First, we prove that
\(\int_{\mathbb{R}_{+}^{N}} |u|^{p+1}\,dx<\infty\). By Lemma
2.1, there exists
\(R_{0}>0\), such that
$$\bigl\langle I^{\prime\prime}(u)u\phi_{R_{0},R},u\phi_{R_{0},R}\bigr\rangle \geq0, $$
for any
\(R>2R_{0}\). That is,
$$ p\int_{\mathbb{R}_{+}^{N}} |u|^{p+1} \phi_{R_{0},R}^{2}\,dx+q\int_{\Gamma_{1}} |u|^{q+1}\phi_{R_{0},R}^{2}\,dx'\leq\int _{\mathbb{R}_{+}^{N}} \bigl|\nabla(u\phi_{R_{0},R})\bigr|^{2}\,dx. $$
(2.2)
Apparently,
$$\int_{\mathbb{R}_{+}^{N}} \bigl|\nabla(u\phi_{R_{0},R})\bigr|^{2}\,dx = \int_{\mathbb{R}_{+}^{N}} |\nabla u|^{2}\phi _{R_{0},R}^{2}+| \nabla \phi_{R_{0},R}|u^{2}+2u\phi_{R_{0},R}\nabla u\nabla \phi_{R_{0},R}\,dx. $$
Multiplying (
1.3) by
\(u\phi_{R_{0},R}^{2}\) and integrating by parts, we obtain
$$ \int_{\mathbb{R}_{+}^{N}} |\nabla u|^{2} \phi_{R_{0},R}^{2}+2u\phi_{R_{0},R}\nabla u\nabla \phi_{R_{0},R}\,dx=\int_{\mathbb{R}_{+}^{N}} |u|^{p+1} \phi_{R_{0},R}^{2}\,dx+\int_{\Gamma_{1}} |u|^{q+1}\phi_{R_{0},R}^{2}\,dx'. $$
(2.3)
By (
2.2) and (
2.3), we have
$$\begin{aligned} &(p-1)\int_{\mathbb{R}_{+}^{N}} |u|^{p+1} \phi_{R_{0},R}^{2}\,dx+(q-1)\int_{\Gamma_{1}} |u|^{q+1}\phi_{R_{0},R}^{2}\,dx' \\ &\quad\leq\int_{\mathbb{R}_{+}^{N}} u^{2}|\nabla \phi_{R_{0},R}|^{2}\,dx \\ &\quad\leq C_{0}+\frac{C}{R^{2}}\int_{\{R\leq|x|\leq 2R\}\cap\mathbb{R}^{N}_{+}}u^{2}\,dx. \end{aligned}$$
(2.4)
In particular, we have
$$ (p-1)\int_{\mathbb{R}_{+}^{N}} |u|^{p+1} \phi_{R_{0},R}^{2}\,dx\leq C_{0}+\frac{C}{R^{2}}\int _{\{R\leq|x|\leq2R\}\cap\mathbb{R}^{N}_{+}}u^{2}\,dx. $$
(2.5)
If
\(N=2\), since
u is a bounded solution, the right hand side of (
2.5) is bounded by a positive constant independent of
R. So we have
\(\int_{\mathbb{R}_{+}^{N}} |u|^{p+1}\,dx<\infty\) by letting
\(R\to \infty\) in (
2.5). Now for the case
\(N\geq3\), we deduce from the Hölder inequality that
$$\begin{aligned} (p-1)\int_{\mathbb{R}_{+}^{N}} |u|^{p+1} \phi_{R_{0},R}^{2}\,dx &\leq C_{0}+\frac{C}{R^{2}}\int_{\{R\leq|x|\leq2R\}\cap\mathbb{R}^{N}_{+}}u^{2}\,dx \\ &\leq C_{0}+C\biggl(\int_{\{R\leq|x|\leq2R\}\cap\mathbb{R}^{N}_{+}}|u|^{p+1}\,dx\biggr)^{\frac{2}{p+1}}\cdot R^{N\frac{p-1}{p+1}-2}. \end{aligned}$$
(2.6)
Suppose that
\(\int_{\mathbb{R}_{+}^{N}} |u|^{p+1}\,dx\) is infinite, then we deduce that
$$ \int_{B_{R}^{+}}|u|^{p+1}\,dx\leq C\biggl( \int_{B_{2R}^{+}}|u|^{p+1}\,dx\biggr)^{\frac{2}{p+1}}\cdot R^{N\frac{p-1}{p+1}-2}. $$
(2.7)
Let
\(\alpha=N\frac{p-1}{p+1}-2\),
\(\beta=\frac{2}{p+1} \), and
\(J(R)=\int_{B_{R}^{+}}|u|^{p+1}\,dx\). Iterating (
2.7), we obtain
$$ J(R)\leq CR^{\alpha\gamma}J\bigl(2^{k+1}R \bigr)^{\beta^{k+1}}, $$
(2.8)
where
\(\gamma=1+\beta+\beta^{2}+\cdots+\beta^{k}\). The boundedness of
u implies that the right hand side of (
2.8) is of order
\(R^{M}\) with
$$M=\alpha\frac{1-\beta^{k+1}}{1-\beta}+N\beta^{k+1}\to\frac{\alpha}{ 1-\beta} $$
as
\(k\to\infty\). Hence, we can choose
k large enough, such that
\(M<0\). Then it follows from (
2.8) that
as
\(R\to\infty\), which is impossible. So we get
\(\int_{\mathbb{R}_{+}^{N}} |u|^{p+1}\,dx<\infty\).
Next, we prove that
\(\int_{\partial\mathbb{R}_{+}^{N}} |u|^{q+1}\,dx'<\infty \). In fact, we deduce from (
2.4) that
$$ (q-1)\int_{\Gamma_{1}} |u|^{q+1} \phi_{R_{0},R}^{2}\,dx' \leq C_{0}+ \frac{C}{R^{2}}\int_{\{R\leq|x|\leq2R\}\cap\mathbb{R}^{N}_{+}}u^{2}\,dx. $$
(2.9)
If
\(N=2\), the right hand side of (
2.9) is bounded by a positive constant independent of
R. So we conclude that
$$\int_{\Gamma_{1}} |u|^{q+1}\,dx'< \infty $$
by letting
\(R\to\infty\) in (
2.9). Now for
\(N\geq3\), we infer from (
2.9) and the Hölder inequality that
$$\begin{aligned} &(q-1)\int_{\Gamma_{1}} |u|^{q+1} \phi_{R_{0},R}^{2}\,dx' \\ &\quad\leq C_{0}+\frac{C}{R^{2}}\int_{\{R\leq|x|\leq2R\}\cap\mathbb{R}^{N}_{+}}u^{2}\,dx \\ &\quad\leq C_{0}+C\biggl(\int_{\mathbb{R}_{+}^{N}} |u|^{p+1}\,dx\biggr)^{\frac{2}{p+1}}R^{N\frac{p-1}{p+1}-2} \\ &\quad< \infty \end{aligned}$$
(2.10)
since
\(\int_{\mathbb{R}_{+}^{N}} |u|^{p+1}\,dx<\infty\) and
\(p\leq\frac{N+2}{N-2}\).