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Singular value

As explained in Math and Architectures of Deep Learning

A nonnegative quantity from singular value decomposition that measures how much a matrix stretches along an orthogonal direction.

A singular value measures how strongly a matrix stretches along an orthogonal direction. In the decomposition A = UΣVᵀ, the diagonal entries of Σ are the nonnegative square roots of the eigenvalues of AᵀA, ordered from largest to smallest. section 4.5 section 4.5.1

The associated right singular vectors are orthogonal directions in the input space. Because AᵀA is symmetric positive semidefinite, its eigenvalues—and therefore the singular values derived from them—are nonnegative. section 4.5.1

Reading singular values in latent semantic analysis

A dataset with correlated terms x₀ and x₁ produces singular values 4867.56982, 118.05858, and 19.68604. The much smaller third value indicates a weak variation that can be ignored, leaving two meaningful topic directions. section 4.6.4

Equation 4.11

A = UΣVT

Use the decomposition equation to connect each singular value in Σ with the matrix transformation represented by UΣVᵀ. section 4.5

How Math and Architectures of Deep Learning covers it

How deep it goes
Defines singular values through SVD and AᵀA, proves nonnegativity, and applies them computationally without developing a full geometric proof.
Examples
Uses a correlated three-term document dataset, computed singular values, and a small-value dimension removed from the LSA representation.
What it assumes
Assumes matrix multiplication, eigenvalues, eigenvectors, and orthogonality while establishing positive semidefiniteness from first principles.
What you can do after
Apply singular values with PyTorch SVD to identify dominant directions and discard weak variation in a dataset.
How it connects
Ties singular values to singular value decomposition, eigenvalues, eigenvectors, PCA, latent semantic analysis, and spectral norms.

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