訂正日: 2006/08/29訂正理由: -訂正箇所: 引用文献情報訂正内容: Right : 1) Cf. N. Bourbaki, Topologie générale, Chap. X. 2) Here 1/n stands for the residue class of 1/n mod. Z. 3) Notice that G is isomorphic with a subgroup of the direct product of all G/Hi. 4) In general, an abelian group G is not regular even when H and G/H are both regular groups. Example: G=the additive group of rationals, H=the additive group of integers. 5) Cf. the author's forthcoming paper “On solvablé extensions of algebraic number fields”.