Conditions for the normal rectilinear congruences whose focal surfaces areminimal

The present work deals with the study of special categories of normal recti linear congruences whose focal surfaces are minimal and the solution to the problem is in the form of the membrane surface [3], but also, of the way o f its supporting, at points beyond its perimeter, that exist. The method ap plied deals with fundamental elements of the Differential Geometry and more precisely, with the theory of normal rectilinear congruence C.