Constructibility and Galois Groups
Date: 04/30/2005 at 12:44:22 From: Erika Subject: constructibility and Galois groups Let a be a complex number and a root of an irreducible polynomial f over the rationals. Show that a is constructible if and only if the Galois group of f is a 2-group. I have proven that if the group is a 2-group, then a is constructible, but I'm having trouble with the other direction. So, if a is constructible, Q(a) is an extension of degree power of 2 (f has degree power of 2) but Q(a) is not necessarily a splitting field. I don't know what I can say about the Galois group. Please help!
Date: 05/01/2005 at 05:09:18 From: Doctor Jacques Subject: Re: constructibility and Galois groups Hi Erika, First, note that we must understand "constructible" in the wide sense. For example, Q(i) is a Galois extension of degree 2, but we cannot really say that i is constructible with ruler and compass in the geometric sense--we must understand that a number is constructible if it can be computed only by taking square roots (it would be more precise to say that the number admits quadrature). We must show that the degree of the splitting field of f is a power of 2. We can compute a root 'a' of f by constructing a chain of quadratic extensions: Q < Q(a1) < Q(a2) < ... Q(an)  where an = a. We can compute any other root 'b' of f by choosing the other sign in some of the square roots, giving another chain of quadratic extensions: Q < Q(b1) < Q(b2) < ... Q(bn) where bn = b; note that Q(b1) = Q(a1), but this will not necessarily be true for the other extensions. For example, if we consider the polynomial: f(x) = x^4 - 2*x^2 - 5 the four roots of f(x) are given by sqrt(1 + sqrt(6)), where we can take both signs for the square roots. In this case, the extensions are: Q < Q(sqrt(6)) < Q(sqrt(1+sqrt(6))) Q < Q(sqrt(6)) < Q(sqrt(1-sqrt(6))) and the latter two extensions are not the same, since the first one is real and the second one is not (in this case, the Galois group is D4, the dihedral group of order 8). The trick is to build the splitting field by merging the chains like  together. More precisely, we build a chain: Q < Q(a1) < Q(a1,a2) < Q(a1,a2,b2) < Q(a1,a2,b2,a3), ...  The chain  will terminate in the splitting field of f. We know that Q(a1,b2) is of degree 2 over Q(a1). This means that Q(a1,a2,b2) is of degree 1 or 2 over Q(a1,a2), because b2 satisfies a quadratic equation over Q(a1), and that equation is also a quadratic equation (not necessarily irreducible) over Q(a1,a2). In general, depending on the degree of f, we can have more than two chains like , and we can merge them together by including elements from one level at a time--each new element is quadratic over a previous extension, and therefore also at most quadratic over the extension just before it in the merged chain. This shows that all the extensions in the chain  are of degree 1 or 2, and therefore the degree of splitting field is a power of 2. Does this help? Write back if you'd like to talk about this some more, or if you have any other questions. - Doctor Jacques, The Math Forum http://mathforum.org/dr.math/
Date: 05/03/2005 at 02:10:32 From: Erika Subject: constructibility and galois groups Hi, thanks for your help. There's little thing I'm still stuck on. I understand that we can build that chain of subfields for a, because a is constructible, but why can we do it for the other roots? Are the other roots all constructible as well? If so, why? If not, why can we do that? Thanks! Erika
Date: 05/03/2005 at 02:32:58 From: Doctor Jacques Subject: Re: constructibility and galois groups Hi again Erika, All the roots of f are constructible, since the only definition of 'a' is that it is a root of the irreducible polynomial f--loosely speaking, we don't know in advance which root we will get. Choosing different signs for the square roots at each step will give you all the roots of f. Note that it is important that we understand "constructible" in the algebraic sense, not the geometric sense. For example, the bi-quadratic equation I gave you has only two real roots. If we attempt to construct the roots geometrically, we will first intersect a line and a circle, giving two choices for the first extension. For one of these choices, we can find a first pair of roots by another similar construction. However, if we select the other intersection in the first step, and attempt to find the other two roots, we will end up trying to find the intersection of a line and a circle that do not intersect each other. For the purpose of computing Galois groups, however, this is irrelevant--all operations take place in the splitting field of f, and we are not restricted to the real part of it. - Doctor Jacques, The Math Forum http://mathforum.org/dr.math/
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