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 A≠0 be a not-necessarily-commutative associative unital K-algebra. Let f1,…,fn be distinct K-algebra homomorphisms from A to K. Let ϕ:A→Kn 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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