The most recent work related with ours is DeepWalk [16],
which deploys a truncated random walk for social network
embedding. Although empirically effective, the DeepWalk
does not provide a clear objective that articulates what net-
work properties are preserved. Intuitively, DeepWalk ex-
pects nodes with higher second-order proximity yield similar
low-dimensional representations, while the LINE preserves
both first-order and second-order proximities. DeepWalk
uses random walks to expand the neighborhood of a vertex,
which is analogical to a depth-first search. We use a breadth-
first search strategy, which is a more reasonable approach to
the second-order proximity. Practically, DeepWalk only ap-
plies to unweighted networks, while our model is applicable
for networks with both weighted and unweighted edges.
In Section 5, we empirically compare the proposed model
with these methods using various real world networks.
3. PROBLEM DEFINITION
We formally define the problem of large-scale information
network embedding using first-order and second-order prox-
imities. We first define an information network as follows:
Definition 1. (Information Network) An informa-
tion network is defined as G = (V, E), where V is the set
of vertices, each representing a data object and E is the
set of edges between the vertices, each representing a re-
lationship between two data objects. Each edge e ∈ E is
an ordered pair e = (u, v) and is associated with a weight
w
uv
> 0, which indicates the strength of the relation. If G
is undirected, we have (u, v) ≡ (v, u) and w
uv
≡ w
vu
; if G
is directed, we have (u, v) 6≡ (v, u) and w
uv
6≡ w
vu
.
In practice, information networks can be either directed
(e.g., citation networks) or undirected (e.g., social network
of users in Facebook). The weights of the edges can be either
binary or take any real value. Note that while negative edge
weights are possible, in this study we only consider non-
negative weights. For example, in citation networks and
social networks, w
uv
takes binary values; in co-occurrence
networks between different objects, w
uv
can take any non-
negative value. The weights of the edges in some networks
may diverge as some objects co-occur many times while oth-
ers may just co-occur a few times.
Embedding an information network into a low-dimensional
space is useful in a variety of applications. To conduct the
embedding, the network structures must be preserved. The
first intuition is that the local network structure, i.e., the
local pairwise proximity between the vertices, must be pre-
served. We define the local network structures as the first-
order proximity between the vertices:
Definition 2. (First-order Proximity) The first-order
proximity in a network is the local pairwise proximity be-
tween two vertices. For each pair of vertices linked by an
edge (u, v), the weight on that edge, w
uv
, indicates the first-
order proximity between u and v. If no edge is observed
between u and v, their first-order proximity is 0.
The first-order proximity usually implies the similarity of
two nodes in a real-world network. For example, people who
are friends with each other in a social network tend to share
similar interests; pages linking to each other in World Wide
Web tend to talk about similar topics. Because of this im-
portance, many existing graph embedding algorithms such
as IsoMap, LLE, Laplacian eigenmap, and graph factoriza-
tion have the objective to preserve the first-order proximity.
However, in a real world information network, the links
observed are only a small proportion, with many others
missing [10]. A pair of nodes on a missing link has a zero
first-order proximity, even though they are intrinsically very
similar to each other. Therefore, first-order proximity alone
is not sufficient for preserving the network structures, and
it is important to seek an alternative notion of proximity
that addresses the problem of sparsity. A natural intuition
is that vertices that share similar neighbors tend to be sim-
ilar to each other. For example, in social networks, people
who share similar friends tend to have similar interests and
thus become friends; in word co-occurrence networks, words
that always co-occur with the same set of words tend to
have similar meanings. We therefore define the second-order
proximity, which complements the first-order proximity and
preserves the network structure.
Definition 3. (Second-order Proximity) The second-
order proximity between a pair of vertices (u, v) in a net-
work is the similarity between their neighborhood network
structures. Mathematically, let p
u
= (w
u,1
, . . . , w
u,|V |
) de-
note the first-order proximity of u with all the other vertices,
then the second-order proximity between u and v is deter-
mined by the similarity between p
u
and p
v
. If no vertex is
linked from/to both u and v, the second-order proximity
between u and v is 0.
We investigate both first-order and second-order proxim-
ity for network embedding, which is defined as follows.
Definition 4. (Large-scale Information Network Em-
bedding) Given a large network G = (V, E), the problem
of Large-scale Information Network Embedding aims
to represent each vertex v ∈ V into a low-dimensional space
R
d
, i.e., learning a function f
G
: V → R
d
, where d |V |.
In the space R
d
, both the first-order proximity and the
second-order proximity between the vertices are preserved.
Next, we introduce a large-scale network embedding model
that preserves both first- and second-order proximities.
4. LINE: LARGE-SCALE INFORMATION
NETWORK EMBEDDING
A desirable embedding model for real world information
networks must satisfy several requirements: first, it must
be able to preserve both the first-order proximity and the
second-order proximity between the vertices; second, it must
scale for very large networks, say millions of vertices and bil-
lions of edges; third, it can deal with networks with arbitrary
types of edges: directed, undirected and/or weighted. In this
section, we present a novel network embedding model called
the “LINE,” which satisfies all the three requirements.
4.1 Model Description
We describe the LINE model to preserve the first-order
proximity and second-order proximity separately, and then
introduce a simple way to combine the two proximity.
4.1.1 LINE with First-order Proximity
The first-order proximity refers to the local pairwise prox-
imity between the vertices in the network. To model the