Lets $x_1, \dots, x_n \in \mathbb{R}$ and $f(x_1, \dots, x_n)=\sum_{i,j}(a_i*x_i - x_j)^2$ is a convex function.
$\mathbb{x}^* = \arg\min f(x1,\dots, x_2)$ with constraints $x_i > b_i, i=1,\dots n$