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.
Keywords:Fučík 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.
One possible answer to these complications could be to concentrate on a special class of matrices corresponding to specific difference operators. In this brief note, we follow this avenue and try to apply Fučík’s pasting technique  to the simplest discrete problem, a direct counterpart of the original continuous problem
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
First, let us briefly recall the pasting technique for the original continuous problem 
Proof The detailed proof could be found, e.g., in [, Chapter 42]. We only provide a seemingly clumsy proof of the construction of the second branch so that we could illustrate the pasting technique. On the solution could be split in the positive part and negative part . Without loss of generality, we assume that , where . Then the negative part must have the form , where is chosen so that and meet at m. The constant C is then chosen so that this connection is continuously differentiable (cf. Figure 1). From this we have that and , which implies that
Figure 1 . Continuous pasting procedure. See Lemma 1 and its proof.
3 Trivial first branch
Let us return back to the discrete problem (1). Following the continuous notation, we denote the discrete spectrum by . We realize first that the problem (1) on points can change sign only -times, which implies that the spectrum on points will have only N branches , i.e.,
As we use the eigenvalues in the sequel, we present a concise proof.
Theorem 2The eigenvalues of (6) are
and the corresponding eigenvectors are
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
Let us move on to the second discrete Fučík branch . It corresponds to the solutions of (1) that change the sign exactly once (see Figure 2 for illustration and basic notation). As in the continuous case, we try to paste together two sine functions. Without loss of generality, we assume that the solution is positive first. We choose and consider solutions which are nonnegative on ,a with values lying on the sine function (cf. Theorem 2 and Figure 2)
Since the solutions on the second branch change sign exactly once and lie on the sine function again, we seek constants (cf. Theorem 2 and Figure 2) in the nonpositive part
Figure 2 . Discrete anchoring procedure. Illustration of the fact that in general.
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.
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)
This equality holds if and only if (see (1))
which verifies (14).
This proves the former part of the statement. The latter follows from the mirror argument. □
Corollary 6 (Necessary condition II)
Proof One can rewrite equalities (13) and (14) into
Isolating C on the right-hand sides of both equations, we get
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).
We have shown that
where (see Lemma 3 and (11))
and the parametrization of is given by (5). Finally, we would like to get the parametrization of , i.e., consider . Corollary 6 implies that one could get the parametrization of if and only if one could solve (15). Equation (15) is equivalent to the problem of solving the transcendent equation
then the nonlinear equation (15) can be rewritten as
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.
Combining this with the following straightforward corollary, we observe that the second branch of the discrete spectrum slides monotonically along its continuous counterpart, coinciding in the ‘symmetric’ values of m, in which (see Figure 3).
Figure 3 . Illustration of the second Fučík branch forwith the corresponding continuous second branch. is not depicted for presentation purposes. In this case, consists only of two points, in which the orange meets the pink , corresponding to and .
Corollary 9The value ofνis decreasing inμ.
and (11) yields the result. □
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.
Our analysis in the previous section implies that one could rephrase this conjecture in the following way.
Conjecture 12The 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).
We compute the following resultant (see [, Definition 7.3]):
Since the roots of are , i.e., not constant, the integral is not elementary (see the Rothstein-Trager theorem [, Theorem 12.9]). □
Multiply the first equation by , the second equation by , add both and divide the result by the non-zero term in the square brackets to obtain the initial problem for the first-order partial differential equation ()
Characteristic equations of (17) are given by
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.
The author declares that they have no competing interests.
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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