“ Following this purely algebraic problem I would like to raise a question that, it seems to me, can be attacked by the same method of continuous coefficient changing, and whose answer is of similar importance to the topology of the families of curves defined by differential equations – that is the question of the upper bound and position of the Poincaré boundary cycles (cycles limites) for a differential equation of first order of the form: d y d x = Y X {\displaystyle {dy \over dx}={Y \over X}} where X, Y are integer, rational functions of nth degree in resp. x, y, or written homogeneously: X ( y d z d t − z d y d t ) + Y ( z d x d t − x d z d t ) + Z ( x d y d t − y d x d t ) = 0 {\displaystyle X\left(y{dz \over dt}-z{dy \over dt}\right)+Y\left(z{dx \over dt}-x{dz \over dt}\right)+Z\left(x{dy \over dt}-y{dx \over dt}\right)=0} where X, Y, Z means integral, rational, homogenic functions of nth degree in x, y, z and the latter are to be considered function of the parameter t. ”