lecture 7 centrality ( cont )
DESCRIPTION
Lecture 7 Centrality ( cont ). Slides modified from Lada Adamic and Dragomir Radev. Eigenvalues and eigenvectors. - PowerPoint PPT PresentationTRANSCRIPT
Lecture 7
Centrality (cont)
Slides modified from Lada Adamic and Dragomir Radev
Eigenvalues and eigenvectors have their origins in physics, in particular in problems where motion is involved, although their uses extend from solutions to stress and strain problems to differential equations and quantum mechanics.
Eigenvectors are vectors that point in directions where there is no rotation. Eigenvalues are the change in length of the eigenvector from the original length.
The basic equation in eigenvalue problems is:
Axx
Eigenvalues and eigenvectors
Slides from Fred K. Duennebier
In words, this deceptively simple equation says that for the square matrix A, there is a vector x such that the product of Ax such that the result is a SCALAR, , that, when multiplied by x, results in the same product. The multiplication of vector x by a scalar constant is the same as stretching or shrinking the coordinates by a constant value.
Axx (E.01)
Eigenvalues and eigenvectors
AxxThe vector x is called an eigenvector and the scalar , is called an eigenvalue.
Do all matrices have real eigenvalues?
No, they must be square and the determinant of A- I must equal zero. This is easy to show:
This can only be true if det(A- I )=|A- I |=0
Are eigenvectors unique?
No, if x is an eigenvector, then x is also an eigenvector and is an eigenvalue.
Ax x0 x A I 0
A(x)= Ax = x = (x)
(E.02)
(E.03)
(E.04)
How do you calculate eigenvectors and eigenvalues?
Expand equation (E.03): det(A- I )=|A- I |=0 for a 2x2 matrix:
A I a11 a12
a21 a22
1 00 1
a11 a12
a21 a22
det A I a11 a12
a21 a22 a11 a22 a12a21 0
0 a11a22 a12a21 a11 a22 2
For a 2-dimensional problem such as this, the equation above is a simple quadratic equation with two solutions for . In fact, there is generally one eigenvalue for each dimension, but some may be zero, and some complex.
(E.05)
The solution to E.05 is:
0 a11a22 a12a21 a11 a22 2
a11 a22 a11 a22 2
4 a11a22 a12a21
(E.06)
This “characteristic equation” does not involve x, and the resulting values of can be used to solve for x.
Consider the following example:
A1 22 4
(E.07)
Eqn. E.07 doesn’t work here because a11a22-a12a12=0, so we use E.06:
0 a11a22 a12a21 a11 a22 2
0 14 22 (14) 2
(14) 2
We see that one solution to this equation is =0, and dividing both sides of the above equation by yields =5.
Thus we have our two eigenvalues, and the eigenvectors for the first eigenvalue, =0 are:
Axx, A I x0
1 22 4
00
xy
1 22 4
xy
1x2y2x4y
00
These equations are multiples of x=-2y, so the smallest whole number values that fit are x=2, y=-1
For the other eigenvalue, =5:
1 22 4
5 00 5
xy
4 22 1
xy
4x2y2x 1y
00
-4x + 2y = 0, and 2x y0, so, x1, y2
This example is rather special; A-1 does not exist, the two rows of A- I are dependent and thus one of the eigenvalues is zero. (Zero is a legitimate eigenvalue!)
EXAMPLE: A more common case is A =[1.05 .05 ; .05 1] used in the strain exercise. Find the eigenvectors and eigenvalues for this A, and then calculate [V,D]=eig[A].
The procedure is:
1) Compute the determinant of A- I2) Find the roots of the polynomial given by | A- I|=0
3) Solve the system of equations (A- I)x=0
A2 .70 .45.30 .55
A3
.65 .525
.35 .475
A100
.600 .600
.400 .400
Or we could find the eigenvalues of A and obtain A100 very quickly using eigenvalues.
What is A100 ?
We can get A100 by multiplying matrices many many times:
What good are such things?
Consider the matrix:
A.8 .3.2 .7
For now, I’ll just tell you that there are two eigenvectors for A:
x1 .6.4
and Ax1
.8 .3
.7 .2
.6
.4
x1 (1 = 1)
x2 1 1
and Ax2
.8 .3
.7 .2
1 1
.5 .5
(2 = 0.5)
The eigenvectors are x1=[.6 ; .4] and x2=[1 ; -1], and the eigenvalues are 1=1 and 2=0.5.
Note that if we multiply x1 by A, we get x1. If we multiply x1 by A again, we STILL get x1. Thus x1 doesn’t change as we mulitiply it by An.
What about x2? When we multiply A by x2, we get x2/2, and if we multiply x2 by A2, we get x2/4 . This number gets very small fast.
Note that when A is squared the eigenvectors stay the same, but the eigenvalues are squared!
Back to our original problem we note that for A100, the eigenvectors will be the same, the eigenvalues 1=1 and 2=(0.5)100, which is effectively zero.
Each eigenvector is multiplied by its eigenvalue whenever A is applied,
Outline Degree centrality
Centralization Betweenness centrality Closeness centrality
Eigenvector centrality Bonacich power centrality
Katz centrality PageRank Hubs and Authorities
Applications to Information Retrieval LexRank
12
Eigenvector Centrality An extension of degree centrality
Centrality increases with number of neighbors
Not all neighbors are equal Having connection to more central nodes increases importance
Eigenvector centrality gives each vertex a score proportional to the sum of scores of its neighbors
where and vi are eigenvectors
13
Eigenvector Centrality As , we get
Hence, A x = k1 x where
Eigenvector centrality of a vertex is large either it has many neighbors and/or it has important neighbors
14
Eigenvector Centrality Can be calculated for directed graphs as well
We need to decide between incoming or outgoing edges
A has no incoming edges, hence a centrality of 0 B has only an incoming edge from A
hence its centrality is also 0 Only vertices that are in a strongly connected component
of two or more vertices or the out-component of such a component have non-zero centrality
15
BA
CD
E
Katz centrality Give each vertex a small amount of centrality
regardless of its position in the network or the centrality of its neighbors
Hence, where
a is a scaling vector, which is set to normalize the score (for the expression to converge a≤1/k1) reflects the extent to which you weight the centrality of people ego is tied toI is the identity matrix (1s down the diagonal) 1 is a matrix of all ones
16
Katz Centrality: The magnitude of reflects the radius of power
• Small values of weight local structure• Larger values weight global structure
If > 0, ego has higher centrality when tied to people who are central
If < 0, then ego has higher centrality when tied to people who are not central
With = 0, you get degree centrality
17
=.25
Katz Centrality: examples
=-.25
Why does the middle node have lower centrality than itsneighbors when is negative?
18
PageRank: bringing order to the web It’s in the links:
links to URLs can be interpreted as endorsements or recommendations the more links a URL receives, the more likely it is to be a
good/entertaining/provocative/authoritative/interesting information source
but not all link sources are created equal a link from a respected information source a link from a page created by a spammer
Many webpages scatteredacross the web
an important page, e.g. slashdot
if a web page isslashdotted, it gains attention
PageRank An issue in Katz centrality measure is that a high
centrality vertex pointing to large number of vertices gives all high centrality Yahoo directory
This can be fixed by dividing the centrality with the out-degree of a vertex
where Dii=max(kiout, 1)
20
Ranking pages by tracking a drunk
A random walker following edges in a network for a very long time will spend a proportion of time at each nodewhich can be used as a measure ofimportance
Trapping a drunk Problem with pure random walk metric:
Drunk can be “trapped” and end up going in circles
Ingenuity of the PageRank algorithm Allow drunk to teleport with some probability
e.g. random websurfer follows links for a while, but with some probability teleports to a “random” page
bookmarked page or uses a search engine to start anew
PageRank algorithm
where p1,p2,...,pN are the pages under consideration,
M(pi) is the set of pages that link to pi,
L(pj) is the number of outbound links on page pj, and
N is the total number of pages.
d is the random jumping probability (d = 0.85 for google)
GUESS PageRank demo
Exercise: PageRank What happens to the
relative PageRank scores of the nodes as you increase the teleportation probability?
Can you construct a network such that a node with low indegree has the highest PageRank?
http://projects.si.umich.edu/netlearn/GUESS/pagerank.html
example: probable location of random walker after 1 step
1
2
3 4
5
7
6 8
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1 2 3 4 5 6 7 8
Page
Ran
k
t=0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1 2 3 4 5 6 7 8
Page
Ran
k
t=1
20% teleportation probability
1
2
3 4
5
7
6 8
example: location probability after 10 steps
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1 2 3 4 5 6 7 8
Page
Ran
k
t=0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.91
1 2 3 4 5 6 7 8
Page
Ran
k
t=1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1 2 3 4 5 6 7 8
Page
Rank
t=10
Matrix-based Centrality measures
28
Divide by out-degree
No division
with constant term without constant term
PageRank Degree centrality
Eigenvector centralityKatz centrality
Hubs and Authorities In directed networks, vertices that point to important
resources should also get a high centrality e.g. review articles, web indexes
recursive definition:
hubs are nodes that links to good authorities
authorities are nodes that are linked to by good hubs
Hyperlink-Induced Topic Search HITS algorithm
start with a set of pages matching a query
expand the set by following forward and back links
take transition matrix E, where the i,jth entry Eij =1/ni
where i links to j, and ni is the number of links from i
then one can compute the authority scores a, and hub scores h through an iterative approach:
hEa T' Eah '
31
Outline Degree centrality
Centralization Betweenness centrality Closeness centrality
Eigenvector centrality Bonacich power centrality
Katz centrality PageRank Hubs and Authorities
Applications to Information Retrieval LexRank
32
Applications to Information Retrieval Can we use the notion of centrality to pick the best
summary sentence?
Can we use the subgraph of query results to infer something about the query?
Can we use a graph of word translations to expand dictionaries? disambiguate word meanings?
How might one use the HITS algorithm for document summarization? Consider a bipartite graph of sentences and words
Centrality in summarization
Extractive summarization pick k sentences that are most representative of a collection of n
sentences
Motivation: capture the most central words in a document or cluster
Centroid score [Radev & al. 2000, 2004a]
Alternative methods for computing centrality?
Sample multidocument cluster
1 (d1s1) Iraqi Vice President Taha Yassin Ramadan announced today, Sunday, that Iraq refuses to back down from its decision to stop cooperating with disarmament inspectors before its demands are met.
2 (d2s1) Iraqi Vice president Taha Yassin Ramadan announced today, Thursday, that Iraq rejects cooperating with the United Nations except on the issue of lifting the blockade imposed upon it since the year 1990.
3 (d2s2) Ramadan told reporters in Baghdad that "Iraq cannot deal positively with whoever represents the Security Council unless there was a clear stance on the issue of lifting the blockade off of it.
4 (d2s3) Baghdad had decided late last October to completely cease cooperating with the inspectors of the United Nations Special Commission (UNSCOM), in charge of disarming Iraq's weapons, and whose work became very limited since the fifth of August, and announced it will not resume its cooperation with the Commission even if it were subjected to a military operation.
5 (d3s1) The Russian Foreign Minister, Igor Ivanov, warned today, Wednesday against using force against Iraq, which will destroy, according to him, seven years of difficult diplomatic work and will complicate the regional situation in the area.
6 (d3s2) Ivanov contended that carrying out air strikes against Iraq, who refuses to cooperate with the United Nations inspectors, ``will end the tremendous work achieved by the international group during the past seven years and will complicate the situation in the region.''
7 (d3s3) Nevertheless, Ivanov stressed that Baghdad must resume working with the Special Commission in charge of disarming the Iraqi weapons of mass destruction (UNSCOM).
8 (d4s1) The Special Representative of the United Nations Secretary-General in Baghdad, Prakash Shah, announced today, Wednesday, after meeting with the Iraqi Deputy Prime Minister Tariq Aziz, that Iraq refuses to back down from its decision to cut off cooperation with the disarmament inspectors.
9 (d5s1) British Prime Minister Tony Blair said today, Sunday, that the crisis between the international community and Iraq ``did not end'' and that Britain is still ``ready, prepared, and able to strike Iraq.''
10 (d5s2) In a gathering with the press held at the Prime Minister's office, Blair contended that the crisis with Iraq ``will not end until Iraq has absolutely and unconditionally respected its commitments'' towards the United Nations.
11 (d5s3) A spokesman for Tony Blair had indicated that the British Prime Minister gave permission to British Air Force Tornado planes stationed in Kuwait to join the aerial bombardment against Iraq.
(DUC cluster d1003t)
Cosine between sentences
Let s1 and s2 be two sentences.
Let x and y be their representations in an n-dimensional vector space
The cosine between is then computed based on the inner product of the two.
yx
yxyx ni
ii ,1),cos(
The cosine ranges from 0 to 1.
LexRank (Cosine centrality)
1 2 3 4 5 6 7 8 9 10 11
1 1.00 0.45 0.02 0.17 0.03 0.22 0.03 0.28 0.06 0.06 0.00
2 0.45 1.00 0.16 0.27 0.03 0.19 0.03 0.21 0.03 0.15 0.00
3 0.02 0.16 1.00 0.03 0.00 0.01 0.03 0.04 0.00 0.01 0.00
4 0.17 0.27 0.03 1.00 0.01 0.16 0.28 0.17 0.00 0.09 0.01
5 0.03 0.03 0.00 0.01 1.00 0.29 0.05 0.15 0.20 0.04 0.18
6 0.22 0.19 0.01 0.16 0.29 1.00 0.05 0.29 0.04 0.20 0.03
7 0.03 0.03 0.03 0.28 0.05 0.05 1.00 0.06 0.00 0.00 0.01
8 0.28 0.21 0.04 0.17 0.15 0.29 0.06 1.00 0.25 0.20 0.17
9 0.06 0.03 0.00 0.00 0.20 0.04 0.00 0.25 1.00 0.26 0.38
10 0.06 0.15 0.01 0.09 0.04 0.20 0.00 0.20 0.26 1.00 0.12
11 0.00 0.00 0.00 0.01 0.18 0.03 0.01 0.17 0.38 0.12 1.00
d4s1
d1s1
d3s2
d3s1
d2s3
d2s1
d2s2
d5s2d5s3
d5s1
d3s3
Lexical centrality (t=0.3)
d4s1
d1s1
d3s2
d3s1
d2s3
d2s1
d2s2
d5s2d5s3
d5s1
d3s3
Lexical centrality (t=0.2)
d4s1
d1s1
d3s2
d3s1
d2s3d3s3
d2s1
d2s2
d5s2d5s3
d5s1
Lexical centrality (t=0.1)
Sentences vote for the most central sentence…
d4s1
d3s2
d2s1
NdTiTETp
TcdTiTETp
TcdTip nn
n
1)()()(
...)()()(
)( ,,111
LexRank
T1…Tn are pages that link to A, c(Ti) is the outdegree of pageTi, and N is the total number of pages.
d is the “damping factor”, or the probability that we “jump” to a far-away node during the random walk. It accounts for disconnected components or periodic graphs.
When d = 0, we have a strict uniform distribution.When d = 1, the method is not guaranteed to converge to a unique solution.
Typical value for d is between [0.1,0.2] (Brin and Page, 1998).
Güneş Erkan and Dragomir R. Radev, LexRank: Graph-based Lexical Centrality as Salience in Text Summarization
lab: Lexrank demo
http://tangra.si.umich.edu/clair/lexrank/
how does the summary change as you:
increase the cosine similarity threshold for an edge how similar two
sentences have to be?
increase the salience threshold (minimum degree of a node)
Menczer, Filippo (2004) The evolution of document networks.
Content similarity distributions forweb pages (DMOZ) and scientific articles (PNAS)
what is that good for? How could you take advantage of the fact that pages that
are similar in content tend to link to one another?
What can networks of query results tell us about the query?
Jure Leskovec, Susan Dumais: Web Projections: Learning from Contextual Subgraphs of the Web
If query results are highly interlinked, is this a narrow or broad query?
How could you use query connection graphs to predict whether a query will be reformulated?
How can bipartite citation graphs be used to find related articles?
co-citation: both A and B are cited by many other papers (C, D, E …)
AB
CD E
bibliographic coupling: both A and B are cite many of the same articles (F,G,H …)
F GH
AB
which of these pairs is more proximate according to cycle free effective conductance:
the probability that you reach the other node before cycling back on yourself, while doing a random walk….
Proximity as cycle free effective conductance
Measuring and Extracting Proximity in Networks by Yehuda Koren, Stephen C. North, Chris Volinsky, KDD 2006
demo: http://public.research.att.com/~volinsky/cgi-bin/prox/prox.pl
Source: undetermined
Using network algorithms (specifically proximity) to improve movie recommendations can pay off
final IR application: machine translation
not all pairwise translations are available e.g. between rare languages
in some applications, e.g. image search, a word may have multiple meanings “spring” is an example in english
But in other languages, the word may be unambiguous.
automated translation could be the key
or or or
final IR application: machine translation
spring English
printemps French
primavera Spanish
ربيعArabic
koanga Maori
udaherri Basque
1
vzmet Slovenian пружина
Russian
ressort French
2
veer Dutch
рысора Belarusian
……
……
3
11 1
1
33
…
…
…3
3
…
444
2
22
4
2
2
4
3
4
1
if we combine all known word pairs, can we construct additional dictionaries between rare languages?
source: Reiter et al., ‘Lexical Translation with Application to Image Search on the Web ’
52
Automatic translation & network structure Two words more likely to have same meaning if there
are multiple indirect paths of length 2 through other languages
spring English
printemps French
primavera Spanish
ربيعArabic
koanga Maori
udaherri Basque
1…
…
3
11 1
13
3…
… 3
33
1
пружина
Russian
…22
summary the web can be studied as a network
this is useful for retrieving relevant content
network concepts can be used in other IR tasks summarization query prediction machine translation