Chapter 3: Problem 6
(a) The matrix equation \(A^{2}=0\) does not imply \(A=0 .\) Show that the most general \(2 \times 2\) matrix whose square is zero may be written as $$ \left(\begin{array}{rr} a b & b^{2} \\ -a^{2} & -a b \end{array}\right) $$ where \(a\) and \(b\) are real or complex numbers. (b) If \(C=A+B\), in general $$ \operatorname{det} C \neq \operatorname{det} A+\operatorname{det} B \text { . } $$ Construct a specific numerical example to illustrate this inequality.
Short Answer
Step by step solution
Understand Matrix Squaring
Calculate A Squared
Set Entries of A Squared to Zero
Simplify Using Specific Conditions
Example for Determinant Inequality
Compute Determinant of C
Unlock Step-by-Step Solutions & Ace Your Exams!
-
Full Textbook Solutions
Get detailed explanations and key concepts
-
Unlimited Al creation
Al flashcards, explanations, exams and more...
-
Ads-free access
To over 500 millions flashcards
-
Money-back guarantee
We refund you if you fail your exam.
Over 30 million students worldwide already upgrade their learning with 91Ó°ÊÓ!
Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Matrix Squaring
In our exercise, we deal with finding a 2x2 matrix \( A \) such that \( A^2 = 0 \), known as a zero matrix.
When we calculate \( A^2 \) for a general matrix \( A = \begin{pmatrix} a & b \ c & d \end{pmatrix} \), we get another 2x2 matrix where each entry is a polynomial of \( a, b, c, \) and \( d \).
We find the matrix \( A \) whose square is zero by setting the resulting entries of \( A^2 \) equal to zero. The conditions derived suggest constraints such as \( a^2 + bc = 0 \) and \( ab + bd = 0 \).
Understanding these constraints helps us find non-trivial solutions to form such matrices. Setting \( b eq 0 \) leads us to a form where \( d = -a \) and \( c = -a^2/b \), resulting in the required zero square matrix form.
Zero Matrix
This property is crucial when considering \( A^2 = 0 \), leading to the conclusion that the square of some matrices isn't trivially zero without the matrix itself being a zero matrix.
Finding a matrix such that \( A^2 = 0 \) is akin to looking for matrices that multiply themselves to become a zero matrix.
It's essential to differentiate between the zero matrix and matrices that have a non-zero structure yet square to become a zero matrix. Only certain specific combinations of elements in a non-zero matrix, satisfying particular conditions, can result in such behavior.
For the given problem, the resulting matrix that squares to zero retains a form dictated by parameters \( a \) and \( b \), showcasing the fascinating complexity behind apparently simple matrices.
Determinant
In a 2x2 matrix \( \begin{pmatrix} a & b \ c & d \end{pmatrix} \), the determinant is computed as \( ad - bc \).
A matrix with a determinant of zero is singular, meaning it does not have an inverse.
In our exercise, the determinant plays a role in analyzing the sum of matrices. Contrary to what might be expected intuitively, the determinant of a sum of two matrices \( C = A + B \) is not simply the sum of their determinants: \( \det(C) eq \det(A) + \det(B) \).
This key concept helps to understand the non-linear nature of matrix operations which can be crucial when dealing with complex matrix equations.
Matrix Addition
For two matrices \( A \) and \( B \) to be added, they must be of the same dimension.
In our example, adding two 2x2 matrices provides a new matrix \( C \) by element-wise summation.
- For example, \( A = \begin{pmatrix} 1 & 2 \ 3 & 4 \end{pmatrix} \) and \( B = \begin{pmatrix} 2 & 0 \ 1 & 3 \end{pmatrix} \), the sum \( C = A + B = \begin{pmatrix} 3 & 2 \ 4 & 7 \end{pmatrix} \).
Understanding the differences between simple arithmetic and matrix operations like addition and determinant ensures accurate manipulation and prediction of matrix behaviors.