A minimizing problem of a polyharmonic operator with Critical Exponent

In this work, we study the two following minimization problems for $r \in \mathbb{N}^{*}$, \begin{equation*} \begin{array}{ccc} S_{0,r}(\varphi)=\displaystyle\inf_{u\in H_{0}^{r}(\Omega)\,|u+\varphi\|_{L^{2^{*r}}}=1}\|u\|_{r}^{2}& \textrm{and}& S_{\theta,r}(\varphi)=\displaystyle\inf_{u\in H_{\theta}^{r}(\Omega)\, \|u+\varphi\|_{L^{2^{*r}}}=1}\|u\|_{r}^{2}, \end{array} \end{equation*} where $\Omega \subset \mathbb{R}^{N},$ $N > 2r$, is a smooth bounded domain, $2^{*r}=\frac{2N}{N-2 r}$, $\varphi\in L^{2^{*r}} (\Omega) \cap C(\Omega)$ and the norm $\|. \|_{r}=\displaystyle{ \int_{\Omega} |(-\Delta)^{\alpha} .|^{2}dx}$ where $\alpha=\frac{r}{2}$ if $r$ is even and $\|. \|_{r}=\displaystyle{ \int_{\Omega} |\nabla(-\Delta)^{\alpha} . |^{2}dx }$ where $\alpha = \frac{r-1}{2}$ if $r$ is odd. Firstly, we prove that, when $\varphi \not\equiv 0,$ the infimum in $S_{0,r}(\varphi)$ and $S_{\theta,r}(\varphi)$ are attained. Secondly, we show that $S_{\theta,r}(\varphi)< S_{0,r}(\varphi)$ for a large class of $\varphi$.