In the last video, you learn about the GRU, the Gated Recurring Unit and how that can allow you to learn very long range connections in a sequence. The other type of unit that allows you to do this very well is the LSTM or the long short term memory units. And this is even more powerful than the GRU, let's take a look. Here the equations from the previous video for the GRU and for the GRU we had a t = c t and two gates the update gate and the relevance gate. c(tilde) t, which is a candidate for replacing the memory cell. And then we use the gate, the update gate gamma Wu to decide whether or not to update c t using c(tilde) t. The LSTM is an even slightly more powerful and more general version of the GRU and it's due to set hook writer and Jurgen Schmidt Huber. And this was a really seminal paper, there's a huge impact on sequence modeling, although I think this paper is one of the more difficult ones to read. It goes quite a lot into the theory of vanishing gradients. And so I think more people have learned about the details of LSTM through maybe other places than from this particular paper, even though I think this paper has had a wonderful impact on the deep learning community. But these are the equations that govern the LSTM, so the will continue to the memory cell c and the candidate value for updating it c(tilde) t will be this. And so notice that for the LSTM, we will no longer have the case that a t is equal to c t. So this is what we use and so this is like the equation on the left except that with now more use a t there or a t-1, c t-1 and we are not using this gamma r this relevance. Although you can have a deviation of
the LSTM and we put that back in but with a more common version of the LSTM doesn't bother with that. And then we will have an update gate same as before. So w update and I'm going to use a t-1 here, ct +bu and one new property of the LSTM is instead of having one update gate control both of these terms, we're going to have two separate terms. So instead of gamma u and 1- gamma u were going to have gamma u here and for gate k we should call gamma f. So this gate gamma f is going to be sigmoid of, pretty much what you'd expect. c t plus bf, and then we're going to have a new output gate which is sigmoid of Wo and then again, pretty much what you'd expect plus bo. And then the update value to the memory cell will be c t equals gamma u then this asterisk in those element wise multiplication. There's a vector vector, element wise multiplication plus and instead of one minus gamma u were going to have a separate for gate gamma f times c t-1. So this gives the memory cell the option of keeping the old value c t-1 and then just adding to it this new value c(tilde) t. So use a separate update and forget gates right? So this stands on update. Forget and output gates and then finally instead of 80 equals c t. Is a t equal to the output gate element wise multiply with c t. So these are the equations that govern the LSTM. And you can tell it has three gates instead of two. So it's a bit more complicated and places against in slightly different places. So here again are the equations governing the behavior of the LSTM. Once again it's traditional to explain these things using pictures, so let me draw one here. And if these pictures are too complicated don't worry about it, I probably find the equations easier to understand than the picture but just show the picture here for the intuitions it conveys. The particular picture here was very much inspired by a blog post due to Chris Kohler titled Understanding LSTM networks. And the diagram drawn here is quite similar to one that he drew in his blog post. But the key things to take away from this picture or maybe that you use a t-1 and x t to compute all the gate values. So in this picture you have a t-1 and x t coming together to compute a forget gate to compute the update gates and the computer the output gate. And they also go through a tarnish to compute a c(tilde) t. And then these values are combined in these complicated ways with element wise multiplies and so on to get a c t from the previous c t -1. Now one element of this is interesting is if you hook up a bunch of these in parallel so that's one of them and you connect them, connect these temporarily. So there's the input x 1, then x 2, x 3. So you can take these units and just hook them up as follows where the output a for a period of time, 70 input at the next time set. And similarly for C and I've simplified the diagrams a little bit at the bottom. And one cool thing about this, you notice is that this is a line at the top that shows how so long as you said the forget and the update gates, appropriately, it is relatively easy for the LSTM to have some value C0 and have that be passed all the way to the right to have, maybe C3 equals C0. And this is why the LSTM as well as the GRU is very good at memorizing certain values. Even for a long time for certain real values stored in the memory cells even for many, many times steps. So that's it for the LSTM, as you can imagine, there are also a few variations on this that people use. Perhaps the most common one, is that instead of just having the gate values be dependent only on a t-1, xt. Sometimes people also sneak in there the value c t -1 as well. This is called a peephole connection. Not a great name maybe, but if you see peephole connection, what that means is that the gate values may depend not just on a t-1 but and on x t but also on the previous memory cell value. And the peephole connection can go into all three of these gates computations. So that's one common variation you see of LSTMs one technical detail is that these are say 100 dimensional vectors. If you have 100 dimensional hidden memory cell union. So is this and so say fifth element of c t-1 affects only the fifth element of the correspondent gates. So that relationship is 1 to 1 where not every element of the 100 dimensional c t-1 can affect all elements of the gates, but instead the first element of c t-1 affects the first element of the gates. Second element affects second elements and so on. But if you ever read the paper and see someone talk about the peephole connection, that's what they mean, that c t -1 is used to affect the gate value as well. So that's it for the LSTM, when should you use a GRU and when should you use an LSTM. There is a widespread consensus in this. And even though I presented GRUs first in the history of deep learning, LSTMs actually came much earlier and then GRUs were relatively recent invention that were maybe derived as partly a simplification of the more complicated LSTM model. Researchers have tried both of these models on many different problems and on different problems the different algorithms will win out. So there isn't a universally superior algorithm, which is why I want to show you both of them. But I feel like when I am using these, the advantage of the GRU is that it's a simpler model. And so it's actually easier to build a much bigger network only has two gates, so computation runs a bit faster so it scales the building, somewhat bigger models. But the LSTM is more powerful and more flexible since there's three gates instead of two. If you want to pick one to use, I think LSTM has been the historically more proven choice. So if you had to pick one, I think most people today will still use the LSTM as the default first thing to try. Although I think the last few years GRUs have been gaining a lot of momentum and I feel like more and more teams are also using GRUs because they're a bit simpler but often were, just as well and it might be easier to scale them to even bigger problems. So that's it for LSTMs with either GRUs or LSTMS, you'll be able to build new networks that can capture much longer range dependencies.
