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Mathmos6
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"Product Field"
Let K and L be subfields of a field M such that M/K (the field extension M of K) is finite. Denote by KL the set of all finite sums ∑xiyi with xi ∈ K and yi ∈ L. Show that KL is a subfield of M, and that
[KL : K] ≤ [L : K ∩ L].
[KL : K] etc. is the standard notation for the dimension of KL as a vector space over K (includes infinity).
I've done the first part about the subfield fine, it's the inequality I'm struggling with. I've tried using the tower law ([K:M]=[K:L][L:M] where M is a subfield of L is a subfield of K. However, I can't seem to appropriately 'link up' the right subfields to relate the left-hand and right-hand side of the inequality; despite this, since it's an inequality I'm not even convinced the tower law is the right way to go: perhaps there's a nicer way to solve the problem, like finding an isomorphism between KL/K and a subfield of L/(K ∩ L) or something similar - but I have a habit of overthinking things, so maybe I'm overdoing it!
Any suggestions would be muchly appreciated - thanks in advance :)
Homework Statement
Let K and L be subfields of a field M such that M/K (the field extension M of K) is finite. Denote by KL the set of all finite sums ∑xiyi with xi ∈ K and yi ∈ L. Show that KL is a subfield of M, and that
[KL : K] ≤ [L : K ∩ L].
Homework Equations
[KL : K] etc. is the standard notation for the dimension of KL as a vector space over K (includes infinity).
The Attempt at a Solution
I've done the first part about the subfield fine, it's the inequality I'm struggling with. I've tried using the tower law ([K:M]=[K:L][L:M] where M is a subfield of L is a subfield of K. However, I can't seem to appropriately 'link up' the right subfields to relate the left-hand and right-hand side of the inequality; despite this, since it's an inequality I'm not even convinced the tower law is the right way to go: perhaps there's a nicer way to solve the problem, like finding an isomorphism between KL/K and a subfield of L/(K ∩ L) or something similar - but I have a habit of overthinking things, so maybe I'm overdoing it!
Any suggestions would be muchly appreciated - thanks in advance :)