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单词 ExampleOfNormalExtension
释义

example of normal extension


Let F=(2). Then the extension F/ is normal because F is clearly the splitting fieldMathworldPlanetmath of the polynomialPlanetmathPlanetmath f(x)=x2-2. Furthermore F/ is a Galois extensionMathworldPlanetmath with Gal(F/)/2.

Now, let 21/4 denote the positive real fourth root of 2 and define K=F(21/4). Then the extension K/F is normal because K is the splitting field of k(x)=x2-2, and as before K/F is a Galois extension with Gal(K/F)/2.

However, the extension K/ is neither normal nor Galois. Indeed, the polynomial g(x)=x4-2 has one root in K (actually two), namely 21/4, and yet g(x) does not split in K into linear factors.

g(x)=x4-2=(x2-2)(x2+2)=(x-21/4)(x+21/4)(x2+2)

The Galois closure of K over is L=(21/4,i).

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更新时间:2025/5/4 9:23:50