Linear Algebra: Orthogonal Transformations and Matrices

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Questions and Answers

What is a defining characteristic of an orthogonal matrix?

  • It has zero elements only above the diagonal.
  • It has all non-zero elements.
  • Its transpose is equal to its inverse. (correct)
  • It can only be a rectangular matrix.

Which statement about triangular matrices is true?

  • They can only have zeros above the diagonal.
  • They must be symmetric matrices.
  • They are always orthogonal.
  • They are square matrices with zeros either below or above the diagonal. (correct)

What does diagonalization of a matrix involve?

  • Representing the matrix as a sum of identity matrices.
  • Transforming the matrix to a rectangular form.
  • Finding all eigenvalues and eigenvectors only.
  • Writing the matrix in the form $PDP^{-1}$ where $D$ is a diagonal matrix. (correct)

Which property is preserved under orthogonal transformations?

<p>The eigenvalues of the matrix. (C)</p>
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How is matrix multiplication characterized in terms of commutativity?

<p>It is associative but not commutative. (B)</p>
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What is the relationship between an eigenvector and eigenvalue of a matrix?

<p>An eigenvector changes by a factor of its eigenvalue under the matrix operation. (B)</p>
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Which of the following correctly describes an orthogonal transformation?

<p>It preserves the dot product and the length of vectors. (C)</p>
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For a vector $x$ and orthogonal matrix $Q$, what can be inferred about $||Qx||$?

<p>$||Qx||$ is equal to $||x||$. (B)</p>
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Study Notes

Triangular Matrices and Transpose

  • Triangular matrices are square matrices with zero elements either below (lower triangular) or above (upper triangular) the main diagonal.
  • A matrix is orthogonal if its transpose equals its inverse, i.e., ( Q^T = Q^{-1} ).

Diagonalization of Matrices

  • A matrix is diagonalizable if it can be expressed in the form ( PDP^{-1} ), where ( D ) is a diagonal matrix.
  • Orthogonal matrices are always diagonalizable.

Orthogonal Transformation

  • An orthogonal transformation is a linear transformation that maintains both the dot product and the lengths of vectors.
  • This transformation is represented by an orthogonal matrix, satisfying the condition ( Q^TQ = I ).

Linear Systems and Eigenvalues

  • Eigenvectors are nonzero vectors that transform under a matrix's action by only scaling, characterized by the eigenvalue.
  • Orthogonal transformations preserve the eigenvalues of a matrix, meaning the eigenvalues remain unchanged.

Matrix Addition, Subtraction, and Multiplication Rules

  • Matrix addition and subtraction are performed element-wise, meaning corresponding elements are added or subtracted.
  • Matrix multiplication is achieved by taking the dot product of rows from the first matrix with columns from the second.
  • While matrix multiplication is associative, it isn't commutative, i.e., ( AB \neq BA ) in general.
  • Orthogonal matrices preserve the lengths of vectors during multiplication, indicating ( ||Qx|| = ||x|| ) for any vector ( x ).

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