• Kyungpook Mathematical Journal
• /
• 제63권1호
• /
• pp.15-27
• /
• 2023

A purely extending module is a generalization of an extending module. In this paper, we study several properties of purely extending modules and introduce the notion of purely essentially Baer modules. A module M is said to be a purely essentially Baer if the right annihilator in M of any left ideal of the endomorphism ring of M is essential in a pure submodule of M. We study some properties of purely essentially Baer modules and characterize von Neumann regular rings in terms of purely essentially Baer modules.

• 대한수학교육학회지:수학교육학연구
• /
• 제15권2호
• /
• pp.197-213
• /
• 2005

초등학교 수학 교과서에서는 적지 않은 수의 용어가 정의 없이 사용되고 있다. 이 연구에서는 초등학교 수학 교과서에서 무정의 용어로 사용되는 용어들을 추출하고 그 사용의 적절성을 비판적으로 검토하였다. 검토 결과 도출된 시사점을 제시하면 다음과 같다. 첫째, '등식'과 같이 교육과정에서 초등학교의 해당 단계에서 사용하게 되어 있지 않은 용어를 교과서에서 무정의 용어로 사용하는 것은 지양해야 한다. 둘째, 일상 기반 용어라 하더라도, 그 용어가 나타내는 일상적 실재물과 수학적 실재물 사이에 괴리가 있는 경우에는 그 용어를 정의하여야 한다. 셋째, 학교수학에서 무정의 용어의 사용에 있어, 일관성과 형평성을 고려해야 한다. 넷째, 용어가 사용되는 맥락이 바뀌거나 확장되는 경우에는 그 용어를 새롭게 정의하여야 한다. 다섯째, 학생들이 정의 없이도 그 의미를 잘 포착할 수 있다고 볼 근거가 없는 순수 용어는 정의하여야 한다

• 대한수학회보
• /
• 제31권2호
• /
• pp.167-172
• /
• 1994

For a given $C^{*}$-dynamical system (A, G, .alpha.) with a G-simple $C^{*}$-algebra A (that is A has no proper .alpha.-invariant ideal) many authors have studied the simplicity of a $C^{*}$-crossed product A $x_{\alpha{r}}$ G. In [1] topological freeness of an action is shown to guarantee the simplicity of the reduced $C^{*}$-crossed product A $x_{\alpha{r}}$ G when A is G-simple. In this paper we investigate the pure infiniteness of a simple $C^{*}$-crossed product A $x_{\alpha}$ G of a purely infinite simple $C^{*}$-algebra A and a topologically free action .alpha. of a finite group G, and find a sufficient condition in terms of the action on the spectrum of the multiplier algebra M(A) of A. Showing this we also prove that some extension of a topologically free action is still topologically free.

• 대한수학회지
• /
• 제52권4호
• /
• pp.853-868
• /
• 2015

Let G be a complex semisimple simply connected linear algebraic group. The main result of this note is to give several equivalent criteria for the untwistedness of the twisted cubes introduced by Grossberg and Karshon. In certain cases arising from representation theory, Grossberg and Karshon obtained a Demazure-type character formula for irreducible G-representations as a sum over lattice points (counted with sign according to a density function) of these twisted cubes. A twisted cube is untwisted when it is a "true" (i.e., closed, convex) polytope; in this case, Grossberg and Karshon's character formula becomes a purely positive formula with no multiplicities, i.e., each lattice point appears precisely once in the formula, with coefficient +1. One of our equivalent conditions for untwistedness is that a certain divisor on the special fiber of a toric degeneration of a Bott-Samelson variety, as constructed by Pasquier, is basepoint-free. We also show that the strict positivity of some of the defining constants for the twisted cube, together with convexity (of its support), is enough to guarantee untwistedness. Finally, in the special case when the twisted cube arises from the representation-theoretic data of $\lambda$ an integral weight and $\underline{w}$ a choice of word decomposition of a Weyl group element, we give two simple necessary conditions for untwistedness which is stated in terms of $\lambda$ and $\underline{w}$.