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Chapter 5: Q25E (page 267)

Question: Let \(\lambda \) be an eigenvalue of an invertible matrix A. Show that \({\lambda ^{ - {\bf{1}}}}\) is an eigenvalue of \({A^{ - {\bf{1}}}}\). (Hint: Suppose a nonzero x satisfies \(A{\bf{x}} = \lambda {\bf{x}}\))

Short Answer

Expert verified

\({\lambda ^{ - 1}}\) is an eigenvalue of \({A^{ - 1}}\).

Step by step solution

01

Write the given information

For an invertible matrix A, \(\lambda \) is an eigenvalue.

02

Check for the eigenvalue of \({A^{ - {\bf{1}}}}\)

If \(\lambda \) is an eigenvalue of A, then there exists a nonzero vector \(x\) such as that \(A{\bf{x}} = \lambda {\bf{x}}\).

Since A is invertible, then \({A^{ - 1}}A{\bf{x}} = {A^{ - 1}}\left( {\lambda {\bf{x}}} \right)\).

As \(x \ne 0\), so \(\lambda \) cannot be equal to zero. Therefore, \({\lambda ^{ - 1}}{\bf{x}} = {A^{ - 1}}{\bf{x}}\).

The above equation shows that, \({\lambda ^{ - 1}}\) is an eigenvalue of \({A^{ - 1}}\).

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Most popular questions from this chapter

Question: Let \(A = \left( {\begin{array}{*{20}{c}}{.6}&{.3}\\{.4}&{.7}\end{array}} \right)\), \({v_1} = \left( {\begin{array}{*{20}{c}}{3/7}\\{4/7}\end{array}} \right)\), \({x_0} = \left( {\begin{array}{*{20}{c}}{.5}\\{.5}\end{array}} \right)\). (Note: \(A\) is the stochastic matrix studied in Example 5 of Section 4.9.)

  1. Find a basic for \({\mathbb{R}^2}\) consisting of \({{\rm{v}}_1}\) and anther eigenvector \({{\rm{v}}_2}\) of \(A\).
  2. Verify that \({{\rm{x}}_0}\) may be written in the form \({{\rm{x}}_0} = {{\rm{v}}_1} + c{{\rm{v}}_2}\).
  3. For \(k = 1,2, \ldots \), define \({x_k} = {A^k}{x_0}\). Compute \({x_1}\) and \({x_2}\), and write a formula for \({x_k}\). Then show that \({{\bf{x}}_k} \to {{\bf{v}}_1}\) as \(k\) increases.

Question: In Exercises 21 and 22, \(A\) and \(B\) are \(n \times n\) matrices. Mark each statement True or False. Justify each answer.

  1. If \(A\) is \(3 \times 3\), with columns \({{\rm{a}}_1}\), \({{\rm{a}}_2}\), and \({{\rm{a}}_3}\), then \(\det A\) equals the volume of the parallelepiped determined by \({{\rm{a}}_1}\), \({{\rm{a}}_2}\), and \({{\rm{a}}_3}\).
  2. \(\det {A^T} = \left( { - 1} \right)\det A\).
  3. The multiplicity of a root \(r\) of the characteristic equation of \(A\) is called the algebraic multiplicity of \(r\) as an eigenvalue of \(A\).
  4. A row replacement operation on \(A\) does not change the eigenvalues.

Question: Find the characteristic polynomial and the eigenvalues of the matrices in Exercises 1-8.

  1. \(\left[ {\begin{array}{*{20}{c}}2&7\\7&2\end{array}} \right]\)

Question: In Exercises 21 and 22, \(A\) and \(B\) are \(n \times n\) matrices. Mark each statement True or False. Justify each answer.

  1. The determinant of \(A\) is the product of the diagonal entries in \(A\).
  2. An elementary row operation on \(A\) does not change the determinant.
  3. \(\left( {\det A} \right)\left( {\det B} \right) = \det AB\)
  4. If \(\lambda + 5\) is a factor of the characteristic polynomial of \(A\), then 5 is an eigenvalue of \(A\).

Use Exercise 12 to find the eigenvalues of the matrices in Exercises 13 and 14.

13. \(A = \left( {\begin{array}{*{20}{c}}3&{ - 2}&8\\0&5&{ - 2}\\0&{ - 4}&3\end{array}} \right)\)
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