  We present a necessary and sufficient condition for Alt's system to be
represented by a concave utility function. This condition can be seen as an
extension of Gossen's first law, and thus has an economic interpretation.
Together with the above result, we present a rigorous proof of Alt's
representation theorem on a Hausdorff, separable, and path-connected
topological space, and provide a necessary and sufficient condition for Alt's
utility to be continuously differentiable.
