# Network Propagation

written by Eric J. Ma on 2017-10-22

## Introduction

I recently read a review paper on network propagation. It looks like a very useful thing in data science, particularly where networks are involved, and I wanted to share it with everybody.

Before I go on, I will start off by assuming we have a dataset modelled as a "network", or a "graph", in which the nodes are entities in the graph, and edges are relationships between those entities. In the context of my work, I encounter biological networks of many kinds - gene-gene interactions, protein-protein interactions, protein-RNA interactions, evolutionary networks, and more.

What kind of problems can network propagation solve? The problem class generally follows this logic: I start with some nodes of interest, and I'm most interested in finding other nodes of interest based on these "seed" nodes. Knowing that path-based methods can often end up over-prioritizing highly connected nodes, we need a different principled method for finding these nodes. One way would be to take a random walk on the graph, and find out how often we land on a particular set of nodes on the graph. This random walk is what we call "network propagation".

## How network propagation works

Network propagation follows this intuition: Imagine nodes contain information, and edges dictate which nodes information can be shared with. Network propagation shares the information with other nodes, according to the following rules:

1. I can only share by copying to my neighbours in the graph,
2. I must share everything that I have with my neighbours, and
3. Exchanges in the graph happen simultaneously.

An animation of how this works is shown below.

If you take my DataCamp course, you'll learn that networks can be represented as matrices on a computer. The adjacency matrix can be a 1/0 (i.e. binary) matrix that describes how nodes (rows and columns) are connected, and nodes can have their own vector of information (i.e. whether they are "seed" nodes or not). As it turns out, network propagation has a very nice matrix representation:

## How to implement network propagation

So, how do we do network propagation with NetworkX, numpy and Python? Let's take a look at code. Firstly, how do you convert a graph to a numpy matrix?

# Assume we have a graph G. Convert it to a numpy matrix.
In : A = nx.to_numpy_matrix(G)

In : A
Out:
matrix([[ 0.,  0.,  1.,  1.,  1.],
[ 0.,  0.,  0.,  1.,  0.],
[ 1.,  0.,  0.,  0.,  1.],
[ 1.,  1.,  0.,  0.,  1.],
[ 1.,  0.,  1.,  1.,  0.]])


This basically corresponds to the following graph: Great! Now, let's mark out some "seed" nodes. These are the "nodes" of interest. To represent this, we use a binary vector - each slot in the vector represents one node in the graph. Let's start by highlighting two nodes, the second and third one:

In : M = np.array([0, 1, 1, 0, 0]).reshape(A.shape, 1)


The .reshape(A.shape, 1) is done for convenience, so that each axis is aligned appropriately for the following matrix multiplication:

In : A @ M
matrix([[ 1.],
[ 0.],
[ 0.],
[ 1.],
[ 1.]])


After the end of one round, nodes 2 and 3 have no information held on them, but their neighbours do. Let's do two rounds of propagation, rather than just one.

In : A @ (A @ M)
matrix([[ 2.],
[ 1.],
[ 2.],
[ 2.],
[ 2.]])


After two rounds of information, information is shared back with the original nodes, but not necessarily evenly - it all depends on the connectivity of each node.

It'll get really verbose doing 100 rounds; imagine doing A @ (A @ (A @ (A @ .....))). Instead of doing that, a recursive function may be better. At the same time, with many rounds of network propagation, we will end up with really large numbers. Therefore, it will be helpful to also normalize the result of matrix multiplication.

In : def normalize(arr):
...:     return arr / arr.sum()

In : def recursive_propagate(A, M, n):
...:     if n > 0:
...:         return recursive_propagate(A, normalize(A @ M), n-1)
...:     else:
...:         return normalize(A @ M)
...:


On a previous blog post, I've alluded to the fact that recursive functions are pretty darn useful!

## Result

So, let's take a look at the result of a few rounds of propagation. The first thing that I'd like to highlight is that the information propagation process converges on a final result fairly quickly. The second thing I'd like to highlight is that the purple nodes (there are two lines!), which weren't highlighted at the start, ended up having the highest propagation scores, followed by the red node.

For those who are familiar with Markov chains, I believe network propagation has strong parallels with that.

## Conclusion

Network propagation is conceptually simple, and easy to implement in Python.