# Applied Geometrical Matrix Computations Alan Edelman Dept of Mathematics: MIT

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```Applied Geometrical
Matrix Computations
Alan Edelman
Dept of Mathematics: MIT
MIT Laboratory for Computer Science
Householder Symposium XV
June 21, 2002
Outline
• Geometrical Matrix Computations
• Illustration with 2x2 matrices:
• Excursions into eigenland (or why tangency
and curvature matter!!)
• Where do matrix factorizations come from?
• Application to Color Science
• Matrix Animations
Geometrical Matrix Computations
Working definition:
•Concerns geometry of matrix space
(n2 dimensions rather than n)
•Involves numerical computation (probably MATLAB)
•Relates to an NLA problem
Some Other GMC People
Absil, Demmel, Elmroth, Huhtanen, Kagstrom, Kahan,
Lippert, Ma, Mahony, Malyshev, Sepulchre, Tisseur,
Trefethen, Van Dooren
Vector Space Diagrams
•Points are vectors (not matrices!)
•Geometric relationships for vectors, subspaces,
and linear transformations
Outline
• Geometrical Matrix Computations
• Illustration with 2x2 matrices:
• Excursions into eigenland (or why tangency
and curvature matter!!)
• Where do matrix factorizations come from?
• Application to Color Science
• Matrix Animations
x
M =  z

Eigenland (in 2d)
Isoeig surfaces are hyperbolas
2
   x z
2
z
x
-z 
-x 
The Eigenvalue Map
x
M =  z

-z 
-x 
2
   x z
zz
2
cos    z x  Eigvec Angle
xx
/2
 Zero Matrix
0
0

The Eigenvalue Map
x
M =  z

-z 
-x 
2
   x z
zz
2
cos    z x  Eigvec Angle
xx
•M
/2
 Zero Matrix
0
0

•M
The Eigenvalue Map
x
M =  z

-z 
-x 
zz
2
   x z
2
cos    z x  Eigvec Angle
xx
Uniformly /2
•M
 Zero Matrix
0
0

?•M
Pseudospectra (Trefethen)
 ( A)  {zz  C ::zz  eig( A  E ), with ||E||2   }
“z is an eigenvalue of a matrix near A”
 {z  C :  min (zI - A)  }
Pseudoportraits = pictures of contours of z
Pseudoportraits
1
1
1
1
0
A=
-1
0
1 -1
1
1
1
1
1
1
0
0
-1
1
1
0
0
0
-1
1
Random Points
pseudospectra &amp; geometry
matrix space
C
eig (w/singularity) spectral portrait
n2
A  XX
1
C
X

n2

Outline
• Geometrical Matrix Computations
• Illustration with 2x2 matrices:
• Excursions into eigenland (or why tangency
and curvature matter!!)
• Where do matrix factorizations come from?
• Application to Color Science
• Matrix Animations
Circle/Hyperbola Tangency = High Density
*
*
*
*
*
*
*
*
*
eigenvalue
have eigenvalue
distributions with 3
2
4
spikes.
eigenvalue
frequency
frequency
frequency
•Circles tangent to 2
4
3
hyperbolas…
eigenvalue
Radius of Curvature = Highest Density
•Circles are tangent to 3
hyperbolas when two
tangency points collide
*
•The circle also shares a
radius of curvature with the
hyperbola at this point
*
eigenvalue
frequency
frequency
frequency
•This is even better than
tangency, which means a
higher spike
*
eigenvalue
eigenvalue
Outline
• Geometrical Matrix Computations
• Illustration with 2x2 matrices:
• Excursions into eigenland (or why tangency
and curvature matter!!)
• Where do matrix factorizations come from?
• Application to Color Science
• Matrix Animations
Where do Matrix Factorizations Come
From?
Classical Answer:
A=UV’
Representation
Theory of
Semisimple Groups
Semisimple group recipe
•Nicely links factorizations
•Three Examples
Nonsingulars
Unitary
SVD
UeiV’
SPD Eigen
UeiU’
One more example
Hyperbolic Svd as in last talk
Group = SO(p,q) ( XJ=JX)
Orthogonal
CS decomp
Essentially Sym Orth
Matrix Factorizations
Where can we look for new factorizations?
• The Mathematics Literature
– Lie Algebra: Cartan, Iwasawa, Bruhat
– Representation Theory: Quivers
• Nearness Problems
• Applications
– Engineering: A factorization is useful if someone can use it
– Mathematics: The useful factorizations are characterized by an
abstract criterion
Ideas to Generalize
1: Cartan Decomposition
E = (antisymmetric) + (symmetric)
expm
M=
Non-singular
expm
Q
Orthogonal
expm
*
S
Pos Definite
[polar]
Ideas to Generalize
1: Cartan Decomposition
E = (antisymmetric) + (symmetric)
expm
M=
Non-singular
expm
Q
Orthogonal
expm
*
S
Pos Definite
2:KAK Decomposition
Positive Diagonals = Maximal Group
M=UV’
Conjugates give S=QQ’
[polar]
Ideas to Generalize
1: Cartan Decomposition
E = (antisymmetric) + (symmetric)
expm
M=
Non-singular
expm
Q
Orthogonal
expm
*
S
[polar]
Pos Definite
2:KAK Decomposition
Positive Diagonals = Maximal Group
M=UV’
Conjugates give S=QQ’
3:Iwasawa, Bruhat
Above not unique at I. Gives
M=LU, other permutations, totally positive, etc
Ideas to Generalize
1: Cartan Decomposition
E = (antisymmetric) + (symmetric)
expm
M=
Non-singular
expm
Q
Orthogonal
expm
*
S
[polar]
Pos Definite
2:KAK Decomposition
Positive Diagonals = Maximal Group
M=UV’
Conjugates give S=QQ’
3:Iwasawa, Bruhat
Above not unique at I. Gives
M=LU, other permutations, totally positive, etc
4:Eigenvalue, Jordan Schur
Step 1:Cartan Decomposition
•Group: non-singular matrices
•Involution: ((M))=M (M1M2)= (M1)(M2)
(M)=M-T
•Fixed Points (M)=M are a group K
K = orthogonal matrices
•Near I
M = (antisymmetric) + (symmetric)
•Cartan: expm
M = QS (S&gt;0)
(polar)
Step 1:Cartan Decomposition (U/O)
•Group: unitary matrices
•Near I
M = (antisymmetric) + (i*symmetric)
•Cartan:
M= (real orth)(unitary symmetric)
Step 2:KAK Decomposition
P = sym pos def
• A = biggest group inside P (abelian)
e.g.  diagonal &gt; 0, or conjugates UU’ (fix U)
•KAK
M=UV’
•P = union of conjugates
S=QQ’
Step 2:KAK Decomposition (U/Q)
P = unitary symmetric
• A = biggest group inside P (abelian)
e.g. diagonals (ei) or conjugates
•KAK
M=UeiV’ (U, V real orthogonal)
•P = union of conjugates
S=QeiQ’ (Q real orthogonal)
Step 2:KAK Decomposition (On/Op X Oq )
C
P = matrices orthogonally similar to ( -S
• A = biggest group inside P (abelian)
C S
e.g. =( -S C ) or conjugates
•KAK
The CS Decomposition
S
C
)
Missing
•The constructible decompositions
Tridiagonalization, Bidiagonalization
•The NNMF (Lee, Seung 1999)
•V  WH Input: Vij&gt;0
Output: Wij&gt;0 Hij&gt;0 (low rank)
Algorithm:
H  H .* (W’V)./(W’WH)
W  W .* (VH’)./(WHH’)
Original Application: Eigenfaces
Another Example: Color Science
Outline
• Geometrical Matrix Computations
• Illustration with 2x2 matrices:
• Excursions into eigenland (or why tangency
and curvature matter!!)
• Where do matrix factorizations come from?
• Application to Color Science
• Matrix Animations
Color Science: Light Spectra from film
wavelength vs density
Reds
Greens
Grays
Blues
Film Recording and measurements
• Solid colors sent to film recorder, e.g. reds
• Negative is produced: film appears as cyans
• Negative sent through projector to spectrometer
• Energy data at each wavelength
Reds
• Log ratio with no film (only bulb)
film density =
log(no film / with film)
The Data
•
•
•
•
Inputs (r,g,b) for 1r,g,b 10 scaled (1000 frames)
Output Space: Densities at 400:3:700 nm’s
Data Structure: 101 x 1000 matrix “A”
Compute SVD(A) •Project onto best 3 space
svd
Three significant
singular values
index
SVD Basis = no physical meaning
The NNMF Basis = primary colors
Outline
• Geometrical Matrix Computations
• Illustration with 2x2 matrices:
• Excursions into eigenland (or why tangency
and curvature matter!!)
• Where do matrix factorizations come from?
• Application to Color Science
• Matrix Animations
Singular 2x2 Matrices (by svd)
 cos  _ sin   
 cos  sin  
A



0   sin  cos  
  sin  cos  
A=
(
cos  cos 
-sin  cos 
cos  sin 
-sin  sin 
 and  

 
 scales cone
)
Torus!
Torus Cone?
All isoeig surfaces are translates of the =0 surface!
hyperpolas and hyperboloids are cross sections!
Bohemian Dome
Linear Algebra with movies
Horizontal
A=QQT
Vertical
A=QQ
Villarceau
A=QR
Hopf Fibration
Challenges
Incorporate 3d graphics tools directly into Matrix
computations. Include geometry of matrix space.
How should this look?
Generalize everything and incorporate into software
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