Abstract Algebra: Theory and Applications · Problem 21.8.5

Finite Fields — Thomas Judson

Mathematics — Abstract Algebra · splitting-field-tower-sage

Problem

Form the number field, KK, which contains a root of the irreducible polynomial q(x)=x3+3x2+3x−2q(x) = x^3 + 3x^2 + 3x - 2. Name your root a. Verify that q(x)q(x) factors, but does not split, over KK. With KK now as the base field, form an extension of KK where the quadratic factor of q(x)q(x) has a root. Name this root b, and call this second extension of the tower LL. Use M.<c> = L.absolute\_field() to form the flattened tower that is the absolute number field MM. Find the defining polynomial of MM with the .polynomial() method. From this polynomial, which must have the generator c as a root, you should be able to use elementary algebra to write the generator as a fairly simple expression. MM should be the splitting field of q(x)q(x). To see this, start over, and build from scratch a new number field, PP, using the simple expression for c that you just found. Use d as the name of the root used to construct PP. Since d is a root of the simple minimal polynomial for c, you should be able to write an expression for d that a pre-calculus student would recognize. Now factor the original polynomial q(x)q(x) (with rational coefficients) over PP, to see the polynomial split (as expected). Using this factorization, and your simple expression for d write simplified expressions for the three roots of q(x)q(x). See if you can convert between the two versions of the roots ``by hand'', and without using the isomorphisms provided by the .structure() method on MM.

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