We have found the global minimum.
The objective function, d(x), is convex
a(x) - b(x) is convex, so the first constraint
defines a convex region.
f(x) is linear and convex, so the second constraint
defines a convex region.
e(x) is concave so - e(x) is convex.
The feasible region is convex, so the local minimum
is the global minimum.