Let d ⋅ ⋅ / d t be a derivation w.r.t. the continuous independent variable t . We consider two sets X = ( x 1 , … … , x k ) and Θ Θ = ( θ θ 1 , … … , θ θ l ) . The associated coordinate set is defined by Z = ( z 1 , … … , z n ) = ( t , x 1 , … … , x k , θ θ 1 , … … , θ θ l ) and its cardinal is n = 1 + k + l . With these notations, a system of first-order ODEs is a system where: