Tuesday, January 15, 2019

field theory - A proof of Artin's linear independence of characters

I came up with a proof of Artin's linear independence of characters in field theory. The usual proof uses a clever trick devised by Artin. Since I'm not as clever as him, I prefer a proof which doesn't use a clever trick. Is this proof well-known? The proof consists of a few easy steps.



Step 1.



Let K be a field. Let A0 be a not-necessarily-commutative associative unital K-algebra. Let f1,,fn be distinct K-algebra homomorphisms from A to K. Let ϕ:AKn be the map defined by ϕ(x)=(f1(x),,fn(x)). Then ϕ is surjective.



The proof is an easy consequence of Chinese remainder theorem.




Step 2.



Let f1,,fn be as above. There are elements x1,,xn of A such that fj(xi)=δ(i,j) where δ(i,j) is Kronecker's delta.



The proof is an easy consequence of Step 1.



Step 3



Let K and A be as above. Let Homalg(A,K) be the set of K-algebra homomorphisms from A to K. Let Hom(A,K) be the set of K-linear maps from A to K. Then Homalg(A,K) is a linearly independent subset of Hom(A,K).




The proof is an easy consequence of Step 2.



Step 4 (Artin's linear independence of characters)



Let K be a field. K is regarded as a monoid by multiplication. Let M be a not-necessarily-commutative monoid. Let Hom(M,K) be the set of monoid homomorphisms. Let KM be the set of maps from M to K. KM is regarded as a vector space over K. Then Hom(M,K) is a linearly independent subset of KM.



The proof is an easy consequence of Step 3 if one considers the monoid algebra K[M].

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