# Lecture 2 - 2 Singular Value Decomposition The singular...

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2 Singular Value Decomposition The singular value decomposition (SVD) allows us to transform a matrix A C m × n to diagonal form using unitary matrices, i.e., A = ˆ U ˆ Σ V * . (4) Here ˆ U C m × n has orthonormal columns, ˆ Σ C n × n is diagonal, and V C n × n is unitary. This is the practical version of the SVD also known as the reduced SVD . We will discuss the full SVD later. It is of the form A = U Σ V * with unitary matrices U and V and Σ C m × n . Before we worry about how to find the matrix factors of A we give a geometric interpretation. First note that since V is unitary (i.e., V * = V - 1 ) we have the equiva- lence A = ˆ U ˆ Σ V * ⇐⇒ AV = ˆ U ˆ Σ . Considering each column of V separately the latter is the same as A v j = σ j u j , j = 1 , . . . , n. (5) Thus, the unit vectors of an orthogonal coordinate system { v 1 , . . . , v n } are mapped under A onto a new “scaled” orthogonal coordinate system { σ 1 u 1 , . . . , σ n u n } . In other words, the unit sphere with respect to the matrix 2-norm (which is a perfectly round sphere in the v -system) is transformed to an ellipsoid with semi-axes σ j u j (see Fig- ure 2). We will see below that, depending on the rank of A , some of the σ j may be zero. Therefore, yet another geometrical interpretation of the SVD is: Any m × n matrix A maps the 2-norm unit sphere in R n to an ellipsoid in R r ( r min( m, n )). 1 v 2 v –1 –0.5 0.5 1 –1 –0.5 0.5 1 2 v 2 σ 1 u 1 σ –2 –1 0 1 2 –2 –1 1 2 Figure 2: Geometrical interpretation of singular value decomposition. In (5) we refer to the σ j as singular values of A (the diagonal entries of ˆ Σ). They are usually ordered such that σ 1 σ 2 . . . σ n . The orthonormal vectors u j (the columns of ˆ U ) are called the left singular vectors of A , and the orthonormal vectors v j (the columns of V ) are called the right singular vectors of A ). Remark For most practical purposes it suffices to compute the reduced SVD (4). We will give examples of its use, and explain how to compute it later. 23

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QR Compression ratio 0.2000, 25 columns used 50 100 150 200 250 300 20 40 60 80 100 120 140 160 180 200 Compression ratio 0.2000, relative error 0.0320, 25 singular values used 50 100 150 200 250 300 20 40 60 80 100 120 140 160 180 200 Figure 3: Image compressed using QR factorization (left) and SVD (right). Besides applications to inconsistent and underdetermined linear systems and least squares problems, the SVD has important applications in image and data compression (see our discussion of low-rank approximation below). Figure 3 shows the difference between using the SVD and the QR factorization (to be introduced later) for com- pression of the same image. In both cases the same amount (20%) of information was retained. Clearly, the SVD does a much better job in picking out what information is “important”. We will also see below that a number of theoretical facts about the matrix A can be obtained via the SVD.
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