Deep learning, and in particular deep neural networks, is one of the most popular and powerful tools of modern data science. Using analogies from neuroscience, neural networks provide a framework for building predictive models whose predictive power is seemingly limitless. They are at the heart of modern “artificial intelligence” wonders like smart assistants (Siri, Cortana, Google Assistant), smart speakers, self-driving cars and most modern image detection algorithms (Google image search, IBM Watson).
This post will give a mathematically-slanted overview to neural networks, with the goal of proving what could be referred to as the “Fundamental Theorem of Neural Networks”. This remarkable theorem is fairly accessible, and explains beautifully and precisely why neural networks are so powerful.
The mathematics of neural networks
A neural network is essentially a function approximation algorithm, modelled at a basic level on the behaviour of neurons in the human brain. Although it is not a particularly common way of thinking as a data scientist, from a mathematical perspective machine learning is nothing more than function approximation with a probabilistic flavour. Under the assumption that the “true” relationship between a set of features \(X\) and an output variable \(y\) can be specified by a function \(f(X, \theta)\) which may depend further on some parameters \(\theta\), the problem of machine learning can be expressed as an attempt to approximate \(f\) in a way which minimizes a specified cost function (the choice of cost function is usually motivated probabilistically).
For a given input \(X\in\mathbb{R}^n\), a neural network with a single hidden layer is a finite linear combination of the form
\[\sum_{j=1}^N\alpha_j\sigma(y_j^TX+\theta_j)\]where \(N,\alpha_j\in \mathbb{R}\), \(y_j\in\mathbb{R}^n\) and \(\sigma\) is a special type of non-linear function we will discuss later. The \(\alpha_j\)’s are called the weights, the \(\theta_j\) the biases and \(\sigma\) is called the activation function.
The above picture explains the analogy with the human brain; given an input vector $X$ (touch input from skin, audio input from the ears, etc.), signals are sent to various neurons which “activate” according to these inputs and a collection of weights. This behaviour then cascades through various layers of the network, resulting finally in an output (don’t touch the fire, say “yes”, etc.).
We will be less concerned in this article with the “machine learning” theory behind neural networks – for example I don’t want to discuss the algorithms which are used to train the networks, or techniques for feature selection or choosing the right activation function or number of “hidden layers”. Rather, we will discuss a mathematical theorem which provides a solid theoretical basis for why neural networks are so powerful.
The universal approximation theorem
Universality theorems are omnipresent in mathematics. The setup is classic, and gives the illusion of a genius mind at work, inventing a solution from thin air and then proceeding to prove it is the unique one which suits a given set of circumstances. In actuality, the workflow is entirely opposite: first a family of functions or a mathematical object is studied by its occurrences in specific examples. Through careful study of these examples, its utility becomes clear and the question of generalisation and universality arises.
So seeing the above family of functions occurring in many different places, the natural question is: what can we use them for? The answer turns out to be just about everything: in a way which can be made mathematically precise, neural networks with a single layer can be used to approximate any continuous function. Let’s state this theorem a little more clearly. The following theorem comes from a paper of Cybenko. A sigmoidal function is one which “looks” like a sigmoid: \(\sigma(x)\to1\) as \(x\to\infty\) and \(\sigma(x)\to0\) as \(x\to-\infty\).
Universal approximation theorem for neural networks (Cybenko)
Let \(\sigma\) be any continuous sigmoidal function. Then finite sums of the form
\[G(x) = \sum_{j=1}^N \alpha_j\sigma(y_j^Tx + \theta_j)\]are dense in the set \(C(I_n)\) of continuous functions on the unit cube.
For those who don’t remember their undergraduate analysis, there is a theoretical definition of density here, but for our purposes it is enough to think of it as meaning that there are “enough” of these functions to be able to use them as “building blocks” to build any function to any required level of accuracy.
The proof of this remarkable theorem uses fairly standard functional analysis techniques. First we show that a slightly weaker but easier-to-work with family of functions (called discriminatory functions) satisfy the conditions of the theorem. Then it proceeds to prove, using a standard functional analysis method, that sigmoidal functions belong to this category.