Algebra. Fields and Galois Theory by Falko Lorenz

By Falko Lorenz

From Math reports: "This is an enthralling textbook, introducing the reader to the classical elements of algebra. The exposition is admirably transparent and lucidly written with merely minimum must haves from linear algebra. the hot techniques are, no less than within the first a part of the e-book, outlined within the framework of the improvement of conscientiously chosen difficulties. therefore, for example, the transformation of the classical geometrical difficulties on buildings with ruler and compass of their algebraic atmosphere within the first bankruptcy introduces the reader spontaneously to such basic algebraic notions as box extension, the measure of an extension, etc... The booklet ends with an appendix containing workouts and notes at the prior elements of the booklet. despite the fact that, short old reviews and recommendations for extra analyzing also are scattered during the text."

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Example text

Since all the coefficients of g and h belong to R (see Chapter 4, F12). was arbitrary, ˜ F8. Let R be a UFD and K D Frac R. Let f 2 RŒX  be a normalized polynomial with coefficients in R. Then any root ˛ of f that lies in K actually lies in R, and moreover divides the constant term of f . Proof. X / in KŒX . Since f is normalized, so is h. X / are in R. 0/ D . 0/ we have ˛ j a0 . ˜ Application. Consider the special case R D ‫ ޚ‬, K D ‫ ޑ‬. X / D X 3 2 is irreducible. Suppose f were reducible. Since it has degree 3, it would have a linear factor, and therefore a root ˛ in ‫ ޑ‬.

By assumption there exist homomorphisms W F ! F 0 and 0 W F 0 ! F with ı à D Ã0 and 0 ı Ã0 D Ã. It follows that 0 ı ı à D 0 ı Ã0 D Ã, and thus, because of the uniqueness requirement, 0 ı D idF ; analogously we have ı 0 D idF 0 . Therefore is an isomorphism. In view of F8 what is left to show is that there is a field K and an injective ring homomorphism à W R ! K. c; d / means ad D bc: It is easy to prove that this is an equivalence relation; let K D M= the set of equivalence classes. a; b/ 2 M by Œa=b.

We will give a proof of the Hermite–Lindemann Theorem in Chapter 17. Proof of F14. e i' /, and suppose t D e i' is transcendental. We must show that z D e i'=3 does not belong to K. z/ W K D 3 Irreducibility criteria and applications (see Chapter 1, F9). Now, z is certainly a root of the polynomial X 3 To prove (17) therefore we have to show that (18) X3 53 t over K. 4), which is to say, the fraction field of the polynomial ring R WD ‫ ޑ‬Œt . Since R is a UFD and t is prime in R, an application of the Eisenstein criterion to the polynomial X 3 t proves (18).

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