[SOUND] So the Galois correspondents. Let's take a Galois extension. Let L over K be a Galois extension. The group of automorphisms of L over K is code by definition the Galois group. And is denoted like Galois of L over K. Some theorem, By Galois. So if L is finite, Over K. Then there is a bijection between the subfields of L, sub-extensions of L and the subgroups in the Galois group. Or sub extensions Correspond objectively to the subgroups. H in Galois. So how do they correspond? If we have a subfield F, then we just send it to the Galois group. Of l over f. L over f is Galois, because it's separable and normal. It's f, which is not necessarily Galois over K, okay? And then, in the other direction, you will have H, then we send it to its fixed field, LH. The second part, as I just said, F is not necessarily Galois but one can see when it's Galois. So the second part, f is Galois over k. If I don't leave for any g in Galois in l over k. G preserves F globally. G of F is F, so if G fixes F more than twice than this means of course that F is equal to K, but here we ask for something with her we ask that g fix f as we said little bunny. Okay and this is also equivalent to seen the Galois group of l over f. Is a normal subgroup in Galois L over K. In this case G goes to g restricted to f. Is this rejection? From Galois of l over k to Galois of f over k. And the kernel is just Galois of l over f. Okay so let's prove it. In fact the first part has been more or less done right? So l, fixed fueled by Galois of l over f is f, this has been done. So n is operable extension. [SOUND] For separable extensions. Then also H is a part of the Galois group of L over LH. This is obvious by definition. But also by Artin's theorem. They have the same cardinality. The degree of L over LH is equal to cardinality of H. But this degree, as we have seen, For normal extensions they have as many automorphisms as to the degree of the extension is, right? So this is Galois the cardinality of Galois group of l over L h so by normality. And separability of course. We have seen that normal and separable extensions have many automorphisms. Okay but in general the cardinality of the group of automorphism is less or equal than the degree of the extension and the Galois case they are equal. So, we have a subgroup which has the same cardinality as the group. And so they must be equal. So one must have H is equal to the Galois group of l over l h. So this means that the maps which we had in the theorem, the map which sends f. To the Galois group of L over F which sends H to the fixed field LH. Well, this means these are mutually inverse. Inverse. Other than that, it's invertible, it's a bijection. So, in particular These are bijections. So the first part was more or less older median observed. Before we were discussing urgency and other things, now the second part, something has to be done here. So we had. Three equivalences. FIrst statement was that f was Galois. The second statement was that a and g. G of F was F for a of g and Galois and the third statement was, of course that the Galois group of L over F was a normal sub group. This is a notation for normal subgroup in Galos, L over K. So we must prove equivalence. Well let's show that 1 implies 2. Let's take an X in F. We have the minimal polynomial of X/K. It splits in L, but it has a root in F. And has a root. In F. So it must have all rules in F by normality. So, F is normal since it's the one. This means that this P minimal Splits in F. This means, of course, that any map from Galois group preserves F since it premutes the roots. Any g in the Galois group, permutes the root of the minimal of f over k right? Since this is true for any x in F, this means that g of F must be contained in F. Right since these f is generated by all sectional. F is generated for the conceive of e. Right. Now it's also very easy to remark that 2 implies 1. Now if g(f) is an F, then, All roots Of the minimal x k for x and f are in f. Since again, g permits those rules. Well, since the Galois group, Acts transitively on the roots of an irreducible polynomial. This is something which is very very important. We have seen this last time, it results from this theorem about extension of homomorphisms and it is very important to memorize it, that the Galois group is transitively on roots of any irreducible polynomial. Okay. So this means that any polynomial which has a root in f is split over f and this is the definition of normality. This is the definition. I will continue after break. [SOUND] [MUSIC]