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**The Eckart-Young Theorem**

##
**The Eckart-Young Theorem**: Given an n by m matrix **X** of rank
**r £ m £ n**,
and its singular value decomposition,
**ULV'**, where
**U** is an n by m matrix,
**L** is an m by m diagonal matrix of singular values,
and **V** is an m by m matrix such that

**U'U = I**_{n} and
**V'V = VV' = I**_{m}

with the
singular values arranged in decreasing sequence

**l**_{1} ³
l_{2} ³
l_{3} ³ ...
³
l_{m} ³ 0

then there exists an n by m matrix **B** of rank s,
**s £ r**, which minimizes the sum of the squared
error between the elements of **X** and the corresponding elements of
**B** when

**B = UL**_{s}V'

where the diagonal elements of the m by m diagonal matrix
**L**_{s} are

**l**_{1} ³
l_{2} ³
l_{3} ³ ...
³
l_{s} >
l_{s+1} =
l_{s+2} = ... =
l_{m} = 0

Geometrically, the Eckart-Young Theorem states that the least squares
approximation in s dimensions of a matrix **X** can be found by replacing
the smallest m-s roots of
**L** with zeroes and remultiplying
**ULV'**.

For example, given:

the rank 2 approximation is:

svd_example.r -- **R**
Program to illustrate simple SVD.

**svd_example.r** looks like this:**#
# file: svd_example.r
#
# Purpose: Simple R program showing Singular
# Value Decomposition
#
#
rm(list=ls(all=TRUE))
#
#
rc.file <- "c:/ucsd_course/svd_matrix.txt"
#
# Standard fields and their widths
#
rc.fields <- c("y1","y2","y3")
#
rc.fieldWidths <- c(3,3,3)
#
# Read the vote data from fwf
#
T <- read.fwf(file=rc.file,widths=rc.fieldWidths,as.is=TRUE,col.names=rc.fields)
dim(T)
#
nrow <- length(T[,1])
ncol <- length(T[1,])
#
xsvd <- svd(T)
#
# The Two Lines Below Put the Singular Values in a
# Diagonal Matrix -- The first one creates an
# identity matrix and the second command puts
# the singular values on the diagonal
#
Lambda <- diag(ncol)
diag(Lambda) <- xsvd$d
#
# Compute U*LAMBDA*V' for check below
#
XX <- xsvd$u %*% Lambda %*% t(xsvd$v)
#
xerror <- sum(abs(T-XX))
#**