Title: Extremal functions for the Moser--Trudinger inequality of Adimurthi--Druet type in $W^{1,N}(\mathbb R^N)$,
Abstract: We study the existence and nonexistence of maximizers for variational problem
concerning to the Moser--Trudinger inequality of Adimurthi--Druet type in
$W^{1,N}(\mathbb R^N)$ \[ MT(N,\beta, \alpha) =\sup_{u\in W^{1,N}(\mathbb R^N),
\|\nabla u\|_N^N + \|u\|_N^N\leq 1} \int_{\mathbb R^N} \Phi_N(\beta(1+\alpha
\|u\|_N^N)^{\frac1{N-1}} |u|^{\frac N{N-1}}) dx, \] where $\Phi_N(t) =e^{t}
-\sum_{k=0}^{N-2} \frac{t^k}{k!}$, $0\leq \alpha < 1$ both in the subcritical
case $\beta < \beta_N$ and critical case $\beta =\beta_N$ with $\beta_N = N
\omega_{N-1}^{\frac1{N-1}}$ and $\omega_{N-1}$ denotes the surface area of the
unit sphere in $\mathbb R^N$. We will show that $MT(N,\beta,\alpha)$ is
attained in the subcritical case if $N\geq 3$ or $N=2$ and $\beta \in
(\frac{2(1+2\alpha)}{(1+\alpha)^2 B_2},\beta_2)$ with $B_2$ is the best
constant in a Gagliardo--Nirenberg inequality in $W^{1,2}(\mathbb R^2)$. We
also show that $MT(2,\beta,\alpha)$ is not attained for $\beta$ small which is
different from the context of bounded domains. In the critical case, we prove
that $MT(N,\beta_N,\alpha)$ is attained for $\alpha\geq 0$ small enough. To
prove our results, we first establish a lower bound for $MT(N,\beta,\alpha)$
which excludes the concentrating or vanishing behaviors of their maximizer
sequences. This implies the attainability of $MT(N,\beta,\alpha)$ in the
subcritical case. The proof in the critical case is based on the blow-up
analysis method. Finally, by using the Moser sequence together the scaling
argument, we show that $MT(N,\beta_N,1) =\infty$. Our results settle the
questions left open in \cite{doO2015,doO2016}.