My question is regarding the paper "Improved Rectangular Matrix Multiplication using Powers of the Coppersmith-Winograd Tensor". In the paper, the authors show an algorithm for multiplying a $n \times n^c$ and a $n^c \times n$ matrix. The authors show a graph of how the exponent in the time complexity depends on $c$. This graph is clearly convex. This would mean that one can upper bound the time complexity by linearly interpolating the exponent between that case of $c = 0.31389$ (where the exponent is 2) and $c=1$ (where the exponent is $\omega$). This would be useful for stating the time complexity of an algorithm I came up with.
However, they do not prove that the graph is convex. Is it convex? If so, any ideas as to how to cite this fact?