d^2y dy
---- + P-- + Qy = 0,
dx^2 dx
by writing y = v exp. (-1/2 [int] Pdx), is at once seen to be reduced to
d^2v/dx^2 + 1v = 0, where 1 = Q - 1/2dP/dx - 1/4P^2. If [eta] = - 1/v dv/dx,
the equation d^2v/dx^2 + 1v = 0 becomes d[eta]/dx = 1 + [eta]^2, a
non-linear equation of the first order.