Alphabeta Math
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The irreducible complex characters of a finite cyclic group are the n powers of a primitive nth root

Example

Let G=g be a cyclic group of order n1, and let ζC be a primitive n-th root of unity. The irreducible complex characters of G are exactly

χk(gm):=ζkm(0kn1),

and they are pairwise distinct.

Facts & Assumptions

Given: A cyclic group G=g of order n1.

[F2]

Choosing a basis identifies degree-one representations with homomorphisms GC×, and two such representations are equivalent exactly when the homomorphisms agree (Equivalence classes of degree-one representations are exactly homomorphisms Gk×; equivalently they factor through G/G, and they form an abelian group).

[F3]

The roots of xn1 in a field form a finite cyclic subgroup μn whose order divides n. This group contains a primitive n-th root exactly when it has order n, and in that case its generators are exactly the primitive n-th roots. (μn(K) is cyclic of order dividing n, and has a primitive n-th root of unity exactly when its order is n)

[F4]

Over a field of characteristic not dividing n, a splitting field of tn1 has exactly n distinct n-th roots of unity, and they form a cyclic group of order exactly n (tn1 is separable over K exactly when the characteristic does not divide n, and then a splitting field carries n distinct n-th roots of unity).

[F5]

Over an algebraically closed field, every endomorphism of an irreducible representation is scalar (Over an algebraically closed field, every endomorphism of an irreducible representation is scalar).

[F6]

A field is a splitting field for a finite group exactly when every irreducible representation has scalar endomorphism ring (A splitting field for a finite group: every irreducible representation has scalar endomorphism ring).

[F7]

Every irreducible representation of a finite abelian group over a splitting field has degree 1 (Every irreducible representation of a finite abelian group over a splitting field is one-dimensional).

Verification

technique · direct
1.1

Since gn=1, a homomorphism φ:GC× sends g to an n-th root of unity: φ(g)n=φ(gn)=φ(1)=1. By [F4] the n-th roots of unity in C are exactly the n distinct powers ζ0,,ζn1 of a primitive root ζ of [F3].

F3F4given
1.2

For each k, the formula χk(gm)=ζkm is well defined by [F1] and is a homomorphism, since χk(gagb)=ζk(a+b)=ζkaζkb. They are pairwise distinct because ζkk1 for 0<kk<n by primitivity of [F3].

F1F3givenalgebra
2.1

By [F5] and [F6], C is a splitting field for the finite abelian group G; then [F7] shows every irreducible representation of G has degree 1. By [F2] it corresponds to a homomorphism φ:GC×, and step 1.1 shows φ(g)=ζk for some k, so φ=χk. Hence the irreducible characters are exactly the χk.

F2F5F6F7step 1.1step 1.2

Depends on

Used by

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Sources