  In this article we consider a special type of degenerate elliptic partial
differential equations of second order in convex domains that satisfy the
interior sphere condition. We show that any positive viscosity solution $u$ of
$-|\nabla u|^\alpha \Delta_p^N u = 1$ has the property that $u^\frac{\alpha +
1}{\alpha + 2}$ is a concave function. Secondly we consider positive solutions
of the eigenvalue problem $-|\nabla u|^\alpha \Delta_p^N u = \lambda
|u|^{\alpha} u$, in which case $\log u$ turns out to be concave. The methods
provided include a weak comparison principle and a Hopf-type Lemma.
