In the previous post, the convolution of the graph Laplacian is defined in its graph Fourier space as outlined in the paper of Bruna et. al. (arXiv:1312.6203) However, the eigenmodes of the graph Laplacian are not ideal because it makes the bases to be graph-dependent. A lot of works were done in order to solve this problem, with the help of various special functions to express the filter functions. Examples include Chebyshev polynomials and Cayley transform.

## ChebNet

Kipf and Welling proposed the ChebNet (arXiv:1609.02907) to approximate the filter using Chebyshev polynomial, using the result proved by Hammond et. al. (arXiv:0912.3848) With a convolutional layer $g_{\theta} \circ x$, instead of computing it directly by $\Phi^T g_{\theta} \Phi x$, it approximates the filter in terms of Chebyshev polynomial:

$g_{\theta} (\Lambda) \approx \sum_{k=0}^K \theta_k T_k(\Lambda)$,

where $T_k(x)$ are the Chebyshev polynomials. It has been proved by Hammond et. al. that by well truncating this representation, Chebyshev polynomials are very good to approximate the function. Then the convolution is:

$g_{\theta} \circ x \approx \sum_{k=0}^K \theta_k T_k (\Lambda) x$.

Here, $\theta$’s are the parameters to be trained. This fixed the bases of the representation, and it speeds up computation. The disadvantage is that eigenvalues are clusters in a few values with large gaps.

## CayleyNet

The problem of ChebNet led to the work of Levie et. al., (arXiv:1705.07664) who proposed another approximation is used using Cayley transform. They made use of the Cayley function:

$C(\lambda) = \frac{\lambda - i}{\lambda + i}$,

which is a bijective function from $\mathbb{R}^+$ to complex unit half-circle. Instead of Chebyshev polynomials, it approximates the filter as:

$g(\lambda) = c_0 + \sum_{j=1}^r \left[ c_j C^j (h \lambda) + c_j^{\ast} C^{j \ast} (h \lambda) \right]$,

where $c_0$ is real and other $c_j$’s are generally complex, and $h$ is a zoom parameter, and $\lambda$’s are the eigenvalues of the graph Laplacian. Tuning $h$ makes one find the best zoom that spread the top eigenvalues. $c$‘s are computed by training. This solves the problem of unfavorable clusters in ChebNet.

## MotifNet

All previous works are undirected graph. How do we deal with directed graph with an asymmetric graph Laplacian? Benson, Gleich, Leskovec published an important work on Science in 2016 (arXiv:1612.08447) to address this problem. Their approach is reducing a directed graph to a higher order structure called network motifs. There are 13 network motifs. For each network motif, one can define an adjacency matrix for that motif by $\mathcal{M}_k$, with elements $\mathcal{M}_{k, ij}$ being the number of motifs in the graph the the edge $(i, j)$ that it belongs to.

Then one computes 13 graph Laplacians from these 13 adjacency matrices. These graph Laplacians are symmetric, like those of undirected graphs. Then any filters can be approximated by the following multivariate matrix polynomial, as suggested by Monti, Otness, and Bronstein in their MotifNet paper (arXiv:1802.01572):

$f_{\Theta} (\Delta_1, \dots, \Delta_k) = \sum_{j=0}^P \sum_{k_1, \dots, k_j \in {1, \ldots, K}} \theta_{\theta_1, \ldots, \theta_k} \Delta_{k_1}, \ldots, \Delta_{k_j}$.

Applications of image processing, citation networks etc.

Suggested by some friends, I have been reading graph convolutional neural network. This is not a recent interest, as I have been interested in some graph-related problems (as I developed the graphflow package) and topological data analysis (TDA, as I developed the mogutda package). Graph theory is not a new field as well, but it is interesting to see how it is being applied in deep learning, as a lot of real-world relational data can be expressed in terms of graphs. (Examples: word relations, organizational structure, genes, internet etc.) With the increasing attention to neo4j, we know graph is coming to be hot.

It would be helpful to review some basic graph theory:

• A graph is represented by $\mathcal{G} = (V, E)$, where $V$ and $E$ are the nodes (vertices) and edges respectively.
• A graph can be directed or undirected. In graph convolutional neural network, they are undirected usually.
• The adjacency matrix $A$ describes how nodes are connected: $A_{ij} = 1$ if there is an edge connecting from node $i$ to node $j$, and $0$ otherwise. $A$ is a symmetric matrix for an undirected graph.
• The incidence matrix $B$ is another way to describe how nodes are connected: $B_{ij} = 1$ if a node $i$ is connected with edge $j$. This is useful for undirected graph.
• The degree matrix $D$ is a diagonal matrix, with elements $D_{ii}$ denotes the number of neighbors for node $i$ in undirected matrix.
• The function acting on the nodes is called the filter.
• The graph Laplacian, or Kirchhoff matrix, is defined by $L = D - A$, and the normalized graph Laplacian is $\tilde{L} = I - D^{-\frac{1}{2}} A D^{-\frac{1}{2}}$.

The graph Laplacian is the most important matrix in graph convolutional neural network. It is analogous to the Laplacian operator in Euclidean space, $\nabla^2$. The reader can easily verify this by constructing a graph of 2D lattice and compute the graph Laplacian matrix, and find that it is the same as the discretized Laplacian operator.

We can also get some insights from the Euclidean analogue. In physics, the solution to the Laplacian equation is harmonic: the basis of the solution can be described in the spectral/Fourier space, as:

$\nabla^2 \psi = - \omega^2 \psi$,

And $\psi \propto e^{\pm i \omega x}$. In graph convolutional neural network, as Bruna et. al. suggested in 2013, the graph is calculated in the graph Fourier space, instead of directly dealing with the Laplacian matrix in all layers of network.

On the other hand, we know that for the convolution

$(f \circ g) (x) = \int dy f(x-y) g(y)$,

its Fourier transform is given by

$\tilde{( f \circ g)} (\omega) = \tilde{f} (\omega) \tilde{g} (\omega)$.

In Fourier space, the convolution of two functions are just their products. Similarly, in graph convolutional neural network, convolutions can be computed in the Fourier space as the mere product of two filters in the Fourier space. More specifically, for finding the convolution of the filters $f$ and $g$, with $\Phi$ being the unitary eigenmatrix,

$f \circ g = \Phi ((\Phi^T g) \circ (\Phi^T f)) = \Phi \textbf{diag}(g_1, g_2, \ldots, g_n) f$.

However, such general description is basis-dependent, and it is still computationally expensive. More work has been proposed to smooth the representation, which will be covered in the upcoming blogs.

A side note, the readers can verify themselves that

$\sum_{ij} A_{ij} | f_i - g_j |^2 = f \tilde{L} g$

In these few days, Facebook published a new research paper, regarding the use of sequence to sequence (seq2seq) model for machine translation. What is special about this seq2seq model is that it uses convolutional neural networks (ConvNet, or CNN), instead of recurrent neural networks (RNN).

The original seq2seq model is implemented with Long Short-Term Memory (LSTM) model, published by Google.(see their paper) It is basically a character-based model that generates texts according to a sequence of input characters. And the same author constructed a neural conversational model, (see their paper) as mentioned in a previous blog post. Daewoo Chong, from Booz Allen Hamilton, presented its implementation using Tensorflow in DC Data Education Meetup on April 13, 2017. Johns Hopkins also published a spell correction algorithm implemented in seq2seq. (see their paper) The real advantage of RNN over CNN is that there is no limit about the size of the tokens input or output.

While the fixing of the size of vectors for CNN is obvious, using CNN serves the purpose of limiting the size of input vectors, and thus limiting the size of contexts. This limits the contents, and speeds up the training process. RNN is known to be trained slow. Facebook uses this CNN seq2seq model for their machine translation model. For more details, take a look at their paper and their Github repository.

On November 21, 2016, the Python package shorttext’ was published. Until today, more than seven versions have been published. There have been a drastic architecture change, but the overall purpose is still the same, as summarized in the first introduction entry:

This package shorttext‘ was designed to tackle all these problems… It contains the following features:

• example data provided (including subject keywords and NIH RePORT);
• text preprocessing;
• pre-trained word-embedding support;
• gensim topic models (LDA, LSI, Random Projections) and autoencoder;
• topic model representation supported for supervised learning using scikit-learn;
• cosine distance classification; and
• neural network classification (including ConvNet, and C-LSTM).

And since the first version, there have been updates, as summarized in the documention (News):

## Version 0.3.3 (Apr 19, 2017)

• Deleted CNNEmbedVecClassifier.

## Version 0.3.2 (Mar 28, 2017)

• Bug fixed for gensim model I/O;
• Console scripts update;
• Neural networks up to Keras 2 standard (refer to this).

## Version 0.3.1 (Mar 14, 2017)

• Compact model I/O: all models are in single files;
• Implementation of stacked generalization using logistic regression.

## Version 0.2.1 (Feb 23, 2017)

• Removal attempts of loading GloVe model, as it can be run using gensim script;
• Confirmed compatibility of the package with tensorflow;
• Use of spacy for tokenization, instead of nltk;
• Use of stemming for Porter stemmer, instead of nltk;
• Removal of nltk dependencies;
• Simplifying the directory and module structures;
• Module packages updated.

Although there are still additions that I would love to add, but it would not change the overall architecture. I may add some more supervised learning algorithms, but under the same network. The upcoming big additions will be generative models or seq2seq models, but I do not see them coming in the short term. I will add corpuses.

I may add tutorials if I have time.

I am thankful that there is probably some external collaboration with other Python packages. Some people have already made some useful contributions. It will be updated if more things are confirmed.

There has been a lot of methods for natural language processing and text mining. However, in tweets, surveys, Facebook, or many online data, texts are short, lacking data to build enough information. Traditional bag-of-words (BOW) model gives sparse vector representation.

Semantic relations between words are important, because we usually do not have enough data to capture the similarity between words. We do not want “drive” and “drives,” or “driver” and “chauffeur” to be completely different.

The relation between or order of words become important as well. Or we want to capture the concepts that may be correlated in our training dataset.

We have to represent these texts in a special way and perform supervised learning with traditional machine learning algorithms or deep learning algorithms.

This package shorttext‘ was designed to tackle all these problems. It is not a completely new invention, but putting everything known together. It contains the following features:

• example data provided (including subject keywords and NIH RePORT);
• text preprocessing;
• pre-trained word-embedding support;
• gensim topic models (LDA, LSI, Random Projections) and autoencoder;
• topic model representation supported for supervised learning using scikit-learn;
• cosine distance classification; and
• neural network classification (including ConvNet, and C-LSTM).

Readers can refer this to the documentation.

There are situations that we deal with short text, probably messy, without a lot of training data. In that case, we need external semantic information. Instead of using the conventional bag-of-words (BOW) model, we should employ word-embedding models, such as Word2Vec, GloVe etc.

Suppose we want to perform supervised learning, with three subjects, described by the following Python dictionary:

classdict={'mathematics': ['linear algebra',
'topology',
'algebra',
'calculus',
'variational calculus',
'functional field',
'real analysis',
'complex analysis',
'differential equation',
'statistics',
'statistical optimization',
'probability',
'stochastic calculus',
'numerical analysis',
'differential geometry'],
'physics': ['renormalization',
'classical mechanics',
'quantum mechanics',
'statistical mechanics',
'functional field',
'path integral',
'quantum field theory',
'electrodynamics',
'condensed matter',
'particle physics',
'topological solitons',
'astrophysics',
'spontaneous symmetry breaking',
'atomic molecular and optical physics',
'quantum chaos'],
'theology': ['divine providence',
'soteriology',
'anthropology',
'pneumatology',
'Christology',
'Holy Trinity',
'eschatology',
'scripture',
'ecclesiology',
'predestination',
'divine degree',
'creedal confessionalism',
'scholasticism',
'prayer',
'eucharist']}


And we implemented Word2Vec here. To add external information, we use a pre-trained Word2Vec model from Google, downloaded here. We can use it with Python package gensim. To load it, enter

from gensim.models import Word2Vec


How do we represent a phrase in Word2Vec? How do we do the classification? Here I wrote two classes to do it.

#### Average

We can represent a sentence by summing the word-embedding representations of each word. The class, inside SumWord2VecClassification.py, is coded as follow:

from collections import defaultdict

import numpy as np
from nltk import word_tokenize
from scipy.spatial.distance import cosine

from utils import ModelNotTrainedException

class SumEmbeddedVecClassifier:
def __init__(self, wvmodel, classdict, vecsize=300):
self.wvmodel = wvmodel
self.classdict = classdict
self.vecsize = vecsize
self.trained = False

def train(self):
for classtype in self.classdict:
for shorttext in self.classdict[classtype]:
self.trained = True

def shorttext_to_embedvec(self, shorttext):
vec = np.zeros(self.vecsize)
tokens = word_tokenize(shorttext)
for token in tokens:
if token in self.wvmodel:
vec += self.wvmodel[token]
norm = np.linalg.norm(vec)
if norm!=0:
vec /= np.linalg.norm(vec)
return vec

def score(self, shorttext):
if not self.trained:
raise ModelNotTrainedException()
vec = self.shorttext_to_embedvec(shorttext)
scoredict = {}
try:
scoredict[classtype] = 1 - cosine(vec, self.addvec[classtype])
except ValueError:
scoredict[classtype] = np.nan
return scoredict


Here the exception ModelNotTrainedException is just an exception raised if the model has not been trained yet, but scoring function was called by the user. (Codes listed in my Github repository.) The similarity will be calculated by cosine similarity.

Such an implementation is easy to understand and carry out. It is good enough for a lot of application. However, it has the problem that it does not take the relation between words or word order into account.

#### Convolutional Neural Network

To tackle the problem of word relations, we have to use deeper neural networks. Yoon Kim published a well cited paper regarding this in EMNLP in 2014, titled “Convolutional Neural Networks for Sentence Classification.” The model architecture is as follow: (taken from his paper)

Each word is represented by an embedded vector, but neighboring words are related through the convolutional matrix. And MaxPooling and a dense neural network were implemented afterwards. His paper involves multiple filters with variable window sizes / spatial extent, but for our cases of short phrases, I just use one window of size 2 (similar to dealing with bigram). While Kim implemented using Theano (see his Github repository), I implemented using keras with Theano backend. The codes, inside CNNEmbedVecClassification.py, are as follow:

import numpy as np
from keras.layers import Convolution1D, MaxPooling1D, Flatten, Dense
from keras.models import Sequential
from nltk import word_tokenize

from utils import ModelNotTrainedException

class CNNEmbeddedVecClassifier:
def __init__(self,
wvmodel,
classdict,
n_gram,
vecsize=300,
nb_filters=1200,
maxlen=15):
self.wvmodel = wvmodel
self.classdict = classdict
self.n_gram = n_gram
self.vecsize = vecsize
self.nb_filters = nb_filters
self.maxlen = maxlen
self.trained = False

def convert_trainingdata_matrix(self):
classlabels = self.classdict.keys()
lblidx_dict = dict(zip(classlabels, range(len(classlabels))))

# tokenize the words, and determine the word length
phrases = []
indices = []
for label in classlabels:
for shorttext in self.classdict[label]:
category_bucket = [0]*len(classlabels)
category_bucket[lblidx_dict[label]] = 1
indices.append(category_bucket)
phrases.append(word_tokenize(shorttext))

# store embedded vectors
train_embedvec = np.zeros(shape=(len(phrases), self.maxlen, self.vecsize))
for i in range(len(phrases)):
for j in range(min(self.maxlen, len(phrases[i]))):
train_embedvec[i, j] = self.word_to_embedvec(phrases[i][j])
indices = np.array(indices, dtype=np.int)

return classlabels, train_embedvec, indices

def train(self):
# convert classdict to training input vectors
self.classlabels, train_embedvec, indices = self.convert_trainingdata_matrix()

# build the deep neural network model
model = Sequential()
filter_length=self.n_gram,
border_mode='valid',
activation='relu',
input_shape=(self.maxlen, self.vecsize)))
model.compile(loss='categorical_crossentropy', optimizer='rmsprop')

# train the model
model.fit(train_embedvec, indices)

# flag switch
self.model = model
self.trained = True

def word_to_embedvec(self, word):
return self.wvmodel[word] if word in self.wvmodel else np.zeros(self.vecsize)

def shorttext_to_matrix(self, shorttext):
tokens = word_tokenize(shorttext)
matrix = np.zeros((self.maxlen, self.vecsize))
for i in range(min(self.maxlen, len(tokens))):
matrix[i] = self.word_to_embedvec(tokens[i])
return matrix

def score(self, shorttext):
if not self.trained:
raise ModelNotTrainedException()

# retrieve vector
matrix = np.array([self.shorttext_to_matrix(shorttext)])

# classification using the neural network
predictions = self.model.predict(matrix)

# wrangle output result
scoredict = {}
for idx, classlabel in zip(range(len(self.classlabels)), self.classlabels):
scoredict[classlabel] = predictions[0][idx]
return scoredict
`

The output is a vector of length equal to the number of class labels, 3 in our example. The elements of the output vector add up to one, indicating its score, and a nature of probability.

#### Evaluation

A simple cross-validation to the example data set does not tell a difference between the two algorithms:

However, we can test the algorithm with a few examples:

Example 1: “renormalization”

• Average: {‘mathematics’: 0.54135105096749336, ‘physics’: 0.63665460856632494, ‘theology’: 0.31014049736087901}
• CNN: {‘mathematics’: 0.093827009201049805, ‘physics’: 0.85451591014862061, ‘theology’: 0.051657050848007202}

As renormalization was a strong word in the training data, it gives an easy result. CNN can distinguish much more clearly.

Example 2: “salvation”

• Average: {‘mathematics’: 0.14939650156482298, ‘physics’: 0.21692765541184023, ‘theology’: 0.5698233329716329}
• CNN: {‘mathematics’: 0.012395491823554039, ‘physics’: 0.022725773975253105, ‘theology’: 0.96487873792648315}

“Salvation” is not found in the training data, but it is closely related to “soteriology,” which means the doctrine of salvation. So it correctly identifies it with theology.

Example 3: “coffee”

• Average: {‘mathematics’: 0.096820211601723272, ‘physics’: 0.081567332119268032, ‘theology’: 0.15962682945135631}
• CNN: {‘mathematics’: 0.27321341633796692, ‘physics’: 0.1950736939907074, ‘theology’: 0.53171288967132568}

Coffee is not related to all subjects. The first architecture correctly indicates the fact, but CNN, with its probabilistic nature, has to roughly equally distribute it (but not so well.)

The code can be found in my Github repository: stephenhky/PyShortTextCategorization. (This repository has been updated since this article was published. The link shows the version of the code when this appeared online.)

At the end of last month, Yahoo opened the sources of training a model to classify not suitable/safe for work (NSFW) images, particularly pornographic images, using convolutional neural network (CNN). It was implemented with Caffe. Users need to supply the training data, positive being the NSFW images, and negative being the suitable/safe for work (SFW) images, to train the model. The model takes an image as the input, and output a score between 0 and 1.

The codes are available on Github, with the README.md about the installation.