We prove the nonexistence of smooth stable solutions to the biharmonic problem
,
in
for
and
,
where
is the largest root of the equation
In particular, as
when
,
we obtain the nonexistence of smooth stable solutions for any
and
.
Moreover, we consider also the corresponding problem in the half-space
, and the elliptic
problem
on a bounded
smooth domain
with the Navier boundary conditions. We prove the regularity of the extremal
solution in lower dimensions.