1. Degree and Eigenvector Centrality
The “Centralities”
Degree and Eigenvector Centrality

2. The “Centralities”
Degree Centrality

3. Degree Centrality Quality: what makes a node important (central)
Mathematical Description
Appropriate Usage
Identification
Lots of one-hop connections from 𝑣
The number of vertices that 𝑣 influences directly
Local influence matters
Small diameter
Degree centrality
deg⁡(𝑣)
Lots of one-hop connections from 𝑣 relative to the size of the graph
The proportion of the vertices that 𝑣 influences directly
Normalized degree centrality
deg⁡(𝑣) |V(G)|

4. Degree Centrality

5. Degree Centrality per vertex the sequence gives {vertex label: degree, …}
{0: 4, 1: 7, 2: 2, 3: 4, 4: 2, 5: 1, 6: 1, 7: 1, 8: 3, 9: 1, 10: 1, 11: 3, 12: 1, 13: 1, 14: 1, 15: 1, 16: 1, 17: 1, 18: 1, 19: 1}

6. Degree Centrality per vertex as a vector

7. Degree centrality (distribution)

8. Normalized Degree Centrality per vertex
4/ /20 4/20 2/20 1/20 1/20 1/20 3/20 1/20 1/20 3/20 1/20 1/20 1/20 1/20 1/20 1/20 1/20 1/20
Or divide by 𝑛−1 not counting the vertex at which centrality is computed.
Or divide by Δ(𝐺) to normalize it since we only care for the relative centrality. 8

9. Eigenvector Centrality
The “Centralities”
Eigenvector Centrality

10. Recall: Quality: what makes a node important (central)
Mathematical Description
Appropriate Usage
Identification
Lots of one-hop connections from 𝑣
The number of vertices that 𝑣 influences directly
Local influence matters
Small diameter
Degree centrality (or simply the C i = deg⁡(𝑖))
Lots of one-hop connections from 𝑣 relative to the size of the graph
The proportion of the vertices that 𝑣 influences directly
Normalized degree centrality
C i = deg⁡(𝑖) |V(G)|
Lots of one-hop connections to high centrality vertices
A weighted degree centrality based on the weight of the neighbors (instead of a weight of 1 as in degree centrality)
For example when the people you are connected to matter.

11. Recall: HOW? Quality: what makes a node important (central)
Mathematical Description
Appropriate Usage
Identification
Lots of one-hop connections from 𝑣
The number of vertices that 𝑣 influences directly
Local influence matters
Small diameter
Degree centrality (or simply the C i = deg⁡(𝑖))
Lots of one-hop connections from 𝑣 relative to the size of the graph
The proportion of the vertices that 𝑣 influences directly
Normalized degree centrality
C i = deg⁡(𝑖) |V(G)|
Lots of one-hop connections to high centrality vertices
A weighted degree centrality based on the weight of the neighbors (instead of a weight of 1 as in degree centrality)
For example when the people you are connected to matter.
HOW?

12. Recall: Quality: what makes a node important (central)
Mathematical Description
Appropriate Usage
Identification
Lots of one-hop connections from 𝑣
The number of vertices that 𝑣 influences directly
Local influence matters
Small diameter
Degree centrality (or simply the C i = deg⁡(𝑖))
Lots of one-hop connections from 𝑣 relative to the size of the graph
The proportion of the vertices that 𝑣 influences directly
Normalized degree centrality
C i = deg⁡(𝑖) |V(G)|
Lots of one-hop connections to high centrality vertices
A weighted degree centrality based on the weight of the neighbors (instead of a weight of 1 as in degree centrality)
For example when the people you are connected to matter.
Eigenvector centrality
(recursive formula):
𝐶 𝑖 ∝ j 𝐶 𝑗

13. Eigenvector Centrality
A generalization of the degree centrality: a weighted degree vector that depends on the centrality of its neighbors (rather than every neighbor having a centrality of 1)
How do we find it? By finding the largest eigenvalue and its associated eigenvector (leading eigenvector) of the adjacency matrix.
We will get to the “why eigenvectors work” part.

14. Example 1 (Eigenvector centrality)
Node 𝑖: Eigenvector centrality 𝐶 𝑖
0: 0,
1: ,
2: ,
3: ,
4: , 5:

15. Example 1 (Eigenvector centrality)
Node 𝑖: Eigenvector centrality 𝐶 𝑖
0: 0,
1: ,
2: ,
3: ,
4: , 5:
Notice that deg(5) < deg (4). Why 𝐶 5 > 𝐶 4 ?

16. Example 2 (Eigenvector centrality)
Node 𝑖: Eigenvector centrality 𝐶 𝑖
0: ,
1: ,
2: ,
3: ,
4: , 5:
Slightly lower for vertex 4 here,
but in a large graph it may make a bigger difference.
deg(4) > deg(3)

17. Example 3 (Adjacency matrix)
[[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]]

18. Example 3 (Eigenvector centrality)
0: ,
1: ,
2: ,
3: ,
4: ,
5: ,
6: ,
7: ,
8: ,
9: ,
10: ,
11: ,
12: ,
13: ,
14:

19. Example 3 (Eigenvector centrality)
2: ,
4: ,
5: ,
6: ,
8: ,
10: ,
12: ,
13: ,
14:
Also, deg(4) = deg(2) = Δ(G), yet…

20. Example 3 (Eigenvector centrality)
2: ,
4: ,
5: ,
6: ,
8: ,
10: ,
12: ,
13: ,
14:
Also, deg(4) = deg(2) = Δ(G), yet 𝐶 4 > 𝐶 2 .

21. Example 3 (Eigenvector centrality)
Adjacent to vertices of small degree
Adjacent to vertices of large degree

22. Example 3 (Eigenvector centrality)
{0: ,
1: ,
2: ,
3: ,
4: ,
5: ,
6: ,
7: ,
8: ,
9: ,
10: ,
11: ,
12: ,
13: ,
14: }
deg(13) < deg(8)

23. Example 3 (Eigenvector centrality)
{0: ,
1: ,
2: ,
3: ,
4: ,
5: ,
6: ,
7: ,
8: ,
9: ,
10: ,
11: ,
12: ,
13: ,
14: }

24. Eigenvector Centrality
Define the centrality 𝑥′ 𝑖 of 𝑖 recursively in terms of the centrality of its neighbors
𝑥 𝑖 ′ = 𝑘∈𝑁(𝑖) 𝑥 𝑘 𝑜𝑟 𝑥 𝑖 ′ = 𝑗 𝐴 𝑖𝑗 𝑥 𝑗
With initial vertex centrality 𝑥 𝑗 =1, ∀𝑗 (𝑖𝑛𝑐𝑙𝑢𝑑𝑖𝑛𝑔 𝑖)—we’ll see why on next slide
That is equivalent to:
𝑥 𝑖 (𝑡)= 𝑗 𝐴 𝑖𝑗 𝑥 𝑗 (𝑡−1)
with the centrality at time t=0 being 𝑥 𝑗 0 =1, ∀𝑗
The centrality of vertices
𝑖 and 𝑗 at time t and t-1, respectively

25. In class: Eigenvector Centrality
Adjacency matrix A for the graph to the right: A=
Then the vector x(t) = 𝑥 1 𝑥 2 : 𝑥 𝑛 gives a random surfer’s behavior Answer the following questions based on the information above

26. In class: Eigenvector Centrality
Q1: Find x(1). What does it represent?
Answer: 𝑥(1)= = ?

27. In class: Eigenvector Centrality
Q1: Find x(1). What does it represent?
Answer: 𝑥(1)= =
The Degree Centrality vector

28. In class: Eigenvector Centrality
Q2: Find x(2). What does it represent?
Answer: 𝑥(2)= = ?

29. In class: Eigenvector Centrality
Q2: Find x(2). What does it represent?
Answer: 𝑥(2)= =
A weighted Degree Centrality vector (distance 2 or less)

30. In class: Eigenvector Centrality
Q3: Find x(3). What does it represent?
Answer: 𝑥(3)= = ?

31. In class: Eigenvector Centrality
Q3: Find x(3). What does it represent?
Answer: 𝑥(3)= =
Another weighted Degree Centrality vector (distance 3 or less)

32. In class: Eigenvector Centrality
0: ,
1: ,
2: ,
3: 0,
4: , 5:

33. Discussion: What did you notice?
What is x(3)?
Answ: 𝑥 3 =𝐴(𝐴(𝐴 ))) = 𝐴 3 𝑥 0
𝑥 3 depends on the centrality of its distance 3 or less neighbors

34. Discussion: Can you generalize it?
What is x(t)?
Answer: 𝑥 t =𝐴(𝐴…(𝐴 ))) = 𝐴 𝑡 𝑥 0 , 𝑡>0
𝑥 t depends on the centrality of its distance t or less neighbors

35. Eigenvector Centrality Derivation
We can consolidate the eigenvector centralities of all the nodes in a recursive formula with vectors:
x 𝑡 =𝐴 ∙ 𝐱(𝑡−1)
with the centrality at time t=0 being x 0 =𝟏 (as a vector)
Then, we solve: x 𝑡 = 𝐴 𝑡 ∙ 𝐱(0), with x 0 =𝟏
Let 𝒗 𝑘 be the eigenvectors of the adjacency matrix 𝐴
Let 𝜆 1 be the largest eigenvalue.
Let x 0 = 𝑘 𝑐 𝑘 𝑣 𝑘 , be a linear combination of 𝒗 𝑘 (eigenvectors are orthogonal since 𝐴 is real and symmetric)

36. Eigenvector Centrality Derivation
Facts from previous page:
x 𝑡 = 𝐴 𝑡 ∙ 𝐱(0), with x 0 =1
𝒗 𝒌 are the eigenvectors of the adjacency matrix A
x 0 = 𝑘 𝑐 𝑘 𝑣 𝑘 is a linear combination of 𝒗 𝒌
𝜆 1 be the largest eigenvalue.
Then
x 𝑡 = 𝐴 𝑡 ∙ 𝐱 0 = 𝐴 𝑡 𝑘 𝑐 𝑘 𝑣 𝑘 = 𝑘 𝑐 𝑘 𝜆 𝑘 𝑡 𝑣 𝑘 =
= 𝜆 1 𝑡 𝑘 𝑐 𝑘 𝜆 𝑘 𝑡 𝜆 1 𝑡 𝑣 𝑘 = 𝜆 1 𝑡 ( 𝑐 1 𝜆 1 𝑡 𝜆 1 𝑡 𝑣 𝑐 2 𝜆 2 𝑡 𝜆 1 𝑡 𝑣 𝑐 3 𝜆 3 𝑡 𝜆 1 𝑡 𝑣 3 …) x 𝑡  𝜆 1 𝑡 𝑐 1 𝑣 1 since 𝜆 𝑘 𝑡 𝜆 1 𝑡 0 as t ∞ (as you repeat the process)

37. Eigenvector Centrality
Thus the eigenvector centrality is
𝒙 𝑡 = 𝜆 1 𝑡 𝑐 1 𝒗 1
where 𝒗 𝟏 is the eigenvector corresponding to the largest eigenvalue 𝜆 1 𝑡
So the eigenvector centrality (as a vector), 𝐱 𝑡 , is a multiple of the eigenvector 𝒗 1 , i.e. 𝐱 𝑡 is an eigenvector of 𝐴.
A x 𝑡 = 𝜆 1 𝑡 x 𝑡
Meaning that the eigenvector centrality of each node is given by the entries of the leading eigenvector (the one corresponding to the largest eigenvalue λ= 𝜆 1 𝑡 )

38. Is it well defined?
That is: Is the vector guaranteed to exist? Is it unique? Is the eigenvalue unique? Can we have negative entries in the eigenvector?
We say that a matrix/vector is positive if all of its entries are positive
Perron-Frobenius theorem: A real square matrix with positive entries has a unique largest real eigenvalue and that the corresponding eigenvector has strictly positive components
Perron-Frobenius theorem applies to positive matrices (but it gives similar information for nonnegative ones)

39. Perron-Frobenius theorem for nonnegative symmetric (0,1)-matrices
Let A ∈ 𝑅 𝑛 𝑋 𝑛 be symmetric (0,1)-nonnegative, then
there is a unique maximal eigenvalue λ 1 of the matrix A (for any other eigenvalue λ, we have λ < λ 1 , with the possibility of |λ| = λ 1 for nonnegative matrices)
λ 1 is real, simple (i.e., has multiplicity one), and positive (trace is zero so some are positive and some negative),
the associated eigenvector is nonnegative (and there are no other nonnegative ones since all eigenvectors are orthogonal)
If you have not seen this and its proof in linear algebra, see a proof on pages of Newman’s textbook

40. In class problem: Let G be an r-regular graph.
Find the eigenvector centrality,
Find the leading eigenvalue.
Solution: use a matrix A and the vector 1 to begin with, and follow a similar argument as the earlier in class problem
Take away: even this centrality may not differentiate vertices
that it should, such as:

41. In class problem (2)
Note that 𝑥 𝑖 2 𝑥 𝑖 1 ≠ 𝑥 𝑖 3 𝑥 𝑖 2 , where 𝑥 𝑖 is the 𝑖 𝑡ℎ entry. For example: , , , (in finding the eigenvectors, these vectors get normalized as they are computed using the power method from Linear Algebra - see page 248 in Newman’s book)
However, the ratios will converge to λ 1

42. Eigenvector Centrality
Therefore:
it is a generalized degree centrality (takes into consideration the global network)
It is extremely useful, one of the most common ones used for non-oriented networks
𝐶 𝑖 ∝ j 𝐶 𝑗 or 𝐶 𝑖 = 𝜆 −1 𝑗 𝐴 𝑖𝑗 𝐶 𝑗 or 𝐶 𝑖 = 𝑖𝑗 є 𝐸(𝐺) 𝐶 𝑗
Why is Eigenvector Centrality not often used for directed graphs?
Adjacency matrix is asymmetric…use left or right leading eigenvector?
Choose right leading eigenvector…importance bestowed by vertices pointing toward you (same problem with left).
Any vertex with in degree zero has centrality value zero and “passes” that value to all vertices to which it points.
The fix: Katz centrality