- Invasion that started in March 19, 2003. Many of the regime's high ranking officials, including Saddam Hussein, avoided capture.
Hussein was last spotted kissing a baby in Baghdad in April 2003, and then his trace went cold.

Designed a deck of cards, each card engraved with the images of the 55 most wanted.

- It worked: by May 1, 2003, 15 men on the cards were captured, and by the end of the month another 12 were under custody.
- Yet, the ace of spades, i.e. Hussein himself, remained at large.

shows the strong predictive power of networks.

underlies the need to obtain accurate maps of the networks we aim to study; and the often heroic difficulties we encounter during the mapping process.

demonstrates the remarkable stability of these networks: The capture of Hussein was not based on fresh intelligence, but rather on his pre-invasion social links, unearthed from old photos stacked in his family album.

shows that the choice of network we focus on makes a huge difference: the hierarchical tree, that captured the official organization of the Iraqi government, was of no use when it came to Saddam Hussein's whereabouts.

DUE TO INTERCONNECTIVITY

- The 2003 blackout is a typical example of a cascading failure.
- 1997, when the International Monetary Fund pressured the central banks of several Pacific nations to limit their credit.
- 2009-2011 financial melt-down

An important theme of this class:

we must understand

**how network structure affects the robustness of a complex system**.develop quantitative tools to assess the interplay between network structure and the dynamical processes on the networks, and their impact on failures.

We will learn that failures reality failures follow reproducible laws, that can be quantified and even predicted using the tools of network science.

NETWORKS AT THE HEART OF

[adj., v. kuh m-pleks, kom-pleks; n. kom-pleks] –adjective

- composed of many interconnected parts; compound; composite: a complex highway system.
- characterized by a very complicated or involved arrangement of parts, units, etc.: complex machinery.
- so complicated or intricate as to be hard to understand or deal with: a complex problem.
`Source: Dictionary.com`

a scientific theory which asserts that some systems display behavioral phenomena that are completely inexplicable by any conventional analysis of the systems’ constituent parts. These phenomena, commonly referred to as emergent behaviour, seem to occur in many complex systems involving living organisms, such as a stock market or the human brain. Source: John L. Casti, Encyclopædia Britannica

- Social graph
- Organization
- Brain
- finantial network
- business
- Internet
- Genes

Behind each system studied in complexity there is an intricate wiring diagram, or a network, that defines the interactions between the component.

Graph theory: 1735, Euler

Social Network Research: 1930s, Moreno

Communication networks/internet: 1960s

Ecological Networks: May, 1979.

While the study of networks has a long history from graph theory to sociology, **the modern chapter of network science emerged only during the first decade of the 21st century, following the publication of two seminal papers in 1998 and 1999**.

The explosive interest in network science is well documented by the citation pattern of two classic network papers, the 1959 paper by Paul Erdos and Alfréd Rényi that marks the beginning of the study of random networks in graph theory [4] and the 1973 paper by Mark Granovetter, the most cited social network paper [5].

Both papers were hardly or only moderately cited before 2000. The explosive growth of citations to these papers in the 21st century documents the emergence of network science, drawing a new, interdisciplinary audience to these classic publications.

- Movie Actor Network, 1998;
- World Wide Web, 1999.
- C elegans neural wiring diagram 1990
- Citation Network, 1998
- Metabolic Network, 2000;
- PPI network, 2001

The architecture of networks emerging in various domains of science, nature, and technology are more similar to each other than one would have expected.

- Interdisciplinary
- Empirical
- Quantitative and Mathematical
- Computational

The human genome project, completed in 2001, offered the first comprehensive list of all human genes.

- Yet, to fully understand how our cells function, and the origin of disease, we need accurate maps that tell us how these genes and other cellular components interact with each other.

Terrorism is one of the maladies of the 21st century, absorbing significant resources to combat it worldwide.

**Network thinking**is increasingly present in the arsenal of various law enforcement agencies in charge of limiting terrorist activities.- It is used to disrupt the financial network of terrorist organizations, to map terrorist networks, and to uncover the role of their members and their capabilities.
- While much of the work in this area is classified, several success stories have surfaced. Examples include the use of social networks to capture Saddam Hussein or
- the capture of the individuals behind the March 11, 2004 Madrid train bombings through the examination of the mobile call network.

While the H1N1 pandemic was not as devastating as it was feared at the beginning of the outbreak in 2009, it gained a special role in the history of epidemics: it was the first pandemic whose course and time evolution was accurately predicted months before the pandemic reached its peak.

- This was possible thanks to fundamental advances in understanding the role of networks in the spread of viruses.
- Indeed, before 2000 epidemic modeling was dominated by compartment models, assuming that everyone can infect everyone else one word the same socio-physical compartment.
- The emergence of a network-based framework has fundamentally changed this, offering a new level of predictability in epidemic phenomena.

In January 2010 network science tools have predicted the conditions necessary for the emergence of viruses spreading through mobile phones.

`- The first major mobile epidemic outbreak that started in the fall of 2010 in China, infecting over 300,000 phones each day, closely followed the predicted scenario. `

The human brain, consisting of hundreds of billions of interlinked neurons, is one of the least understood networks from the perspective of network science.

The reason is simple:

- we lack maps telling us which neurons link to each other.
- The only fully mapped neural map available for research is that of the C.Elegans worm, with only 300 neurons.

Driven by the potential impact of such maps, in 2010 the **National Institutes of Health** has initiated the Connectome project, aimed at developing the technologies that could provide an accurate neuron-level map of mammalian brains.

- If a graph has more than two nodes of odd degree, there is no path.
- If a graph is connected and has no odd degree nodes, it has at least one path.

COMPONENTS OF A COMPLEX SYSTEM

- components: nodes, vertices N
- interactions: links, edges L
- system: network, graph (N,L)

network often refers to real systems

- www,
- social network
- metabolic network.

Language: (Network, node, link)

graph: mathematical representation of a network

- web graph,
- social graph (a Facebook term)

Language: (Graph, vertex, edge)

The choice of the proper network representation determines our ability to use network theory successfully.

In some cases there is a unique, unambiguous representation. In other cases, the representation is by no means unique. For example, the way we assign the links between a group of individuals will determine the nature of the question we can study.

If you connect individuals that work with each other, you will explore the professional network.

If you connect individuals based on their first name (all Peters connected to each other), you will be exploring what?

In [174]:

```
%matplotlib inline
import matplotlib.pyplot as plt
import networkx as nx
Gu = nx.Graph()
for i, j in [(1, 2), (1, 4), (4, 2), (4, 3)]:
Gu.add_edge(i,j)
nx.draw(Gu, with_labels = True)
```

In [173]:

```
import networkx as nx
Gd = nx.DiGraph()
for i, j in [(1, 2), (1, 4), (4, 2), (4, 3)]:
Gd.add_edge(i,j)
nx.draw(Gd, with_labels = True)
```

In [175]:

```
nx.draw(Gu, with_labels = True)
```

In [177]:

```
nx.draw(Gd, with_labels = True)
```

For a sample of N values: $x_1, x_2, ..., x_N$:

For a sample of N values: $x_1, x_2, ..., x_N$:

For a sample of N values: $x_1, x_2, ..., x_N$:

In [20]:

```
import numpy as np
x = [1, 1, 1, 2, 2, 3]
np.mean(x), np.sum(x), np.std(x)
```

Out[20]:

(1.6666666666666667, 10, 0.7453559924999299)

In [22]:

```
plt.hist(x)
plt.show()
```

In [113]:

```
from collections import defaultdict, Counter
freq = defaultdict(int)
for i in x:
freq[i] +=1
freq
```

Out[113]:

defaultdict(int, {1: 3, 2: 2, 3: 1})

In [37]:

```
freq_sum = np.sum(freq.values())
freq_sum
```

Out[37]:

6

In [38]:

```
px = [float(i)/freq_sum for i in freq.values()]
px
```

Out[38]:

[0.5, 0.3333333333333333, 0.16666666666666666]

In [41]:

```
plt.plot(freq.keys(), px, 'r-o')
plt.show()
```

$A_{ij} =1$ if there is a link between node i and j

$A_{ij} =0$ if there is no link between node i and j

In [182]:

```
plt.figure(1)
plt.subplot(121)
pos = nx.spring_layout(Gu) #定义一个布局，此处采用了spring布局方式
nx.draw(Gu, pos, with_labels = True)
plt.subplot(122)
nx.draw(Gd, pos, with_labels = True)
```

$A_{ij} =1$ if there is a link between node i and j

$A_{ij} =0$ if there is no link between node i and j

$A_{ij} =1$ if there is a link between node i and j

$A_{ij} =0$ if there is no link between node i and j

Note that for a directed graph the matrix is not symmetric.

bipartite graph (or bigraph) is a graph whose nodes can be divided into two disjoint sets U and V such that every link connects a node in U to one in V; that is, U and V are independent sets.

- Hits algorithm
- recommendation system

A path is a sequence of nodes in which each node is adjacent to the next one

`- In a directed network, the path can follow only the direction of an arrow. `

The distance (shortest path, geodesic path) between two nodes is defined as the number of edges along the shortest path connecting them.

`- *If the two nodes are disconnected, the distance is infinity.`

**Diameter $d_{max}$** is the maximum distance between any pair of nodes in the graph.

The path with the shortest length between two nodes (distance).

- for a connected graph: where $d_{ij}$ is the distance from node i to node j

- In an undirected graph $d_{ij} =d_{ji}$ , so we only need to count them once:

A path with the same start and end node.

has a path from each node to every other node and vice versa (e.g. AB path and BA path).

it is connected if we disregard the edge directions.

Strongly connected components can be identified, but not every node is partof a nontrivial strongly connected component.

- In-component: nodes that can reach the scc,
- Out-component: nodes that can be reached from the scc.

In [215]:

```
G1 = nx.complete_graph(4)
pos = nx.spring_layout(G1) #定义一个布局，此处采用了spring布局方式
nx.draw(G1, pos = pos, with_labels = True)
```

In [206]:

```
print(nx.transitivity(G1))
```

1.0

In [216]:

```
G2 = nx.Graph()
for i, j in [(1, 2), (1, 3), (1, 0), (3, 0)]:
G2.add_edge(i,j)
nx.draw(G2,pos = pos, with_labels = True)
```

In [198]:

```
print(nx.transitivity(G2))
```

0.6

In [219]:

```
G3 = nx.Graph()
for i, j in [(1, 2), (1, 3), (1, 0)]:
G3.add_edge(i,j)
nx.draw(G3, pos =pos, with_labels = True)
```

In [200]:

```
print(nx.transitivity(G3))
```

0.0

THREE CENTRAL QUANTITIES IN NETWORK SCIENCE

- A. Degree distribution: $p_k$
- B. Path length: $
$ - C. Clustering coefficient: $C_i$

- Discovering, Modeling, Verification
- WATTSDJ,STROGATZSH.Collective dynamics of‘small-world’ networks. Nature, 1998, 393(6684): 440–442.
- BARABÁSI A-L, ALBERT R. Emergence of scaling in random networks. Science, 1999, 286(5439): 509-512.

Fan Chung & Linyuan Lu, The average distance in random graphs with given expected degree,. PNAS, 19, 15879-15882 (2002).

A.-L.Barabási,R.Albert,H.Jeong Mean-field theory for scale-free random networks. Physica A 272, 173–187 (1999).

- Community detection
- Link prediction
- Recommendation algorithms

Controllability of Complex Networks

Liu Y Y, Slotine J J, Barabási A L. Nature, 2011, 473(7346): 167-173.

- Barabasi 2016 Network Science. Cambridge
- 汪小帆、李翔、陈关荣 2012 网络科学导论. 高等教育出版社
- 梅拉妮·米歇尔 2011 复杂,湖南科学技术出版社
- 菲利普-鲍尔 2004 预知社会：群体行为的内在法则，当代中国出版社
- 巴拉巴西 2007 链接：网络新科学 湖南科技出版社

In [ ]:

```
```