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- AI-adapted: a semantically identical restatement of literature-sourced material, modulo indexing, notation, and boundary cases adopted by the library.
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The irreducible complex characters of a finite cyclic group are the powers of a primitive th root
Example
Let be a cyclic group of order , and let be a primitive -th root of unity. The irreducible complex characters of are exactly
and they are pairwise distinct.
Facts & Assumptions
Given: A cyclic group of order .
A cyclic group of order is isomorphic to (Every cyclic group is isomorphic to or to for its finite order ).
Choosing a basis identifies degree-one representations with homomorphisms , and two such representations are equivalent exactly when the homomorphisms agree (Equivalence classes of degree-one representations are exactly homomorphisms ; equivalently they factor through , and they form an abelian group).
The roots of in a field form a finite cyclic subgroup whose order divides . This group contains a primitive -th root exactly when it has order , and in that case its generators are exactly the primitive -th roots. ( is cyclic of order dividing , and has a primitive -th root of unity exactly when its order is )
Over a field of characteristic not dividing , a splitting field of has exactly distinct -th roots of unity, and they form a cyclic group of order exactly ( is separable over exactly when the characteristic does not divide , and then a splitting field carries distinct -th roots of unity).
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).
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).
Every irreducible representation of a finite abelian group over a splitting field has degree (Every irreducible representation of a finite abelian group over a splitting field is one-dimensional).
Verification
Since , a homomorphism sends to an -th root of unity: . By [F4] the -th roots of unity in are exactly the distinct powers of a primitive root of [F3].
For each , the formula is well defined by [F1] and is a homomorphism, since . They are pairwise distinct because for by primitivity of [F3].
By [F5] and [F6], is a splitting field for the finite abelian group ; then [F7] shows every irreducible representation of has degree . By [F2] it corresponds to a homomorphism , and step 1.1 shows for some , so . Hence the irreducible characters are exactly the .
Depends on
- Over an algebraically closed field, every endomorphism of an irreducible representation is scalar
- A splitting field for a finite group: every irreducible representation has scalar endomorphism ring
- Every cyclic group is isomorphic to $(\mathbb Z,+)$ or to $(\mathbb Z/n,+)$ for its finite order $n\ge1$
- Equivalence classes of degree-one representations are exactly homomorphisms $G\to k^{\times}$; equivalently they factor through $G/G'$, and they form an abelian group
- A complex polynomial of degree $n$ has exactly $n$ roots counted with multiplicity
- $\mu_n(K)$ is cyclic of order dividing $n$, and has a primitive $n$-th root of unity exactly when its order is $n$
- $t^{n}-1$ 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
- Every irreducible representation of a finite abelian group over a splitting field is one-dimensional
Used by
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Sources
- Pavel Etingof et al., Introduction to Representation Theory, Section 3.3 (standard reference, not scraped)
- Peter Webb, A Course in Finite Group Representation Theory, Section 4.1 (standard reference, not scraped)