INTRODUCTION 5 (0.9) such that (0.22) σ(λ, d) = ∞ and limt→∞ v(t, λ, d) = 0, or σ(λ, d) ∞ and v (σ(λ, d), λ, d) = 0. In chapter 4 we prove the following result concerning ground states of (0.9). Theorem 5. The equation ( 0.9) has a ground state if and only if −1 q p 2∗ − 1. Moreover, such a ground state is unique. Ground states for (0.9) have been studied by several authors in the last two decades (see [16, 17, 18, 20, 23]). In fact it is pointed out in [16] the interest of this problem in plasma physics. Our proof of Theorem 5 borrows ideas developed in the latter papers. In Chapter 4 we study the stability of stationary solutions for the constant density case (see (0.6)) under either (0.4) or (0.5). Our main result are: Theorem 6. Let (λ, d) ∈ Σ with λ = ξ1(d). If i) d 1 and ξ1 is strictly decreasing in a neighborhood of d, or ii) d 1 and ξ1 is strictly increasing in a neighborhood of d, then u(r, λ, d) = v 1 k+1 (r, λ, d), with r = |x|, is a radial steady state solution of (0.9) subject to (0.7) stable under non-necessarily radial uniform perturbations of the initial data. Theorem 7. The steady state solution u ≡ 1 of (0.3) subject to (0.5) is unsta- ble. Moreover, any solution u(x, t) of (0.3) subject to (0.5) such that 0 u(x, 0) 1 for all x ∈ ¯(0, 1) (respectively u(x, 0) 1 for all x ∈ ¯(0, 1)) satisfies u(x, t) → 0 as t → ∞ uniformly (respectively u(x, t) → ∞ as t → ∞ uniformly.) Theorem 8. Let v(r, d) be a solution of (0.9) subject to (0.7), respectively (0.9) subject to (0.8), such that vd(r, d) := ∂dv(r, d) 0 for some r ∈ (0, 1]. Then u(r, d) = v 1 k+1 (r, d), with r = |x|, is a stationary solution of (0.3) subject to (0.4) or to (0.5) respectively, that is linearly unstable. In particular, if (λ, d) ∈ Σ and i) d 1, λ = ξ1(d), and ξ1 is strictly increasing in a neighborhood of d, or ii) d 1, λ = ξ1(d), and ξ1 is strictly decreasing in a neighborhood of d, or iii) λ = ξi(d) for i = 2, 3, . . ., then u(r, λ, d) = v 1 k+1 (r, λ, d), with r = |x|, is a radial steady state solution of (0.9) subject to (0.7) linearly unstable, respectively a radial steady state solution of (0.9) subject to (0.8) linearly unstable.

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