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# Simplest perceptron

## Simple perceptron

Let's consider the following simple perceptron with a transfert function given by $$f(x)=x$$ to keep the maths simple:

## Transfert function

The perceptron global transfert function is given by the following equation:

$$$$y= w_1.x_1 + w_2.x_2 + ... + w_N.x_N = \sum\limits_{i=1}^N w_i.x_i \label{eq:transfert-function}$$$$

## Error (or loss)

In artificial neural networks, the error we want to minimize is:

$$$$E=(y'-y)^2 \label{eq:error}$$$$

with:

• $$E$$ the error
• $$y′$$ the expected output (from training data set)
• $$y$$ the real output of the network (from network)

In practice and to simplify the maths, this error is divided by two:

$$E=\frac{1}{2}(y'-y)^2$$

The algorithm (gradient descent) used to train the network (i.e. updating the weights) is given by:

$$w_i'=w_i-\eta.\frac{dE}{dw_i}$$

where:

• $$wi$$ the weight before update
• $$w′i$$ the weight after update
• $$η$$ the learning rate

## Derivating the error

Let's derivate the error:

$$$$\frac{dE}{dw_i} = \frac{1}{2}\frac{d}{dw_i}(y'-y)^2 \label{eq:eq-error}$$$$

Thanks to the chain rule

$$(f \circ g)'=(f' \circ g).g'$$

equation $$\eqref{eq:eq-error}$$ can be rewritten as:

$$\frac{dE}{dw_i} = \frac{2}{2}(y'-y)\frac{d}{dw_i} (y'-y) = -(y'-y)\frac{dy}{dw_i}$$

As $$y= w_1.x_1 + w_2.x_2 + ... + w_N.x_N$$:

$$\frac{dE}{dw_i} = -(y'-y)\frac{d}{dw_i}(w_1.x_1 + w_2.x_2 + ... + w_N.x_N) = -(y'-y)x_i$$

## Updating the weights

The weights can be updated with the following formula:

$$w_i'=w_i-\eta.\frac{dE}{dw_i} = w_i+\eta(y'-y)x_i$$

In conclusion:

$$w_i'= w_i + \eta(y'-y)x_i$$