On the validity of Prager's example of nonunique Michell structures

Prager (1974) demonstrated through an example that the optimal layout of least-weight trusses for a stress constraint — termed also Michell trusses—can be nonunique. The strain field for the above example, however, seems to violate Michell's optimality criteria. It is shown in this note that the above example of nonuniqueness is completely correct if we restrict the truss members to a smaller subset of the plane.