https://en.wikipedia.org/wiki/Kronecker%E2%80%93Weber_theorem Kronecker?Weber theorem (抜粋) In algebraic number theory, it can be shown that every cyclotomic field is an abelian extension of the rational number field Q, having Galois group of the form (Z/nZ )^x . The Kronecker?Weber theorem provides a partial converse: every finite abelian extension of Q is contained within some cyclotomic field. In other words, every algebraic integer whose Galois group is abelian can be expressed as a sum of roots of unity with rational coefficients. For example, √5=e^2πi/5-e^4πi/5-e^6πi/5+e^8πi/5, √5=e^2πi/5-e^4πi/5-e^6πi/5+e^8πi/5, √-3=e^2πi/3-e^4πi/3,√-3=e^2πi/3-e^4πi/3, and √3=e^2πi/12-e^10πi/12.√3=e^2πi/12-e^10πi/12. The theorem is named after Leopold Kronecker and Heinrich Martin Weber. (引用終り) 以上
