fleurdejoo
Well-known member
It's a good thing you can imagine it!
Because you can't actually see it. :-(
Because you can't actually see it. :-(
OR F2 Solve on my TI-89Also love the equations in the background.......
Suppose that K, L and M form a tower of fields as in the degree formula above, and that both d = [L:K] and e = [M:L] are finite. This means that we may select a basis {u1, ..., ud} for L over K, and a basis {w1, ..., we} for M over L. We will show that the elements umwn, for m ranging through 1, 2, ..., d and n ranging through 1, 2, ..., e, form a basis for M/K; since there are precisely de of them, this proves that the dimension of M/K is de, which is the desired result.
First we check that they span M/K. If x is any element of M, then since the wn form a basis for M over L, we can find elements an in L such that
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Then, since the um form a basis for L over K, we can find elements bm,n in K such that for each n,
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Then using the distributive law and associativity of multiplication in M we have
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which shows that x is a linear combination of the umwn with coefficients from K; in other words they span M over K.
Secondly we must check that they are linearly independent over K. So assume that
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for some coefficients bm,n in K. Using distributivity and associativity again, we can group the terms as
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and we see that the terms in parentheses must be zero, because they are elements of L, and the wn are linearly independent over L. That is,
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for each n. Then, since the bm,n coefficients are in K, and the um are linearly independent over K, we must have that bm,n = 0 for all m and all n. This shows that the elements umwn are linearly independent over K. This concludes the proof.
The answer is 42, Andrew.
42.