the transformed equation X"' + a.X" + b-X' + d-X + c = 0 (4) By making a further transformation of the type X + c /d = Y (5) we obtain 7'"+ aF" + 67'-+ d-7 = 0 (6) Equation (6) is a third-order linear differential equation and hence represents the equivalent linear system in the transformed plane (Y-T). Equation (6) can easily be solved to obtain F as a function of T. Using Eqs. (3) and (5) along with the solution to Eq. (6) gives, say, From Eq. (2),Substitution of Eq. (7) in (8) gives the relation between t and T as, say,Thus, the implicit relation between x and t is established by Eqs. (7) and (9) in terms of a common parameter T. In many cases, it may be possible to obtain the explicit relation, viz., a; as a function of t, if T is eliminated between Eqs.(7) and (9). The method just outlined is illustrated by means of the following suitable example. An equation of the type considered previously, viz., Eq. (1), arises in gyroscopic theory, while considering the inertial motion of a gyro rotating about its center of mass.Euler's equation of motion in body-fixed coordinates for a rigid body rotating about its center of mass under inertial motion (i.e., under no external moments) can be written as 4Equation (10) represents three coupled nonlinear first-order differential equations that on successive differentiation and mutual substitution can be reduced 5 to a set of uncoupled, third-order, nonlinear differential equations of the type x -(xx/x) + bx 2 x = 0 (H) Now Eq. (11) is a particular case of Eq. (1) with f(x) = x and a = c = d = 0. Hence, the necessary transformation function is i.e., dX/dx = dT/dt = x X = z 2 /2 X' = x Equation (13) in (11) gives X"' + bX' = 0 Letting X' = Y reduces Eq. (14) toY" + bY = 0 whose solution can at once be written as(13) (14) (15) (16) (17) Reverting back to the variable X gives X(T) = A 1 cos(bV*T + 6) + A, Using Eq. (13) in (18) gives x = {2[Ai cos(6 1 / 2 T + 0) + ^2]} 1/ and, from Eqs. (13) and (19), dT t J {214! 6) + A 2 ]} 1/2 (18) (19)Here Eqs. (19) and (20) together represent the response of the third-order system given by Eq. (11), and the constants Ai, 6, and A 2 can be chosen to suit the initial conditions.In case T is eliminated between Eqs. (19) and (20), the explicit relationship between x and t can be attained. Similar results can be obtained for the angular velocities along all of the three principal axes of the rigid body. Thus, the practical example just treated illustrates the value of the method outlined.
ConclusionsFor any given nonlinear third-order system of the class represented by Eq. (1), the function f(x) and the constants a, b, c } and d are known, and hence the corresponding linear systems represented by Eq. (4) or (6) can be attained.By solving the resulting linear equations and working backwards, the solution to the original nonlinear equation can be arrived at, as can be seen from the example considered previously. Therefore, the method described here is expected to be of use in the study of many problems in various fields such as gyrodynamics, vi...