuous world. The goal of this paper is to show that the Fučík spectrum is one of the most astonishing examples of the latter type.
The nonlinear generalization of the eigenvalue problem for ODEs by Svatopluk Fučík  was quickly applied in the theory of semilinear boundary problems (e.g., [5, 6]). Since then this concept has been extended to more complicated differential operators (e.g., [7, 8]) and studied in general settings of Banach spaces (e.g., ). Attempts to analyze the Fučík spectrum for matrices and difference operators have been less successful (see [10–13]). Those works reveal the complexity of the matrix problem, which prevents to fully describe the spectrum beyond matrices.
Assuming that , we seek the Fučík spectrum, i.e., pairs such that the problem (1) has a nontrivial solution.
In Section 2, there is a short summary of continuous pasting technique. In Section 3, we deal with the trivial first branch of (1). In Section 4, we show that (i) one should rather talk about anchoring than pasting in the case of the second branch, and that (ii) the problem of finding its parametrization is equivalent to the problem of solving with and . Finally, in Section 5 we state and discuss the conjecture that the parametrization of the second-branch of (1) is not elementary, i.e., it cannot be described by a finite number of elementary functions.
2 Pasting in continuous case
3 Trivial first branch
As we use the eigenvalues in the sequel, we present a concise proof.
Thus, we have N independent eigenvectors, which finishes the proof. □
The first eigenvalue generates the first Fučík branch of (1). Since the corresponding eigenfunction does not change its sign, we obtain that the problem (1) has a nontrivial solution for an arbitrary couple , with . Similarly, the problem (1) has a nontrivial solution for an arbitrary couple , with .
4 Second branch and anchoring
Above, we considered . This follows from the fact that if we had chosen or , there would have been no positive/negative part and the solution would have laid on instead.
Our first observation is trivial and considers integer values of m. In this case, the transition between the positive and negative parts occurs exactly at , and we could easily compute β and C in (10). In other words, we could still talk about pasting in this case.
Moreover, and are equal in , m and .
The analysis gets more complicated once we consider non-integer values of m. In this case, the transition between positive and negative parts occurs between and . Our first result states that the values of and coincide at and (cf. Figure 2).
Lemma 4 (Necessary condition)
which verifies (14).
Using the same argument at for , we obtain that (13) holds as well. □
This result enables us to get both peripheral parts of .
This proves the former part of the statement. The latter follows from the mirror argument. □
Obviously, for m sufficiently close to N. This implies that (11) cannot hold for any β, i.e., not all the solutions on the second branch can be obtained as a composition of sine functions!
Since the problem is solved for , we could turn our attention to in the following. Applying (9) and (10), we can rewrite conditions (13) and (14) in the following way.
Corollary 6 (Necessary condition II)
Now, it suffices to multiply this equality by both denominators to get (15). □
Remark 7 (Anchoring)
Corollary 6 implies that the (continuous extensions of) sine functions (9) and (10) do not intersect at m in general. Indeed, we could see that does not solve (15) for all m. In other words, if we define , we have for almost every (see Figure 2).
Considering , we see that D and C are constant non-zero integers and . Consequently, if we substitute and , we get (16).
In order to get the parametrization of the complete second Fučík branch , we need to solve (15) for β, or, equivalently, (16) for x. While this could be done pretty easily numerically, we state, in the final section, a conjecture that it is not possible to use a finite number of elementary functions to get such a parametrization. Meanwhile, we make two straightforward observations.
Remark 8 Considering non-integer values of m, we can solve equation (15) only in the symmetric case, in which . Either the symmetry of sine functions or the direct computation yields that . Similarly, as in the integer case (cf. Lemma 3), this value coincides with the value of the corresponding continuous problem as the positive and negative parts meet at m.
Corollary 9 The value of ν is decreasing in μ.
and (11) yields the result. □
5 Elementariness of the second branch
Since the second-branch of (1) is the first nontrivial branch of the simplest discrete Fučík spectrum, we believe that its properties can help to explain difficulties with discrete Fučík spectra. Therefore, we discuss possible ways to prove that its part has no elementary parametrization.
Definition 10 We say that a function is elementary if it is a finite composition of rational, algebraic, exponential, logarithmic, trigonometric, inverse trigonometric, hyperbolic and inverse hyperbolic functions. We say that a parametrization of a curve in is elementary if it consists of elementary functions.
Under this definition, our conjecture becomes as follows.
Conjecture 11 The second branch of (1) has no elementary parametrization.
Our analysis in the previous section implies that one could rephrase this conjecture in the following way.
Conjecture 12 The solution of equation (16) cannot be solved in elementary functions.
Since there is a developed theory of elementary integration (see ) and (to our knowledge) there is no suitable tool dealing with elementary parametrizations of transcendent equations like (16), we try to use the theory of elementary integration to attack Conjectures 11 and 12.
Definition 13 We say that the integral is elementary if it can be expressed in terms of elementary functions.
One could use the Risch algorithm [, Chapter 12] to determine whether an integral is elementary or not. We use this procedure to analyze an integral directly connected to (15).
Lemma 14 The integral is not elementary.
Since the roots of are , i.e., not constant, the integral is not elementary (see the Rothstein-Trager theorem [, Theorem 12.9]). □
Let us return back to Conjecture 12 and study (16) in more general settings, considering .
Conjecture 15 Equation cannot be solved for using a finite number of elementary functions.
Let us denote and consider the equation . Since , the implicit function theorem can be applied in points such that and . Therefore, we take into account triplets with , .
where is the value of u along characteristic curves. The third equation of (18) cannot be solved using elementary functions for as the integral is not elementary (Lemma 14).
Unfortunately, the existence of non-elementary parametrization does not imply yet that the solution surfaces of (which arise as a union of the characteristic curves) cannot be expressed using elementary functions.
The goal of this paper was to shed some light on the problems which have arisen in the study of the discrete Fučík spectrum or related resonance problems. Although we were unable to fully prove the nonexistence of elementary parametrization of the second branch of the simplest discrete Fučík spectrum, we believe that the anchoring technique and the relationship to the transcendent equation (16) help to understand better the troubles which occur in this area.
a Subscript ℤ denotes the discrete interval, i.e., .
This research brought the author to the corners of mathematics he had not been familiar with. Therefore, he is very thankful to Jochen Merker and Petr Nečesal for their guidance. His thanks are also directed to Pavel Drábek, Gabriela Holubová and Komil Kuliev. This research has been supported by the grants of Ministry of Education, Youth and Sports of the Czech Republic ME09109 and MSM 4977751301.
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