  We give an interpretation of the hemisphere rigidity theorem of Hang-Wang in
the framework of Gelfand problem. More precisely, Hang-Wang showed that for a
metric $g$ conformal to the standard metric $g_0$ on $S^{n}_{+}$ with $R\geq
n(n-1)$ and whose boundary coincides with $g_0|_{\partial S^{n}_{+}}$, then
$g=g_0$. This is related to the classical Gelfand problem, which investigates
$-\Delta u=\lambda g(u)$ for certain nonlinearity $g$ in a bounded region
$\Omega \subset \mathbb{R}^n$ subject to the Dirichlet boundary condition. It
is well-known that there exists an extremal $\lambda^{*}$, such that for
$\lambda>\lambda^{*}$, the above equation does not admit any solution.
Interestingly, Hang-Wang's hemisphere rigidity theorem yields a precise value
for $\lambda^{*}$ for $g(u)=e^{2u}$ when $n=2$ and
$g(u)=(1+u)^{\frac{n+2}{n-2}}$ for $n\geq 3$. We attempt to generalize the
hemisphere rigidity theorem under $Q$ curvature lower bound and fit this into
the interpretation of fourth order Gelfand problem for bi-Laplacian with
conformal nonlinearity.
