I wonder if there is any paper on the "priori estimate approach" for Yamabe problem.
The Yamabe problem is solving the following equation: On a $C^{\infty}$ compact Riemannian manifold $M_n$ of dimension $n \geq 3$, let us consider the differential equation
$$ \Delta \varphi+h(x) \varphi=\lambda f(x) \varphi^{N-1} $$
where $h(x)$ and $f(x)$ are $C^{\infty}$ functions on $M_n$, with $f(x)$ everywhere strictly positive and $N=2 n /(n-2)$.
As a graduate student, the materials I read are all based on variational method, did any scholars have studied the Yamabe problem through priori estimate approach? For example, the $C^k$ or $W^{k,p}$ estimate by maximum principal or other tools.