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PROFESSOR: All right, an announcement that I must make, after developing all these friendships and working together, we will, next Tuesday, once again, enter a brief adversarial relationship. And we will have quiz number two. What we will cover on that quiz will be about 50%, perhaps a little bit more, on symmetry. And I'll remind you in just a minute what we've covered since the last quiz. And the second half or 40% of the quiz will be on the formalism of tensors and tensor manipulation. And obviously, you've had a chance only to do a very few problems. So these will be pretty basic questions and nothing tricky or very difficult. So let me remind you very, very quickly just by topics what we've covered. We began the period following the first quiz discussing how we can take the plane groups, two-dimensional symmetries that are translationally periodic, and defining a third translation to stack up the plane groups to give us a three-dimensional periodic ar...

One announcement, even though you will have no academic exercises Thursday and Friday, I know you would feel a sense of shock, disruption and dare I say dislocation in your life if I did not stick to schedule, so there will be a skill testing question on Tuesday based on Homework 12. And that will be the last one because the subsequent week is the last week of semester. And, since there is a final exam in this subject, there can be no tests during the last week of class, so we will not have any weekly test. Today I want to talk about biochemistry. We are going to spend the next three lectures on biochemistry. This is the chemistry of living organisms. And I want to make several points by way of introduction. The first one is that living organisms are chemical systems. And they are governed by the same laws that apply to inanimate matter. We don't have a special chemistry. And, in fact, I came across this comment that was made by the Nobel Laureate Richard Feynman who ...

So just a brief announcement, on this Friday you guys have a writing tutorial. It's at 2 p.m. there's only one tutorial this week at 2 p.m., and it's going to be in this room. So make sure you come. We're going to talk about issues involved with preparing and presenting your DP2. And it's really important that you come to pay attention. So today we're going to continue our discussion of networking and network layering. If you remember last time, we talked about the three layers that are in any typical network stack. And these three layers we said were the end-to-end layer, the network layer, and the link layer. And we went to the example of how these three layers interact with each other as, say, a message is sent through a network. So, on a typical sender node, we said there are these three layers -- And there might also be a receiver node. And then there could be several. Each time a message is sent through the network, it might pass through any ...