$$f(x,y)=-x\log(1+y)$$ The Hessian matrix of $f(x,y)$ is $$\left[ \begin{matrix} 0 & -\frac{1}{1+y}\\ -\frac{1}{1+y} & \frac{x}{(1+y)^2} \end{matrix}\right] $$
Then the eigen value is $-\frac{1}{(1+y)^2}$.
Since it is not positive, $f(x,y)$ is not convex. Right??
However, papers said it is convex.
Please let me know why it is convex.