# Unsupervised Learning - PCA The neural approach->PCA; SVD; kernel PCA Hertz chapter 8 Presentation based on Touretzky + various additions.

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Unsupervised Learning - PCA The neural approach->PCA; SVD; kernel PCA Hertz chapter 8 Presentation based on Touretzky + various additions

Use a set of pictures of faces to construct a PCA space. Shown are first 25 principal components. (C. DeCoro)

Variance as function of number of PC

Each iteration, from left to right, corresponds to addition of 8 principal components

Matching general images to faces using eigenfaces space

Singular Value Decomposition Singular Value Decomposition This can be rewritten in the matrix representation where ∑is a (non-square) diagonal matrix, and U,V are orthogonal matrices. Ordering the non-zero elements of ∑ in descending order, we can get an approximation of lower rank r of the matrix X by choosing =0 for j>r leading to SVD involves expanding an mxn matrix X of rank k=min(m,n) into a sum of k unitary matrices of rank 1, in the following way:

Singular Value Decomposition continued This is the best approximation of rank r to X, i.e. it leads to the minimal sum of square deviations

Relation between SVD and PCA SVD uses the same unitary transformations as PCA performed on the rows or columns of X (using the columns or rows as feature spaces). The singular values of SVD are the square toots of the eigenvalues of PCA.

SVD – Singular Value Decomposition

Applications of SVD  Dimensionality reduction, compression  Noise reduction  Pattern search, clustering Example: microarray of expression data, “ DNA chips ”

Affymetrix GeneChip ® probe array Image courtesy of Affymetrix

Hybridization of tagged probes Image courtesy of Affymetrix

Microarray Experiment Result

SVD example: gene expressions of nine rats

SVD – Singular Value Decomposition

Alter et al 2000: identify eigengenes that are responsible for yeast cell cycle

Processing: SVD

Processing: SVD – 1 dimension

Processing: SVD – 10 dimensions

Processing: SVD – 100 dimension

Kernel PCA

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