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heavily we know some of these explanations but not all of these explanations and thats
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a substantial part of what works i will also be working down on why it works and what are
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going through this whole route or if you have a different kind of a candidate search which
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in hand and thats what we will be doing from theory to experiments and eventually that
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you can do is get down any of these and then which which are really good except for lets
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place while we also have done the clustered axis given down for participants of this course
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so that you can get down access to an hpc so or you can obviously buy down this say
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tool boxes all of these are open source as of now so you can use any of this other than
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the one on matlab for which you will definitely have to pay for the licenses but the matlab
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can get this book and thats thats what we will be using as a major reading material
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to what we were doing in the earlier classes was that in the earlier classes while we were
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using feature extractors and feature descriptors which are hard coded functions over there
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neural network on the contrary today when we are going to do it
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so here is when a neutral network itself has to come down to be an end to end learning
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framework which means that input to the neural network itself is an image while the output
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from it is still a classification ah output so it can be a classification output it can
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again and then entering into the multi layer perceptron from there we enter into something
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is my input and my output related and what happens during the learning phase and this
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is a quite critical part over here
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since in the last lecture and the lab which we had done so you were introduced to the
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concept of error back propagation and from there we had a gradient descent based learning
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rule now what exactly happens in telling this as a error back propagation and why it happens
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of code is what we are going to explain you through this signal flow graph representation
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other transformations and cost functions also to exist and then eventually go down to the
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there were three inputs over here in the earlier case last week when we were doing it so these
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three different features but these three can be three pixels so you can consider just three
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space so each component itself is represented as one independent scalar value so your x
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one can be the red value of a pixel x two can be the green value of a pixel x three
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so let the decision associated with a particular pixel over here b p hat and now with the simple
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neuron model what would happen is that we will have a weighted combination of these
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inputs going down to a neuron and from there add down a bias take a summation out over
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them and this summation is what is has what has this form so its w naught plus w one x
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one plus w two x two plus w three x three where each of these weights w one w two and
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w three are three weights associated with each of the three values x one x two and x
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three and w naught is what is called as the bias or the one w naught can also be written
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down as one into b where say b is the weight over there and the constant input to this
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particular edge over there is what is one ok so in its linear algebra form which is
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in its matrix representation this is a form which you would be getting now so you get
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y is equal to the ah inner product or the dot product of two matrices
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one of them is the weight matrix where you have the weights and the bias taken together
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and the other matrix is a column matrix over there so thats why its x , one transpose
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of y tends towards plus infinity this value tends towards plus one as the value of y tends
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towards minus infinity this value tends towards zero and on the contrary with the tan hyperbolic
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now from taking down a perceptron to getting into a neural network formulation which is
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over there and i can map it down to again a different group of scalars so maybe my first
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everything to my p one hat now note over here that as we had also discussed in the earlier
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which you see with these weights one , two one , one one , three now the
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reasoning behind these weights is that the first subscript is to the target where its
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to my target which is called as p one so my first subscript is going to be the subscript
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it is connecting and thats the nomenclature which we are following now if i arrange all
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of these weights w one one w one two and w one three in a row matrix form then that is
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along with that i have my scalar value which is my bias w one zero or b one ok and accordingly
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dot product and then my non linearity applied similarly if i take down my second neuron
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on the output side of it and feed it appropriately
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so i would be getting down this second part of the partial network coming down and my
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called as w k , g and then put a impose non linearity on top of it and then taking
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which we see now if you look into this matrix of weights and biases which are come together
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so i arrange them in a column form in a call in a in a column major format which is that
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it is it just has k number of rows and just one column over there accordingly my bias
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can also be stacked one on top of the other because each is independent of the other one
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and now that would give me some sort of a rectangular matrix now if i clearly look into
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and then that gives me an output matrix over there and this output matrix is a column matrix
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my input matrix is also a column matrix
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so thats my y and then i have a non linearity applied on a matrix which means that each
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all of them together is what i get down as my target output so this was my very basic
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understanding of how a neural network works down as such and then this was what we had
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into the revision part over there so my error in prediction how it was different was that
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if i have one of these predictors p one then i get down one value of a scalar for another
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is the euclidean error over there so a euclidean error or the total error of the network is
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so whatever is my actual ground truth which is p and my predicted value p hat these two
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matrices are subtracted and then you take the amplitude of that or the l two norm of
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matrix is a of all of these scalar xs over there and one of these samples is x subscript
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one
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be getting down a predicted value which is p hat subscript one ok so similarly i take
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training data over there as i feed my last sample through my network over there my output
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i will be getting a different value of error but can we give some sort of a consolidated
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down and accordingly manipulating what happens to your predictions over there
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so because there there isnt anything else on which it can change see my input x is constant
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ah so that that will be different number of samples and across samples and across so between
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two samples it will be a different value thats always known but when i am training across
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over there the only reason why the prediction value bit of putting down the same sample
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my sample and i get down a predicted value p one hat i do all my updates and everything
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and then it comes to my second epoch which is epoch one in my epoch one i put down x
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the epoch one is very different from p one hat at epoch zero and the only reason why
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now that i have my cost function written down in terms of my weights my final point is that
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we need to come down to a point where to a point in the weight space such that my argument
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keep on changing the weights there will be one particular combination of these weights
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such that my error is minimum and thats the exact one which i would like to achieve now
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and how that achieved is through something called as the gradient descent learning rule
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so in this gradient descent learning rule what we do is basically that its an iterative
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your gradient of the weight space over there in terms of your cost function and that is
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a range of zero to one and then say my del del w of jw is in a range of ten power of
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going to be very less and in that case this factor over here comes to your rescue
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that value is something which will actually be impacting significantly how the value of
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w is changing over there
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so this learning rate basically is a fact way of mathematically modulating the gradient
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be in some way significantly impacting the change in w and thats how my w of k plus one
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will be revising at a much better rate then w of k would have if we did not have this
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till you are at the final conclusive step over there ok
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now this was one way of trying to visualize our learning in terms of its cost function
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