All right. So, we've just seen this best approximation property of the finite element method. And hopefully, it gives you a little more, little insight into how the method works in picking solutions, from the space that we have chosen for it. [COUGH] In this segment, I am going to derive another result that is sometimes called the, the equivalent of the, of the Pythagoras' theorem. For this for this particular problem that we are working with. And it has a nice consequence that is useful to note when one is actually looking at numerical solutions, and comparing with exact solutions. Okay? So so right, so, so the title of this segment is The Pythagorean, The Pythagorean Theorem. Right? And it's really in quotes because it's, it's similar to that, but it's not the original Pythagoras' theorem. Okay. Right, here is theorem first of all. This theorem is really a corollary of the fact of consistency, but but, okay, so let me state this as a corollary. Okay? And the corollary is this. It is, that right. If we consider the energy norm, of the exact solution, okay? This is equal to the energy norm of u h comma u h plus the energy norm of the error, okay? For, for a specific type of problem, okay? This holds for a problem where S h equals V h. Now, if you're wondering what that means, it simply means that our Dirichlet boundary conditions, for this particular problem, are homogenous ones. Okay? So, any place that will specify there is a Dirichlet boundary condition, we are saying that, that Dirichlet boundary condition is 0, okay? All right. So what this means is Dirichlet boundary conditions are Homogeneous. Okay? All right. And you note that this actually holds for the Dirichlet-Neumann problem. In fact, something that we often remarked upon during our development of the finite element method, for that particular problem was that, well, yes, if, if the, if we have the left-end of the bar fixed, then S e h and W e h are, are, the same, right? We just [INAUDIBLE] S h, and, and V h aren't the same, right? Or alternately, little u h and w h satisfy the same conditions, right? The same [INAUDIBLE] conditions. Okay, it is for this case, that we, that, that it holds. So, proof. Okay. So, let's in order to start looking at this, let us consider here right. Let us consider the following. For this setting we have u h Minus u equals e. Right? That's our definition of the error. Right? This is the error. The error in the finite element solution. And from this, we can therefore, write that U equals u h minus e. Therefore, when we consider on the left hand side, the energy norm of the exact solution. We see. Of course, we see this. But now, just as we did in the previous segment, we can expand out the term on the right hand side. By just using the fact of bi-linearity. All right. And this gives us a u h comma u h Minus a U h comma e minus a e comma u h Minus, sorry plus a e comma e. All right? Okay. And then as before, we recognize that the second, and third terms are the same because of the fact of fact of what? Fact of symmetry, right? The symmetry of the bi-linear form, right? So, that let's us write it as a. U h comma u h Minus twice a. U h comma e. Plus a e comma e. Okay? But now, if you look at the condition in the statement of the corollary, you should be able to see something about the second term there. It should be able to say something about the second term. What can you say? This is equal to twice a of w h comma e because the spaces S h, and V h are the same. Right? So, u h is the same as w h, right? U h is a w h, it is indeed a weighting function. Okay? And this therefore, is equal to 0, by consistency. Okay? And there we have it, right? We, what we are left with is, is the result we set out to proof. Okay? This is a, the energy norm of the finite element solution plus the energy norm of the error. Okay? This holds of course, for the problems with the Dirichlet boundary conditions are homogeneous, right? So, it's a useful to think to note in some sense this looks just like the Pythagoras' Theorem. Recognizing that the energy norm involves a square. So, it's like saying that w, w, if we use the this bi-linear form to define a more generalized version of an inner product, right? It is, it is indeed an inner product, but we can use it to use to define a more generalized notion of a square, right, of two functions. Right? Then this generalized notion of the square of functions tells us that in as much as we admit, this to represent a square. We are saying something to the affect of u square equals u h square plus e square, okay? All right. So, that is why, we call it the Pythagorean theorem. What good is this? Okay so a corollary of back Okay? So, let's call this corollary Maybe, I should call this corollary 2, and I call that corollary 1. So, let me do that. Let me go back now, and call this one that's corollary 1, okay? Okay, corollary 2. It says that, I mean, corollary 2 is actually let me state it. The corollary 2 says the finite elements solution. Okay? Underestimates The energy norm Of the solution. Well, let me [INAUDIBLE] the solution, let me see the energy norm of the problem. Proof is a single step from corollary 1. We see that a U comma u equals a U h comma u h plus a e comma e. Now, because of the nature of this bi-linear form and the fact, that it leads to an energy norm, when we use the same function, both slots. We know, that this is greater than, or equal to 0. And so, is this, right? This is a fact that we've remarked, upon the previous segment. All right. What this implies for us then, is that if we drop the last stone, what we get for the right-hand side is that a of u h comma u h, the energy norm of the finite elements solution. Is lesser than, or equal to, the energy norm of the exact solution. Okay? And this what we mea,n by saying that the finite element solution, u h, underestimates the energy norm, all right? Of the problem. The exact energy norm of the problem, is what we have on the right-hand side here. Okay? When we get the finite element solution, and we go back and compute the energy norm, which is what we have on the left-hand side. We will be computing an energy norm, which is less than that of the exact solution. Why does this matter? Well, it is, as all norms do the, this is a particular way in which to get some sense of another sense of control over the solution. What is the solution doling, how big is it? Well, you can compute it's energy norm, it turns out that the energy norm that we get from finite element solution is not as big as the energy norm of the exact solution. Why might, where else, when might it matter in a more physical sense? If you are solving a problem in linear elasticity, for instance. Where the ene, the energy norm as, as written out here actually, has physical meaning in it. And in fact, corresponds to, what corresponds to the strain energy, right? What it says is that, if you had the exact solution available to you, and you were to go ahead, and compute the strain energy of the exact solution. And compare the strain energy of your finite element solution, what you, what you ought to, to see is that the finite element solution actually, underestimates that strain energy, right? It gives you a smaller strain energy. Okay? So, that's a particular result of this, of this, of this method. Okay. We are actually going to end this segment here. It's a shorter segment, but we are, we are getting prepared for the main results of, of our, analysis of the finite element method.