# Neural networks

tags
Machine learning

## Two-layers neural network

Mathematically, a simple two-layers neural network with relu non-linearities can be written like below. For an input vector $$x \in \mathbb{R}^D$$, $$\mathbf{a} = (a_1, \cdots, a_N)\in \mathbb{R}^M$$ are the output weights, $$\mathbf{b} = (b_1, \cdots, b_N)\in \mathbb{R}^D$$ are the input weights

$h(x) = \frac{1}{m} \sum_{i=1}^m a_i \max\{ b_i^\top x,0\},$

## Universal approximation theorem

Cybenko showed in 1989 that a neural network of arbitrary width with sigmoid activation function could approximate any continuous function (Cybenko 1989).

Barron added rates of convergence by enforcing smoothness condition on the target function (Barron 1993).

## Bibliography

1. . . "Universal Approximation Bounds for Superpositions of a Sigmoidal Function". IEEE Transactions on Information Theory 39 (3):930–45. DOI.

2. . . “Approximation by Superpositions of a Sigmoidal Function”. Mathematics of Control, Signals, and Systems 2 (4):303–14. DOI.