# AI News, MachineLearning ## MachineLearning

So for example, it may have 25 inputs which are gray-levels of a 5x5 image and a single output that should, for example, tell whether a cat is in the image or not.

If you are familiar with linear algebra you will notice that this can be done with the dot product of the normal vector with the data point.

This operation is exactly the same as weighting the inputs, summing them and determine if the sum is positive or negative.

If a data set with labels is available, a learning rule can be applied to find the correct line.

If you want to understand SVMs make sure to understand Perceptrons correctly, what a dot product is, how the figure with the artificial neuron corresponds to the figure with the 2-dimensional input space and why the learning rule actually works.

In this form the weight vector is expressed as a sum over dot products between all data points and the input vector.

The crucial point here is that that a dot-product of the input data is used to define the weight vector.

Different kernels have different feature spaces and so choosing a kernel function means choosing a feature space.

A mathematical framework called statistical learning theory was developed in order to give a precise definition what &quot;best&quot;

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A 2-input hard limit neuron is trained to classify 5 input vectors into two categories.

Each of the five column vectors in X defines a 2-element input vectors and a row vector T defines the vector's target categories.

Here the input and target data are converted to sequential data (cell array where each column indicates a timestep) and copied three times to form the series XX and TT.

ADAPT updates the network for each timestep in the series and returns a new network object that performs as a better classifier.

The perceptron correctly classified our new point (in red) as category 'zero' (represented by a circle) and not a 'one' (represented by a plus).

## Perceptron

In machine learning, the perceptron is an algorithm for supervised learning of binary classifiers (functions that can decide whether an input, represented by a vector of numbers, belongs to some specific class or not).&#91;1&#93;

a classification algorithm that makes its predictions based on a linear predictor function combining a set of weights with the feature vector.

perceptron was intended to be a machine, rather than a program, and while its first implementation was in software for the IBM 704, it was subsequently implemented in custom-built hardware as the 'Mark 1 perceptron'.

This machine was designed for image recognition: it had an array of 400 photocells, randomly connected to the 'neurons'.

In a 1958 press conference organized by the US Navy, Rosenblatt made statements about the perceptron that caused a heated controversy among the fledgling AI community;

based on Rosenblatt's statements, The New York Times reported the perceptron to be 'the embryo of an electronic computer that [the Navy] expects will be able to walk, talk, see, write, reproduce itself and be conscious of its existence.'&#91;4&#93;

Although the perceptron initially seemed promising, it was quickly proved that perceptrons could not be trained to recognise many classes of patterns.

This caused the field of neural network research to stagnate for many years, before it was recognised that a feedforward neural network with two or more layers (also called a multilayer perceptron) had far greater processing power than perceptrons with one layer (also called a single layer perceptron).&#91;dubious&#32;

in 1969 a famous book entitled Perceptrons by Marvin Minsky and Seymour Papert showed that it was impossible for these classes of network to learn an XOR function.

(See the page on Perceptrons (book) for more information.) Three years later Stephen Grossberg published a series of papers introducing networks capable of modelling differential, contrast-enhancing and XOR functions.

While the complexity of biological neuron models is often required to fully understand neural behavior, research suggests a perceptron-like linear model can produce some behavior seen in real neurons &#91;7&#93;&#91;8&#93;.

In the modern sense, the perceptron is an algorithm for learning a binary classifier: a function that maps its input x (a real-valued vector) to an output value

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(0 or 1) is used to classify x as either a positive or a negative instance, in the case of a binary classification problem.

The most famous example of the perceptron's inability to solve problems with linearly nonseparable vectors is the Boolean exclusive-or problem.

In the context of neural networks, a perceptron is an artificial neuron using the Heaviside step function as the activation function.

The perceptron algorithm is also termed the single-layer perceptron, to distinguish it from a multilayer perceptron, which is a misnomer for a more complicated neural network.

Alternatively, methods such as the delta rule can be used if the function is non-linear and differentiable, although the one below will work as well.

When multiple perceptrons are combined in an artificial neural network, each output neuron operates independently of all the others;

These weights are immediately applied to a pair in the training set, and subsequently updated, rather than waiting until all pairs in the training set have undergone these steps.

The perceptron is a linear classifier, therefore it will never get to the state with all the input vectors classified correctly if the training set D is not linearly separable, i.e.

In this case, no 'approximate' solution will be gradually approached under the standard learning algorithm, but instead learning will fail completely.

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{\displaystyle \mathbf {w} \cdot \mathbf {x} _{j}&gt;\gamma }

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{\displaystyle \mathbf {w} \cdot \mathbf {x} _{j}&lt;-\gamma }

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The idea of the proof is that the weight vector is always adjusted by a bounded amount in a direction with which it has a negative dot product, and thus can be bounded above by O(&#8730;t), where t is the number of changes to the weight vector.

However, it can also be bounded below by O(t) because if there exists an (unknown) satisfactory weight vector, then every change makes progress in this (unknown) direction by a positive amount that depends only on the input vector.

While the perceptron algorithm is guaranteed to converge on some solution in the case of a linearly separable training set, it may still pick any solution and problems may admit many solutions of varying quality.&#91;11&#93;

The perceptron of optimal stability, nowadays better known as the linear support vector machine, was designed to solve this problem (Krauth and Mezard, 1987)&#91;12&#93;.

The pocket algorithm with ratchet (Gallant, 1990) solves the stability problem of perceptron learning by keeping the best solution seen so far 'in its pocket'.

However, these solutions appear purely stochastically and hence the pocket algorithm neither approaches them gradually in the course of learning, nor are they guaranteed to show up within a given number of learning steps.

In the linearly separable case, it will solve the training problem – if desired, even with optimal stability (maximum margin between the classes).

In all cases, the algorithm gradually approaches the solution in the course of learning, without memorizing previous states and without stochastic jumps.

The algorithm starts a new perceptron every time an example is wrongly classified, initializing the weights vector with the final weights of the last perceptron.

Each perceptron will also be given another weight corresponding to how many examples do they correctly classify before wrongly classifying one, and at the end the output will be a weighted vote on all perceptron.

The so-called perceptron of optimal stability can be determined by means of iterative training and optimization schemes, such as the Min-Over algorithm (Krauth and Mezard, 1987)&#91;12&#93;

The perceptron of optimal stability, together with the kernel trick, are the conceptual foundations of the support vector machine.

-perceptron further used a pre-processing layer of fixed random weights, with thresholded output units.

Another way to solve nonlinear problems without using multiple layers is to use higher order networks (sigma-pi unit).

In this type of network, each element in the input vector is extended with each pairwise combination of multiplied inputs (second order).

Indeed, if we had the prior constraint that the data come from equi-variant Gaussian distributions, the linear separation in the input space is optimal, and the nonlinear solution is overfitted.

≈Learning again iterates over the examples, predicting an output for each, leaving the weights unchanged when the predicted output matches the target, and changing them when it does not.

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In recent years, perceptron training has become popular in the field of natural language processing for such tasks as part-of-speech tagging and syntactic parsing (Collins, 2002).

Perceptron Training

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