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Singular Value Decomposition

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Applications of SVD

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SVD is used in signal processing, data reduction, solving inverse problems, and principal component analysis among other applications.

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SVD for Image Compression

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In image compression, SVD can reduce the amount of information needed to reconstruct an image by keeping only a few of the largest singular values and corresponding singular vectors, significantly reducing the data size without substantial quality loss.

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Diagonal Matrix Σ\Sigma in SVD

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The diagonal matrix Σ\Sigma in SVD contains the singular values, which are the lengths of the semi-axes of the ellipsoid represented by the matrix AA, ordered from largest to smallest.

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Full SVD vs Reduced SVD

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Full SVD includes all the singular values (including zeros) and corresponding vectors, resulting in UU and VV matrices of size equal to the original matrix. In contrast, reduced SVD omits the zero singular values and the corresponding vectors.

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SVD and Pseudoinverse

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The pseudoinverse of a matrix AA, denoted A+A^+, can be computed using SVD by taking A+=VΣ+UA^+ = V\Sigma^+U^* where Σ+\Sigma^+ is formed by taking the reciprocal of the non-zero singular values of AA and transposing the matrix.

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Compact SVD

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Compact SVD is a variant of SVD where matrices UU and VV are truncated to include only the columns corresponding to the non-zero singular values, often used when the matrix has more zero singular values.

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Unitary Matrices in SVD

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In SVD, UU and VV are unitary matrices, meaning they satisfy UU=IUU^* = I and VV=IVV^* = I, where II is the identity matrix and * denotes the conjugate transpose operation.

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SVD Equation

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The equation for SVD of a matrix AA is A=UΣVA = U\Sigma V^*, where UU and VV are orthogonal (or unitary, if complex) matrices, Σ\Sigma is a diagonal matrix, and VV^* is the conjugate transpose of VV.

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Definition of SVD

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Singular Value Decomposition is a factorization of a matrix into three matrices UU, Σ\Sigma, and VV^*, where UU and VV are unitary matrices and Σ\Sigma is a diagonal matrix containing the singular values.

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Relation between SVD and Eigen-decomposition

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SVD is related to eigen-decomposition in that the singular values of AA are the square roots of the eigenvalues of AAA^*A or AAAA^*, and the columns of UU and VV are the eigenvectors of AAAA^* and AAA^*A respectively.

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SVD of a Rectangular Matrix

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SVD can be applied to any m×nm \times n rectangular matrix, not just square matrices. The resulting UU, Σ\Sigma, and VV^* matrices will have dimensions m×mm \times m, m×nm \times n, and n×nn \times n respectively in the full SVD case.

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Stability of SVD

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SVD is numerically stable, meaning small changes in the input matrix will cause only small changes in the singular values and singular vectors. This makes SVD a reliable method for numerical computations.

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Singular Values

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Singular values are the non-negative values on the diagonal of matrix Σ\Sigma in the SVD. They are the square roots of the nonzero eigenvalues of AAA^*A where AA is the original matrix.

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SVD for Matrix Rank Determination

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The rank of a matrix AA can be determined using SVD by counting the number of non-zero singular values in Σ\Sigma. This corresponds to the number of linearly independent rows or columns in AA.

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Eckart-Young Theorem

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The Eckart-Young theorem states that the best low-rank approximation of a matrix can be obtained by using the SVD and setting all but the largest kk singular values in Σ\Sigma to zero, forming Σk\Sigma_k.

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