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versus epochs and this can be a typical graph so you would often be seeing that you start
slightly then goes down it may keep on jittering and these are all aspects about whats happening
be a very smooth transition which it will be getting if your value of is very high
to the local minima and these are different issues which we would be tackling down through
experimental processes and some more learning experiences subsequently now looking at the
so here what i am doing is typically i am looking into two different plots so one is
my plot of epoch versus cost function the other is my plot of weight space versus cost
goes to the next one then to the next one and and finally to my convergence now if you
you would be seeing that the error function over there form some sort of a very structure
troughs present over there so as these points of my weights they keep on moving down they
would always be encircling and coming down to my local minimum point as soon as possible
and the way it comes down to this local minima is what is my learning which is happening
is that we can actually initialize a network at any random point over there and based on
that it can start converging and oscillating around any of these trucks and that definitely
these non global minima positions but rather somehow escape into this from these small
all of this what comes down to our mind is something interesting so you have seen that
there is for any kind of a given network if i have three different scalar values over
there then i can take in these scalar values and i can predict out one of these predicted
that i have my weights w one w two and w three i take all of these weights together and then
down to my output over there ok
doing so in order to solve it out the best possible way is basically trying to look into
something which is called as the chain rule of differentiation which you have done in
so i can take a derivative of the cost function with respect to this output next is my output
now be represented as a product of del del p of jw and del del y of p so together these
of the cost function if you look into the second part of the gradient then you see that
of the derivative thats a derivative of the linear network itself and these three things
together are what will be helping me in finding out the gradient part for my whole network
then enter into eventually the deep learning and how to train down these deep neural networks
thank you and stay tuned for the next lecture
so welcome and so in the ah we will be continuing down with the from where we left in the last
lecture and thats on multi layer perceptrons to deep neural networks and this is where
and a recap of the learning rules and how to create down this single model and then
a single perceptron and then a whole collection of perceptrons in terms of its matrix and
the matrix form of representing the data from there going down to the gradient and what
it gets broken down into partial products over there and using these partial derivative
last class so that was on the gradient computation part say i have three scalars x one x two
and x three and then i would like to map it down to another predictor scalar which is
is from x how to get down to p hat the question which we had raised is how to get down this
to y which is the output from the summation block and you take a derivative of y with
respect to w which is known as the derivative of the linear part of the network ok now and
now the point is that this kind of a computation is what holds true for just one single neuron
ok and the next point is that if it is not just one single neuron but you have a collection
of neurons or something which is a deep neural network
as a multi layer perceptron due to its multiple layers form over there so thats exactly what
nodes over there that connects to another set of intermediate nodes and that subsequently
to get down the derivative in its own way but then in order to get this one you see
case is something tricky so lets look into this
small part of the network so what we do is say that i am looking at one of my particular
layer which is say called as the d th layer ok now for my a d th layer what will i will
now that can be done as an extended product of del del w of y d which is output which
is the linear part of summation which comes down to that particular plot now if i go down
to my d minus one th layer so this is my w d which is just connecting down my output
to the d th layer now if i go down to one layer before it now what we can see is that
this linear part of this block with respect to the weights which are connecting these
of the linear part of this one with respect to the block earlier it ok so this is my d
this is my w of d minus one and thats where my expansion happens now and similarly i keep
on repeating this whole thing together on the chain and finally what i would get down
is on the final part which is del del w of y one which is my first output layer over
it which is my x over here so this is a typical way in which we calculate now our whole networks
gradient over there so if i want that my total network has to be solved out so this is exactly
what i would be doing in terms of my calculations so you can typically look so now that i dont
have what is my input coming from here so what i would be doing is i dont know exactly
what values are over here so i will be again differentiating this with respect to this
it stops is a del del w one of y one and thats equal to my input which is x ok so i i believe
this part is quite clear to you guys and and quite intuitive actually not so hard to calculate
down something like this now that i have this form going down so my first part of it is
is what is called as a derivative of the network and finally is the input to the network which
is my x ok so these together is what constitutes of any
sort of a learning mechanism within a multi layer perceptron or any kind of a deep neural
network so what you will have to do is you will have to find out what is my derivative
of my cost function you will have to find out the derivative of the network which together
and derivative of the perceptron together and you will have to find out what is that
now by solving out this complete derivative over here is what we are able to get down
as our neural network learning algorithm in terms of gradient descent and thats where
criteria so what this essentially means is that in order for the total derivative of
fraction of the derivative exists so every single part over there we were doing a chain
rule of expansion so if every single component of the chain
rule exists only in that case you would see that the total derivative of the network as
which is lets take down these two cost functions so the first one is what is called as the
you can do is quite simple i mean you can just take a del del w ah del del p of j over
happen is definitely it does exist for the first case which is euclidean norm
have the second one ok and this is where the fun is so do you think that the derivative
of this one will exist as well or not just just take down a few seconds over here while
at zero you see you have an l one norm or just a mod ok so mod of p minus p hat this
one over there is basically a value which is always a nonzero value and this hat does
of the rest part of the network and now over here one important point is that the derivative
del del y of p that will not come into existence whereas del del y of z that does not have
question is does the derivative of each of them exist or not so lets give you some time
the derivative does exist and thats a perfectly differentiable function whereas look into
the second part of that that again is something which is not differentiable because of the
property so one property was definitely to make it bounded in some form ok but we also
mentioned that the there are other properties and one of those important properties is that
the cost function itself sorry not the cost function but the transfer function itself
end over there until and unless a transfer function is differentiable the derivative
function and thats the reason why these kind of functions cannot be made cannot be used
through intermediate weights w one up to w d and ah go down to my final output which
is p hat and the way of how we are doing down is something of this sort and the first step
which is called as the forward pass of the network and this is something similar to what
pass of this x in order to obtain your p hat now that you have your forward pass and you
that i need to compute out my j which is my cost function the way of computing this j