I got stuck with one task, please any help.
Given the function $f (x_1,x_2,x_3) = ax_1x_2 + bx_1x_3$, $x_1,x_2,x_3\in[L,U], \,L,U\in\mathbb{R}$. Does convex envelope of $f$ ever strictly dominate the sum of conv. envelopes of each terms, i.e., $\mathrm{convenv}(ax_1x_2) + \mathrm{convenv}(bx_1x_3) $?