# 2.5 Complex Eigenvalues

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1 1 25 Complex Eigenvalues Real Canonical Form A semisimple matrix with complex conjugate eigenvalues can be diagonalized using the procedure previously described However, the eigenvectors corresponding to the conjugate eigenvalues are themselves complex conjugate and the calculations involve working in complex n-dimensional space There is nothing wrong with this in principle, however the manipulations may be a bit messy Example: Diagonalize the matrix Eigenvalues are roots of the characteristic polynomial The eigenvalues are and Eigenvectors are solutions of Obtain and Then from we need to compute The transformation matrix Computing requires care since we have to do matrix multiplication and complex arithmetic at the same time If we now want to solve an initial value problem for a linear system involving the matrix, we have to compute and This matrix product is pretty messy to compute by hand Even using a symbolic algebra system, we may have to do some work to convert our answer for into real form Carry out the matrix product in Mathematica instead using ComplexDiagonalization1nb Discuss the commands Eigenvalues, Eigenvectors, notation for parts of expressions, Transpose, MatrixForm, Inverse and the notation for matrix multiplication Obtain and Alternatively, there is the Real Canonical Form that allows us to stay in the real number system Suppose has eigenvalue, eigenvector and their complex conjugates Then writing in real and imaginary parts: Taking real and imaginary parts

2 2 Consider the transformation matrix These equation can be written The exponential of the Eq 231) block on the right was computed at the end of section 23 (Meiss, Example Let Find its real canonical form and compute We have already found the eigenvalues and eigenvectors Setting we have, The transformation matrix and its inverse are, Find, Using Meiss 231 Compute Find, Diagonalizing an arbitrary semisimple matrix

3 3 Suppose has real eigenvalues and pairs of complex conjugate ones Let be the corresponding real eigenvectors and, be the real and imaginary parts of the complex conjugate eigenvectors The transformation matrix is nonsingular and where The solution of the initial value problem will involve the matrix exponential In this way we compute the matrix exponential of any matrix that is diagonalizable 26 Multiple Eigenvalues The commutator of and is If the commutator is zero then and commute Fact If and, then Proof Generalized Eigenspaces Let where Recall that eigenvalue and eigenvector satisfy This can be rewritten as Suppose has algebraic multiplicity 1 Then the associated eigenspace is

4 4 A space is invariant under the action of if implies For example, is invariant under by the fact above Suppose is an eigenvalue of with algebraic multiplicity Define the generalized eigenspace of as The symbol refers to generalized eigenspace but coincides with eigenspace if A nonzero solution to is a generalized eigenvector of Lemma 25 (Invariance) Each of the generalized eigenspaces of a linear operator under is invariant Proof Suppose so that Since and commute Let and be vector spaces The direct sum is the vector space with elements, where and and operations of vector addition and scalar multiplication defined by and, where also and and For example, Theorem 26 (Primary Decomposition) Let be a linear operator on a complex vector space with distinct eigenvalues and let be the generalized eigenspace of with eigenvalue Then is the algebraic multiplicity of and is the direct sum of the generalized eigenspaces, ie

5 5 Proof This is proved in Hirsch and Smale Remark We can choose a basis combined to obtain a basis for for each eigenspace By theorem 26, these can be Warning The labeling for generalized eigenvectors given above is Meiss notation Note that the eigenvectors are relabeled to give the basis for This keeps the notation simple but the labels must be interpreted correctly depending on context Semisimple-Nilpotent Decomposition Shift notation from as linear operator and refer to matrix instead Let or and Let be the diagonal matrix with the eigenvalues of repeated according to multiplicity Let be a basis for of generalized eigenvectors of Consider the transformation matrix and define is a semisimple matrix Multiply by on the right to obtain The i^th component of this result is, where are the distinct eigenvalues of and Think of as the diagonalizable part of Consider an arbitrary Then can be expressed as a linear combination of the basis vectors for : We then have Within, acts as a multiple of the identity operator In particular, is invariant under the action of Lemma 27 Let, where Then commutes with and is nilpotent with order at most, the maximum of the algebraic multiplicities of Remark 1 is nilpotent of order means the same thing as has nilpotency Remark 2 Since the generalized eigenspace of is invariant under the action of both and, it is also invariant under the action of Proof See the text; plan to give it in class Note that there are two parts: (1) show and (2) show nilpotent

6 6 Theorem 28 A matrix on a complex vector space has a unique decomposition, where is semisimple, is nilpotent and Proof Not in lecture See text The Exponential Let, then by lemma 27 where Further, we have, and, where is the maximum algebraic multiplicity of the eigenvalues Then, using the law of exponents for commuting matrices and the series definition of the exponential [1] This formula allows us to compute the exponential of an arbitrary matrix Combine this result with the fundamental theorem to find an analytical solution for any linear system Example Solve the initial value problem with given and By the fundamental theorem, We need to compute and The characteristic equation is The root has multiplicity 2 Then Every matrix commutes with the identity matrix, so that Then Notice that N has nilpotency 2 Then using [1],

7 7 Notice that if the straight line solution is obtained, where is the eigenvector associated with However the full phase portrait is most easily visualized using a computer phase portrait drawn by a computer Example Solve the initial value problem, where Since is upper triangular, the eigenvalues can be read off the main diagonal has multiplicity and has multiplicity The generalized eigenspace associated with is Find A choice for generalized eigenvectors spanning is and The generalized eigenspace associated with is Find Let The transformation matrix is Notice that is block diagonal Its inverse is also block diagonal, with each block the inverse of the corresponding block in Then We are now ready to find and where Obtain and It s easy to check Then is given by

8 8 The solution of the initial value problem is, where Jordan Form Let where or cannot always be diagonalized by a similarity transformation, but it can always be transformed into Jordan canonical form, which gives a simple form for the nilpotent part of Finding a basis of generalized eigenvectors that reduces to this form is generally difficult by hand, but computer algebra systems like Mathematica have built in commands that perform the computation Finding the Jordan form is not necessary for the solution of linear systems and is not described by Meiss in chapter 2 However, it is the starting point of some treatments of center manifolds and normal forms, which systematically simplify and classify systems of nonlinear ODEs This subsection follows the first part of section 18 in Perko closely The following theorem is described by Perko and proved in Hirsch and Smale: Theorem (The Jordan Canonical Form) Let be a real matrix with real eigenvalues and complex eigenvalues and Then there exists a basis for, where, are generalized eigenvectors of, the first of these are real and for The matrix is invertible and, where the elementary Jordan blocks are either of the form, [2] for one of the real eigenvalues of or of the form, [3]

9 9 with, and, for one of the complex eigenvalues of The Jordan form yields some explicit information about the form of the solution on the initial value problem [4] which, according to the Fundamental Solution Theorem, is given by If is an matrix of form [2] and is a real eigenvalue of then where is nilpotent of order and, Then Similarly, if is an matrix of form [3] and is a complex eigenvalue of, then where

10 10 is nilpotent of order and, where is the rotation matrix This form of the solution to [3] leads to the following result Corollary Each coordinate in the solution combination of functions of form of the initial value problem [4] is a linear or, where is an eigenvalue of the matrix and More precisely, we have, where is the largest order of the elementary Jordan blocks 27 Linear Stability Let The solution of, is, and each component is a sum of terms proportional to an exponential, for an eigenvalue of The real parts of these eigenvalues determine whether the terms are exponentially growing or decaying Denote the generalized eigenvectors and define is the unstable eigenspace, is the center eigenspace and is the stable eigenspace According to Lemma 25 each of the generalized eigenspaces is invariant under the action of is the direct sum of the generalized eigenspaces corresponding to eigenvalues with positive

11 11 real part and it is also invariant under the action of Similarly, and are invariant is the direct sum We can consider the action of in each subspace by considering restricted operators denotes the restriction of to, etc This corresponds to the fact that is block diagonal For example, can always be brought to Jordan canonical form A system is linearly stable if all its solutions are bounded as If then is always bounded Lemma 29 If is an matrix and, the stable space of, then there are constants and such that Consequently, Remark This result is very reasonable From [1], each component of the solution will be proportional to for some eigenvalue, and by hypothesis is chosen so that for each such eigenvalue The maximum power of that appears in any component of is, where is the maximum multiplicity of any eigenvalue in For sufficiently large, the exponentially decaying terms must dominate the powers of t For details of the proof see Meiss A linear system is asymptotically linearly stable if all of its solutions approach 0 as Theorem 210 (Asymptotic Linear Stability) for all if and only if all eigenvalues of have negative real parts Proof If all eigenvalues have negative real part, lemma 29 implies If an eigenvalue has positive real part, then there is a straight line solution, where is an eigenvector of, that grows without bound If there is an eigenvalue with zero real part, the solutions in this subspace have terms of the form that do not go to zero

12 A system with no center subspace is hyperbolic Lemma 29 and Theorem 210 describe properties of these systems that depend only on the signs of their eigenvalues 12 In contrast, the stability of systems with a center subspace can be affected by the nilpotent part of The proof of theorem 210 suggests why these systems cannot be asymptotically stable Solutions of the 2D center described in section 22 are bounded However, center systems with nonzero nilpotent parts have solutions that are unbounded (Perko, section 19, problem 5(d)) Routh-Hurwitz Stability Criteria These criteria determine whether the roots of a polynomial have all negative real parts When applied to the characteristic polynomial associated with a linear system of equations, they test for asymptotic stability of the equilibrium point In the 2D case, the characteristic polynomial is It is easy to see that all of the eigenvalues have negative real parts if and All of the coefficients of the characteristic polynomial must be positive In the 3D case, the characteristic polynomial is All of the eigenvalues have negative real parts if and only if and See Meiss, problem 211 The positivity of the coefficients of the characteristic polynomial is necessary but not sufficient Analogous stability criteria are available for higher order polynomials In some cases, it may be much easier to study the stability of a linear system using these criteria than by finding the eigenvalues 28 Nonautonomous Linear Systems and Floquet Theory Let The initial value problem for an autonomous linear system,, [1] can arise from linearization about an equilibrium point As we shall see in chapter 5, when hyperbolic this system gives a good approximation to the behavior of nearby trajectories sketch The solution of [1] is given by the Fundamental Solution Theorem is The initial value problem for the non-autonomous linear system

13 13,, [2] can arise from linearization about a periodic orbit In this case, where is the periodic orbit and is another trajectory close to the orbit sketch Higher order terms are dropped to arrive at [2], but the solution of [2] may give a good approximation to the behavior of the nearby trajectory See chapter 4 Floquet theory discusses the solution of [2] when is periodic Let the period be The fundamental matrix solution corresponding to [2] is the solution of the initial value problem [3] is the solution at time of the initial value problem that begins at time Note that, if solves [3] then solves [2] A trajectory that starts at an initial time and ends at time may be decomposed into two parts The first from time to time and the second from time to time Mathematically, satisfies Then and Suppose that has period and consider the solution of [3] with The monodromy matrix is the solution of the initial value problem after one period Then the solution of [2] with after one period is Consider the trajectory during the second period It is the solution of the initial value problem Define a new time variable, and use to see that this is the same as [2] with and replaced by Therefore the solution of [2], with, after two periods is After periods The eigenvalues of are the Floquet multipliers Suppose the initial condition of [2], with =0, is also an eigenvector of and is the corresponding eigenvalue Then,

14 14 where is the corresponding Floquet exponent Meiss discusses the fact that the monodromy matrix is nonsingular; see Theorem 211 Therefore the Floquet multipliers are all nonzero and the Floquet exponents are well defined However, it is perfectly possible for a Floquet multiplier to be negative, in this case the corresponding exponent would be pure imaginary Floquet theory will be concerned with the logarithm of the monodromy matrix We next define the matrix logarithm Begin with a preliminary remark The matrix exponential was defined by a series expansion patterned after the Maclaurin series for where A similar procedure will be used to define the logarithm of a nilpotent matrix Recall, converging for If we now integrate both sides and evaluate the constant of integration, we find, also converging for Now replace, where is a nilpotent matrix, to obtain [4] There are no convergence issues because Lemma 212 Any nonsingular matrix is nilpotent! has a (possibly complex) logarithm Here, is the semisimple-nilpotent decomposition, with eigenvalues of repeated according to multiplicity, is the maximum algebraic multiplicity of any eigenvalue and is the matrix of generalized eigenvectors of Proof As usual let and define as given above Since is nonsingular, none of the eigenvalues are zero Then, thus general, This is the formula for the logarithm of a semisimple matrix In Since,, so is nilpotent Let in the remark above Then the logarithm of is given by [4] By analogy with, we claim is the logarithm of If and commute, then

15 15, and is the logarithm of To see that and commute, note that in each generalized eigenspace the action of is multiplication by and therefore is proportional to the identity matrix is also invariant under the action of, which given by [4] with Every matrix commutes with the identity matrix, and therefore and commute Remark The logarithm of a complex number is many-valued Consider If then with an integer To obtain the logarithm function, a consistent choice must be made for the imaginary part In the same way, when has an eigenvalue that is not positive, a consistent choice must be made for the imaginary part of when is formed Example Find the logarithm of The roots of the characteristic equation give Then and, recalling Euler s formula, Write and solve for the components of to find and The transformation matrices are and Finally Example Find the logarithm of Since is upper triangular, the eigenvalues are on the main diagonal has 1 eigenvalue,, with multiplicity The associated generalized eigenspace is all of The simplest choice for a basis is and Then the transformation matrices are We thus have has a nilpotent part so we have another term in the logarithm to compute Note

16 Let Then According to [4] 16 Recall that we are trying to characterize the solutions of the initial value problem for a - periodic linear system [2] The fundamental matrix solution is the solution of the matrix initial value problem [3] Any solution of [2] can be written The monodromy matrix is the solution of [3], with, after one period: Theorem 213 (Floquet) Let be the monodromy matrix for a -periodic linear system and its logarithm Then there exists a -periodic matrix such that the fundamental solution is Proof Give as in the text Remark Note that This implies for an integer Then However, when is not an integer multiple of a period the matrices and may be complex (consider when is the square root of ) Alternatively there is a real form of Floquet s theorem It is based upon the fact that the square of any real matrix has a real logarithm (Exercise #21) Theorem 214 Let be the fundamental matrix solution for the time T-periodic linear system [2] Then there exists a real -periodic matrix and a real matrix such that Proof From exercise 21, for any nonsingular matrix there is a real matrix such that Define, and then Therefore, is -periodic

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