3

Here's one possible formulation, where $a_1,\dots, a_n$ are the values of the $n$ integers. Let binary decision variable $x_{i,j}$ indicate whether integer $i$ is assigned to subset $j\in\{1,\dots,k\}$. The problem is to maximize $z$ subject to \begin{align} \sum_j x_{i,j} &= 1 &&\text{for all $i$} \tag1\\ \sum_i a_i x_{i,j} &\ge z &&...


2

For a continuous function, all you need to do is prove that it's (i) non-convex, and (ii) monotonic. (i) can be shown using the eigenvalues of the hessian matrix, and (ii) using the gradient. However, in your case your domain is $\mathbb{Z}$, therefore derivatives are generally not defined, and neither is the concept of (pseudo)convexity. You can show ...


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