“…We mention that the concavity in (x, y) means concavity in two variables x and y jointly, not in each variable separately. e concavity of f(x, y, z, u) is used to find the optimal solution of NSDM (19), namely, function f(x, y, z, u) is concave in (x, y) if and only if for any pair of distinct points (x, y, z, u) and (x, y, z, u) in the domain, we have…”