Solution
= Solution
If $x,y$ are <feasible points> and $0\leq t\leq1$, <concavity> gives
$$
c_i(tx+(1-t)y)\geq tc_i(x)+(1-t)c_i(y)\geq0
$$
for every $i$. Thus the <feasible set> is a <convex set>. Since the <objective function> is <convex>, this is a <convex optimization> problem.