On osculating element-bands associated with loci of surface-elements
Percey F. Smith · Transactions of the American Mathematical Society · 1910
(3) f('Y'l+'t ) °' the integral curves of which are isogonal trajectories of the integral curves of the original equation (1). The first theorem in question is now this: The osculating circles of all the isogonal trajectories through a fixed point pass also through a second fixed point, that is, form a pencil. Furthermore, these circles will osculate at the second point another system of isogonal trajectories. The second theorem is similar in character if the group (2) is replaced by