# FIRST-ORDER SYSTEMS OF ORDINARY DIFFERENTIAL EQUATIONS III: Autonomous Planar Systems David Levermore Department of Mathematics University of Maryland

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1 FIRST-ORDER SYSTEMS OF ORDINARY DIFFERENTIAL EQUATIONS III: Autonomous Planar Systems David Levermore Department of Mathematics University of Maryland 4 May 2012 Because the presentation of this material in lecture will differ from that in the book, I felt that notes that closely follow the lecture presentation might be appreciated Contents 6 Autonomous Planar Systems: Integral Methods 61 Stationary Solutions 2 62 Reduction to First-Order Equations 3 63 Hamiltonian Systems 4 64 Conservative Systems 5 7 Autonomous Planar Systems: Nonintegral Methods 71 Linearization near Stationary Solutions 9 8 Application: Population Dynamics 81 Models of Two Interacting Populations Predator-Prey Models 13 1

2 2 6 Autonomous Planar Systems: Integral Methods For the remainder of the course we will stu first-order, autonomous, planar systems in the normal form 61 fx, y, gx, y dt dt The word autonomous has Greek roots and means self governing Such systems are called autonomous because they only depend on the state x, y and not on time t What makes them planar is the fact they have two dependent variables, x and y In the general discussions throughout this section we will assume that f and g are continuously differentiable over some domain D in the xy-plane Our basic existence and uniqueness theorem for initial-value problems therefore applies whenever the initial data lies in D When D is not specified explicitly, assume it is the entire xy-plane 61 Stationary Solutions A solution of system 61 is said to be stationary if it does not depend in time Because the time derivatives are zero for such a solution, it must satisfy the algebraic system 62 0 fx, y, 0 gx, y Conversely, if x o, y o satisfies this algebraic system then it is easily checked that xt, yt x o, y o is a stationary solution of system 61 You can therefore find all stationary solutions of the first-order system 61 by finding all solutions of the algebraic system 62 System 62 consists of two algebraic equations for the two unknowns x and y However, in general this system can have no solutions, one solution, or many solutions Example Find all stationary solutions of the system dt y, Solution The stationary solutions satisfy dt x x3 0 y, 0 x x 3 x1 x1 + x The first of these equations implies y 0, while the second implies x 0, x 1, or x 1 The stationary solutions are therefore 0, 0, 1, 0, 1, 0 Example Find all stationary solutions of the system y xx 1, dt dt 3 + 2x x2 y Solution The stationary solutions satisfy 0 y xx 1, x x 2 y 3 x1 + xy The first of these equations implies y x or x 1 If y x then the second equation becomes 0 3 x1 + xx, which implies x 3, x 1, or x 0 Hence, y x leads to the stationary solutions 3, 3, 1, 1, 0, 0

3 On the other hand, if x 1 then the second equation becomes 0 4y, which implies y 0 Hence, x 1 leads to the stationary solution 1, 0 We therefore find the four stationary solutions 3, 3, 1, 1, 0, 0, 1, 0 62 Reduction to First-Order Equations Sometimes system 61 can be solved analytically by solving two first-order equations The first of these is the so-called orbit equation It seeks y as a function of x that satisfies 63 gx, y fx, y This first-order equation can be solved if it is linear, is separable, or has an exact differential form If you can find an explicit solution y Y x of the orbit equation then you can try to find x as a function of t by solving the so-called reduced equation, 64 fx, Y x dt This first-order equation is autonomous, so it can be solved implicitly if you can find a primitive of 1/fx, Y x If you are able to find a solution xt of the reduced equation then a solution of system 61 is given by 65 xt, yt xt, Y xt It is often difficult or impossible to solve the orbit equation 63 And even when you can find an explicit solution Y x of the orbit equation, it is often difficult or impossible to find a primitive of 1/fx, Y x in order to solve the reduced equation 64 Example Try to solve the system dt y, Solution The orbit equation is dt x x3 x x3, y which is seperable It has the separated form which can be integrated to obtain y x x 3, 1 2 y2 1 2 x2 1 4 x4 + 1c 2 Here we put the 1 in front of the c because we see that the equation must be multiplied 2 by 2 in order to solve for y Upon solving for y we find the explicit solutions y ± x x4 + c The reduced equation then becomes dt ± x x4 + c, 3

4 4 which can be solved implicitly in terms of an integral as 1 t ± x x4 + c This integral cannot be evaluated analytically for every c by methods from first-year calculus That evaluation requires the use of elliptic functions, which lie beyond the scope of this course It can be evaluated analytically for c 0 by methods from first-year calculus, but we will not do that here Remark In the previous example we could integrate the orbit equation easily, but could not do the same for the reduced equation This is often the case when the orbit equation can be integrated In the next example we cannot even integrate the orbit equation Most orbit equations cannot be integrated Example Try to solve the system y xx 1, dt dt 3 + 2x x2 y Solution The orbit equation is 3 + 2x x2 y y xx 1 This equation is not linear or separable Later we will show that it cannot be integrated 63 Hamiltonian Systems The orbit equation 63 can be put into the differential form gx, y + fx, y 0 This differential form is exact when 66 x fx, y + y gx, y 0 In that case there exists a function Hx, y such that x Hx, y gx, y, y Hx, y fx, y System 61 thereby is seen to have the form 67 dt yhx, y, dt xhx, y Such a system is said to be a Hamiltonian system while H is called its Hamiltonian The Hamiltonian H is determined uniquely up to an additive constant Because we have assumed that f and g are continuously differentiable, H will be twice continuously differentiable Example Consider the system dt x x3 dt y, Show it is Hamiltonian and find its Hamiltonian Hx, y Solution The system is Hamiltonian because x fx, y + y gx, y x y + y x x 3 0

5 You can find Hx, y by solving y Hx, y y, x Hx, y x + x 3 Upon integrating the first equation we find that Hx, y 1 2 y2 + hx By substituting this into the second equation we find that h x x+x 3 By setting hx 1 2 x x4 we obtain the Hamiltonian Hx, y 1 2 y2 1 2 x x4 Many systems can be put into Hamiltonian form For example, every second-order equation of the form d 2 x dt + gx 0, 2 can be recast as the first-order system dt y, dt gx This system is Hamiltonian with Hx, y 1 2 y2 + Gx where G x gx because y Hx, y y, x Hx, y G x gx More generally, consider any first-order system of the form dt fy, dt gx This system is Hamiltonian with Hx, y Fy + Gx where F y fy and G x gx because y Hx, y F y fy, x Hx, y G x gx Remark When a system can be put into the Hamiltonian form 67 then you can determine its phase portrait by just analyzing Hx, y We will postpone the discussion of how this is done until after we have introduced a more general class of systems to which the same techniques can be applied 64 Conservative Systems More generally, the orbit equation 63 can be put into the differential form µx, y gx, y + µx, y fx, y 0, where µx, y is a positive factor that we will assume is also continuously differentiable We hope to find a µx, y that makes this differential form exact, in which case it is called an integrating factor This will be the case when µx, y satisfies 68 x [fx, y µ] + y [gx, y µ] 0 When you can find such a µ then there exists a function Hx, y such that x Hx, y µx, y gx, y, y Hx, y µx, y fx, y System 61 thereby is seen to have the form 69 dt 1 µx, y yhx, y, dt 1 µx, y xhx, y 5

6 6 Such a system is said to be conservative Hamiltonian systems correspond to the cases when µx, y 1 In the more general setting, H is called an integral of the system Given a choice of µ, the integral H is determined uniquely up to an additive constant Because we have assumed that f, g, and µ are continuously differentiable, H will be twice continuously differentiable When a system can be put into the conservative form 69 then you can determine its phase portrait just by analyzing the integral Hx, y This is due to the following facts Fact 1 A point x o, y o is a stationary solution of the conservative system 69 if and only if it is a critical point of Hx, y Reason Recall that a point is a critical point of Hx, y if and only if it satifies x Hx, y 0, y Hx, y 0 But these are exactly the equations that characterize stationary solutions of 69 Fact 2 If xt, yt is any solution to the conservative system 69 then Hxt, yt is a constant Reason The multivariable chain rule and system 69 yield d dt Hx, y xhx, y dt + yhx, y dt 1 x Hx, y µx, y yhx, y + y Hx, y Therefore Hxt, yt is a constant 1 µx, y xhx, y 0 This fact states that orbits of system 69 lie on so-called level sets of Hx, y namely, sets in the xy-plane of the form { } 610 x, y : Hx, y c for some constant c This set will be empty unless c is in the range of H If c is in the range of H then c is called a critical value of H if c Hx o, y o for some point x o, y o that is a critical point of the function H, and is called a noncritical value of H otherwise If c is a noncritical value of H then the associated level set will consist of one or more disjoint curves, each of which is a single orbit These curves will be either a closed loop, corresponding to a periodic orbit, or an unbounded curve, corresponding to an orbit that becomes unbounded as t or as t If c is a critical value of H then the associated level set will consist of one or more critical points and possibly other curves that can either loop from a critical point to itself, connect two critical points, connect a critical point to infinity, run from infinity to infinity, or be a closed loop We will illustrate most of these possibilities with examples below To simplify our duscussion, we consider only functions H whose critical points are nondegenerate Recall that a critical point x o, y o of H is said to be nondegenerate if 611 det Hx o, y o 0,

7 where Hx, y denotes the Hessian matrix of second derivatives, which is given by xx Hx, y 612 Hx, y xy Hx, y yx Hx, y yy Hx, y Because xy Hx, y yx Hx, y, the Hessian matrix is symmetric It therefore has real eigenvalues Recall that det Hx, y will be the product of those eigenvalues If x o, y o is a nondegenerate critical point of H then the eigenvalues of Hx o, y o are nonzero and there are three possibilites: if Hx o, y o has two positive eigenvalues then x o, y o is a local minimizer of H; if Hx o, y o has two negative eigenvalues then x o, y o is a local maximizer of H; if Hx o, y o has one eigenvalue of each sign then x o, y o is a saddle point of H In the first two cases the level sets of H near x o, y o will be closed loops, representing periodic orbits In those cases x o, y o is a center in the phase portrait of system 69 One of these first two cases arises whenever det Hx o, y o > 0 In the third case the level sets of H near x o, y o moves away from x o, y o In that case x o, y o is a saddle in the phase portrait of system 69 This last case arises whenever det Hx o, y o < 0 Example Draw a phase portrait for the system dt y, dt x x3 Solution We have shown alrea that this is a Hamiltonian system with Hx, y 1 2 y2 1 2 x x4 We also have shown alrea that the critical points of H are The associated critical values are The Hessian matrix is Because H0, 0 0, 0, 1, 0, 1, 0 H0, 0 0, H±1, Hx, y 1 + 3x , H±1, , 0 1 we see that 0, 0 is a saddle point while 1, 0 and 1, 0 are local minimizers Hence, the phase portrait is a saddle near 0, 0 and a clockwise center near 1, 0 and 1, 0 Because Hx, y as x 2 + y 2 while 1, 0 and 1, 0 are the only local minimizers and they have the same value H±1, 0 1, these local minimizers are 4 global minimizers Hence, the level set associated with the critical value 1 consists 4 7

8 8 of just the critical points 1, 0 and 1, 0 This can also be seen from the fact the equation Hx, y 1 can be written as y x , which is satisfied only when x, y ±1, 0 The level set associated with the critical value 0 consists of the saddle point 0, 0 plus all other points x, y that satisfy Hx, y 0 This equation can be written as y 2 x x4, which has solutions y ± x x4 for every x in [ 2, 2 ] A graph shows two orbits that each emerge from the origin, moves out to either ± 2, 0, and returns to the origin

9 7 Autonomous Planar Systems: Nonintegral Methods For most autonomous planar systems the orbit equation 63 cannot be integrated In this section we present techniques for understanding the phase portrait of such a system that do not require finding an integral We consider first-order, autonomous, planar systems that have the form 71 fx, y, gx, y dt dt 71 Linearization Near Stationary Solutions Recall from multivariable calculus that the Taylor approximations of fx, y and gx, y near a point x o, y o have the form fx, y fx o, y o + x x o x fx o, y o + y y o y fx o, y o + higher-order terms, gx, y gx o, y o + x x o x gx o, y o + y y o y gx o, y o + higher-order terms If we dorp the higher-order terms then this becomes a linear approximation to fx, y and gx, y that will be valid when x, y is near x o, y o If x o, y o is a stationary solution of our system then fx o, y o gx o, y o 0 and these linear approximations become fx, y x x o x fx o, y o + y y o y fx o, y o, gx, y x x o x gx o, y o + y y o y gx o, y o The idea of linearization is that when the solution xt, yt of system is near x o, y o then it can be approximated by the solution of a linear system obtained by replacing fx, y and gx, y with the above linear approximations Specifically, the idea is that xt, yt x o + xt, y o + ỹt where xt, ỹt satisfy the linear system 72 d dt x ỹ x fx o, y o y fx o, y o x gx o, y o y gx o, y o x ỹ This is called the linearization of system 71 about x o, y o The coefficient matrix of this linearized system is A Jx o, y o where Jx, y is the matrix of partial derivatives given by x fx, y 73 Jx, y y fx, y x gx, y y gx, y This matrix is sometimes called the Jacobian matrix Notice that the partial derivatives of f are the entries of the top row while those of g are the entries of the bottom row We now ask if the phase-plane portrait of system 71 near the stationary point x o, y o looks like the phase-plane portrait of the linearized system with coefficient matrix A Jx o, y o This turns out to be the case in some instances, but not in all If the origin is attracting or repelling for the linearized system then the stationary solution x o, y o of system 71 will have the same property If the linearized system is a saddle, nodal sink, nodal source, spiral sink, or spiral source then the phase-plane portrait of system 71 will look like it near the stationary point If system 71 is conservative and linearized system is a center then the phaseplane portrait of system 71 will look like a center near the stationary point 9

10 10 Example Determine the stability and classify each stationary point of the system y xx 1, dt dt 3 + 2x x2 y Solution We have seen that the stationary points of this system are 1, 1, 0, 0, 1, 0, 3, 3 The Jacobian matrix for the system is y 2x + 1 x 1 Jx, y 2y 2xy 3 + 2x x 2 At 1, 1 the coefficient matrix of the linearized system is A J 1, Its characteristic polynomial is pz z 2 2z 8 z + 2z 4 Its eigenvalues are thereby 2 and 4 Hence, the stationary point 1, 1 is a saddle and is unstable At 0, 0 the coefficient matrix of the linearized system is 1 1 A J0, Because A is upper triangular, we can read off that its eigenvalues are 1 and 3 Hence, the stationary point 0, 0 is a nodal source and is repelling At 1, 0 the coefficient matrix of the linearized system is A J1, Because A is diagonal, we can read off that its eigenvalues are 1 and 4 Hence, the stationary point 1, 0 is a saddle and is unstable At 3, 3 the coefficient matrix of the linearized system is A J3, Its characteristic polynomial is pz z 2 + 2z + 24 z Its eigenvalues are thereby the conjugate pair 1 ± i 23 while a < 0 Hence, the stationary point 3, 3 is a clockwise spiral sink and is attracting Example Determine the stability and classify each stationary point of the system dt y, dt x x3 Solution Because x fx, y+ y gx, y 0, the system is Hamiltonian, and is thereby conservative We have seen that its stationary points are The Jacobian matrix for the system is 1, 0, 0, 0, 1, 0 Jx, y x 2 0

11 At 0, 0 the coefficient matrix of the linearized system is 0 1 A J0, Its characteristic polynomial is pz z 2 1 z + 1z 1 Its eigenvalues are thereby 1 and 1 Hence, the stationary point 0, 0 is a saddle and is unstable At 1, 0 and 1, 0 the coefficient matrix of the linearized system is A J±1, Its characteristic polynomial is pz z 2 +2 Its eigenvalues are thereby the conjugate pair ±i 2 while a 21 2 < 0 Because the system is conservative, we conclude that the stationary points 1, 0 and 1, 0 are clockwise centers and are stable 11

12 12 8 Application: Population Dynamics Earlier in the course we studied models of population namics that were single first-order equations in the form dp dt Rpp, where pt is the size of the population as a function of time, and Rp models the growth rate of the population as a function of p For a population in a closed ecosystem the growth rate is simply the birth rate minus the mortality rate In more complicated situations it must also account for mirgation in to and out of the ecosystem The simplest such model takes Rp r for some constant r, which has the solution pt p I e rt when p0 p I This is the so-called exponential model because when r > 0 its solution grows exponentially, while when r < 0 its solution decays exponentially The ability of a individual to survive and reproduce depends upon the environment experienced by that individual The assumption that Rp is constant assumes that this environment is uneffected by the number of individuals present However once the population grows large enough there might be competition between its individuals for resources, which should reduce the growth rate The simplest model that captures this effect takes Rp to be a linear function of p, Rp r ap, for some constants a and r While this logistic model could be solved analytically, we found it helpful to stu it with phase-line portraits In this section we extend these ideas to stu models of two interacting populations with phase-plane portraits 81 Models of Two Interacting Populations We now consider two populations that interact in a shared environment We let the size of these populations be p 1 t and p 2 t and consider first-order autonomous models of the form dp 1 81 dt R dp 2 1p 1, p 2 p 1, dt R 2p 1, p 2 p 2, where R 1 p 1, p 2 and R 2 p 1, p 2 model the growth rates of the first and second populations as functions of p 1 and p 2 We will specialize further to the case where R 1 p 1, p 2 and R 2 p 1, p 2 depend linearly upon p 1 and p 2 as 82 R 1 p 1, p 2 r 1 a 11 p 1 a 12 p 2, for some constants a 11, a 12, and r 1, R 2 p 1, p 2 r 2 a 21 p 1 a 22 p 2, for some constants a 21, a 22, and r 2 This model assumes that the growth rates for each population can depend upon the size of the two populations, but nothing else It assumes moreover that this dependence is linear Both of these assumptions are huge oversimplifications for any ecosystem Still, there are instances when such models correctly capture phenomena that are observed in real populations Remark When such models are used in practice, p 1 and p 2 often are not the number of individuals in the respective populations, but rather are the biomass of the populations This makes more sense than simply counting individuals because the amount of resources an individual will consume is roughly proportional to its biomass

13 The six constants a 11, a 12, a 21, a 22, r 1, and r 2 are parameters of the model In practice their values are chosen so that the model fits some observed behavior The process of choosing what data to fit and how to fit it is often called calibrating the model We will not address how to calibrate such models in this course Rather, we will be given values of these parameters and asked what behaviors are predicted by a given model The process of testing the model by comparing such predictions with certain observed behavior is often called validating the model We will not address how to validate such models in this course Rather, we will address how to make the kind of predictions needed for the validation process It is important to understand the role each of the six parameters plays in the model We start by examining them one at a time The parameters r 1 and r 2 can be viewed as the bare growth rates for the respective populations Namely, they are the rates at which the populations grow when both populations are small When r k > 0 the k th population will thrive on the resources available in the environment When r k < 0 the k th population will decline without some additional resource, usually the other population which it preys upon The parameters a 11 and a 22 represent how the growth rates respond to increased numbers in their respective populations When a kk > 0 the growth rate for the k th population will reduce as that population grows This might be the case when individuals in the population compete for the same food resource When a kk < 0 the growth rate for the k th population will increase as that population grows This might be the case when a greater population density increases the likelihood of individuals finding mates However, because model exhibits unrealistic behavior in such regimes, we will generally restrict our attention to cases where a kk 0 Model is interactive when the coupling parameters a 12 and a 21 are both nonzero The nature of the interaction can be read off from the signs of these parameters When a 12 and a 21 have opposite signs then one of the populations benefits from and harms the other This would be the case, for example, when one population preys on the other Such models are often called predator-prey models When a 12 > 0 and a 21 > 0 then each population is effected adversely by the other This would be the case, for example, when both populations compete for the same food resource Such models are often called competing species models When a 12 < 0 and a 21 < 0 then each population benefits from the other This would be the case, for example, when the populations cooperate Such models are sometimes called cooperative models In this section we will focus on predator-prey and competing species models 82 Predator-Prey Models In this subsection we will stu predator-prey models in the form 83 x r ax byx, y s + cx y, where the parameters b, c, r, and s are positive while a and d are nonnegative It should be clear to you that x and y represent the populations of prey and predators respectively 13

14 14 Example Sketch a phase portrait for the predator-prey model Solution Stationary points satisfy x 6 3yx, y xy 0 6 3yx, xy The first equation is satisfied when either x 0 or y 2 When x 0 the second equation can only be satisfied when y 0 When y 2 the second equation can only be satisfied when x 3 The stationary points are therefore 0, 0, 3, 2 You should plot these points in the xy-plane Like all population models in this section, this model has special solutions that lie on the x and y axes Notice that if y 0 initially then it will remain zero The x-equation then reduces to x 6x, which has solution xt x I e 6t Therefore if its initial data lies on the x-axis then the solution of the model will remain on the x-axis and has the explicit form xt, yt xi e 6t, 0 You should indicate these solutions on the x-axis with arrows pointing away from the origin This model predicts that without predators to keep its population in check, the population of prey will grow exponentially without bound Similarly, notice that if x 0 initially then it will remain zero The y-equation then reduces to y 15y, which has solution yt y I e 15t Therefore if its initial data lies on the y-axis then the solution of the model will remain on the y-axis and has the explicit form xt, yt 0, yi e 15t You should indicate these solutions on the y-axis with arrows pointing towards the origin This model predicts that without prey to sustain it, the population of predators will die off quickly The orbit equation is xy 6 3yx, which is separable Its separated form is which integrates to 5x 15 x + 3y 6 y 0, 5x 15 log x + 3y 6 log y c This cannot be solved either for y as an explicit function of x or for x as an explicit function of y Rather, the orbits are given implicitly by Hx, y c where the integral Hx, y is Hx, y 5x 15 log x + 3y 6 log y This function is undefined on the x-axis and y-axis

15 Because this is a population model, we only care about the first quadrant of the xy-plane The first partial derivatives of Hx, y are x Hx, y 5 15 x 5x 3 x, y Hx, y 3 6 y 3y 2 y The stationary point 3, 2 is therefore also a critical point of the function Hx, y The Hessian matrix of Hx, y is 15 xx Hx, y Hx, y xy Hx, y 0 yx Hx, y yy Hx, y x y 2 Because det Hx, y 15 x 2 6 y 2, tr Hx, y 15 x y 2, we see that these are both positive off the x-axis and y-axis The function Hx, y is thereby strictly convex concave up in each quadrant Its critical point 3, 2 is therefore a global minimum of Hx, y in the first quadrant Because the orbits of our model are the level sets of Hx, y, we see that the stationary point 3, 2 is a counterclockwise center and that counterclockwise periodic orbits fill out the first quadrant Example Sketch a phase portrait for the predator-prey model Solution Stationary points satisfy x 12 2x 3yx, y xy x 3yx, xy The second equation is satisfied when either x 3 or y 0 When x 3 the first equation can only be satisfied when y 2 When y 0 the first equation can be satisfied when either x 0 or x 6 The stationary points are therefore 0, 0, 6, 0, 3, 2 The matrix of partial derivatives is x fx, y y fx, y 12 4x 3y 3x x gx, y y gx, y 5y x By evaluating this at each of the stationary points you find that the coefficient matrices for the linearizations about them are 12 0 A, A, A

16 16 At 0, 0 the eigenvalues are 12 and 15, so the type is a saddle Because A 12I, A + 15I, the eigenpairs are 1 12,, , 1 At 6, 0 the eigenvalues of A are 12 and 15, so the type is a saddle Because A + 12I, A 15I, the eigenpairs are 1 12,, 0 At 3, 2 the characteristic polynomial of A is 15, 2 3 pz z 2 traz + deta z 2 + 6z + 90 z The eigenvalues of A are therefore 3 ± i9 Because a > 0, the type is a counterclockwise spiral sink Because its phase portrait is a spiral sink near 3, 2, this predator-prey model cannot be conservative

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