Math Problem Statement

Let A and B be real invertible matrices such that AB = −BA. Then

Question 1Answer

a. Trace(A) =Trace(B) = 1

b. Trace(A) = 1, Trace(B) = 0

c. Trace(A) =Trace(B) = 0

d. Trace(A) = 0, Trace(B) = 1

Solution

Given that AA and BB are real invertible matrices such that AB=BAAB = -BA, we want to find the correct relationship between their traces.

Analyzing the Property AB=BAAB = -BA

  1. Trace of a Matrix: The trace of a matrix XX, denoted as Tr(X)\text{Tr}(X), is the sum of its diagonal elements. A useful property of the trace function is that for any two matrices XX and YY, Tr(XY)=Tr(YX).\text{Tr}(XY) = \text{Tr}(YX).

  2. Applying the Trace to the Given Equation: Given AB=BAAB = -BA, we can take the trace of both sides:

    Tr(AB)=Tr(BA).\text{Tr}(AB) = \text{Tr}(-BA).

    Using the property of the trace, we have:

    Tr(AB)=Tr(BA)=Tr(BA).\text{Tr}(AB) = \text{Tr}(-BA) = -\text{Tr}(BA).

    Since Tr(AB)=Tr(BA)\text{Tr}(AB) = \text{Tr}(BA), this gives:

    Tr(AB)=Tr(AB).\text{Tr}(AB) = -\text{Tr}(AB).

    This implies:

    Tr(AB)=0.\text{Tr}(AB) = 0.

  3. Conclusion on Traces of AA and BB: Since AB=BAAB = -BA and both AA and BB are invertible, AA and BB must be matrices with specific properties. In particular, if they are real matrices and satisfy the anti-commutation relation AB=BAAB = -BA, their traces must satisfy:

    Tr(A)=0andTr(B)=0.\text{Tr}(A) = 0 \quad \text{and} \quad \text{Tr}(B) = 0.

    This is because the trace of a product of such matrices will always yield zero. Thus, both matrices have to be traceless.

Correct Answer:

The correct answer is:

c. Tr(A)=Tr(B)=0\text{Tr}(A) = \text{Tr}(B) = 0.

Would you like more details or have any questions? Here are 5 related questions that could further enhance understanding:

  1. Why is the trace of a product of matrices invariant under cyclic permutations?
  2. Can you provide an example of two 2×22 \times 2 matrices AA and BB such that AB=BAAB = -BA and both have a trace of zero?
  3. What are the implications of a matrix being traceless in terms of its eigenvalues?
  4. How does the property AB=BAAB = -BA influence the determinant of the matrices AA and BB?
  5. Are there any geometric interpretations for matrices that satisfy AB=BAAB = -BA?

Tip: When dealing with matrix equations, always consider fundamental properties like trace, determinant, and rank for insight.

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Math Problem Analysis

Mathematical Concepts

Linear Algebra
Matrix Theory
Trace of a Matrix
Matrix Invertibility

Formulas

Tr(XY) = Tr(YX)
AB = -BA implies Tr(AB) = 0

Theorems

Cyclic Property of Trace
Properties of Anti-Commutative Matrices

Suitable Grade Level

College Level (Linear Algebra)