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An exact multiplicity result for a class of semilinear equations. (English) Zbl 0877.35048
A sharp multiplicity result for positive solutions of some semilinear elliptic problems on balls in $$\mathbb{R}^2$$ is proved. The model example is the problem $-\Delta u=\lambda u(u-b)(c-u)\quad\text{in }|x|<\mathbb{R},\quad u=0 \quad\text{on }|x|=r,$ where $$0<b<c$$, $$c>2b$$, and $$\lambda$$ is a real parameter. It is easy to see that all solutions are positive. The main result is that there is a $$\lambda_0>0$$ such that the problem has exactly two solutions if $$\lambda>\lambda_0$$, one if $$\lambda=\lambda_0$$ and no solution for $$\lambda_0>\lambda$$. The main tool used in the proofs is a local inversion theorem by Crandall-Rabinowitz and the key point in order to obtain this interesting result is to show that eigenfunctions corresponding to the zero eigenvalue of the linearized problem at degenerate critical points do not change sign. It is pointed out that this is the only point where dimension two plays a role. One-dimensional arguments are widely used all along the proofs. Most of the properties of the solution branches, including stability are exhibited as well.

##### MSC:
 35J65 Nonlinear boundary value problems for linear elliptic equations 35P30 Nonlinear eigenvalue problems and nonlinear spectral theory for PDEs 35B32 Bifurcations in context of PDEs
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##### References:
 [1] DOI: 10.1007/BF02412022 · Zbl 0288.35020 [2] DOI: 10.1512/iumj.1975.24.24066 · Zbl 0329.35026 [3] DOI: 10.1007/BF00282325 · Zbl 0275.47044 [4] Evans L.C., Berkeley Lecture Notes in Mathematics 3 (1994) [5] Gardner R., Proc. Royal Soc. Edinburgh 104 pp 53– (1994) [6] DOI: 10.1007/BF01221125 · Zbl 0425.35020 [7] DOI: 10.1007/BF01450485 · Zbl 0806.35034 [8] Komornik V., The Multiplier Method. (1994) · Zbl 0937.93003 [9] Korman P., Steady states and long time behavior of some convective reaction-diffusion equations · Zbl 0887.35076 [10] Korman P., Proc. Royal Soc. Edinburgh 126 pp 599– (1996) [11] DOI: 10.1137/S0036141092231872 · Zbl 0824.34028 [12] DOI: 10.1090/S0002-9939-1988-0920985-9 [13] DOI: 10.1090/S0002-9939-1988-0920985-9 [14] Wei J., personal communication
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