# Neural Networks

### From Ufldl

Consider a supervised learning problem where we have access to labeled training
examples (*x*^{(i)},*y*^{(i)}). Neural networks give a way of defining a complex,
non-linear form of hypotheses *h*_{W,b}(*x*), with parameters *W*,*b* that we can
fit to our data.

To describe neural networks, we will begin by describing the simplest possible neural network, one which comprises a single "neuron." We will use the following diagram to denote a single neuron:

This "neuron" is a computational unit that takes as input *x*_{1},*x*_{2},*x*_{3} (and a +1 intercept term), and
outputs , where is
called the **activation function**. In these notes, we will choose
to be the sigmoid function:

Thus, our single neuron corresponds exactly to the input-output mapping defined by logistic regression.

Although these notes will use the sigmoid function, it is worth noting that
another common choice for *f* is the hyperbolic tangent, or tanh, function:

Here are plots of the sigmoid and tanh functions:

The tanh(*z*) function is a rescaled version of the sigmoid, and its output range is
[ − 1,1] instead of [0,1].

Note that unlike CS221 and (parts of) CS229, we are not using the convention
here of *x*_{0} = 1. Instead, the intercept term is handled separately by the parameter *b*.

Finally, one identity that'll be useful later: If *f*(*z*) = 1 / (1 + exp( − *z*)) is the sigmoid
function, then its derivative is given by *f*'(*z*) = *f*(*z*)(1 − *f*(*z*)).
(If *f* is the tanh function, then its derivative is given by
*f*'(*z*) = 1 − (*f*(*z*))^{2}.) You can derive this yourself using the definition of
the sigmoid (or tanh) function.

## Neural Network formulation

A neural network is put together by hooking together many of our simple
``neurons,* so that the output of a neuron can be the input of another. For*
example, here is a small neural network:

In this figure, we have used circles to also denote the inputs to the network. The circles
labeled ``+1* are called bias units, and correspond to the intercept term.*
The leftmost layer of the network is called the

**input layer**, and the rightmost layer the

**output layer**(which, in this example, has only one node). The middle layer of nodes is called the

**hidden layer**, because its values are not observed in the training set. We also say that our example neural network has 3

**input units**(not counting the bias unit), 3

**hidden units**, and 1

**output unit**.

We will let *n*_{l}
denote the number of layers in our network; thus *n*_{l} = 3 in our example. We label layer *l* as
*L*_{l}, so layer *L*_{1} is the input layer, and layer the output layer.
Our neural network has parameters (*W*,*b*) = (*W*^{(1)},*b*^{(1)},*W*^{(2)},*b*^{(2)}), where
we write
to denote the parameter (or weight) associated with the connection
between unit *j* in layer *l*, and unit *i* in layer *l* + 1. (Note the order of the indices.)
Also, is the bias associated with unit *i* in layer *l* + 1.
Thus, in our example, we have , and .
Note that bias units don't have inputs or connections going into them, since they always output
the value +1. We also let *s*_{l} denote the number of nodes in layer *l* (not counting the bias unit).

We will write to denote the **activation** (meaning output value) of
unit *i* in layer *l*. For *l* = 1, we also use to denote the *i*-th input.
Given a fixed setting of
the parameters *W*,*b*, our neural
network defines a hypothesis *h*_{W,b}(*x*) that outputs a real number. Specifically, the
computation that this neural network represents is given by:

In the sequel, we also let denote the total weighted sum of inputs to unit *i* in layer *l*,
including the bias term (e.g., ), so that
.