I asked this question about thermodynamics.
I considered $$T=\frac{\partial U}{\partial S}$$ at constant $V$ and $N$ (number of particles), but now I can't figure out the change of $U$ with respect to $V$ if $T<0$. Can somebody help me?
I also thought that $T = \text{slope of the tangent line to the function } U(S)$, but I can't figure out what the curve $U(V)$ is when $T<0$, as required.
Note: I know the temperature can't be negative, but I want to know what would theoretically happen.
