出典:Wikipedia
出典:『Wikipedia』 (2010/10/26 08:27 UTC 版)
In order theory, a partially ordered set X is said to satisfy the countable chain condition, or to be ccc, if every strong antichain in X is countable. There are really two conditions: the upwards and downwards countable chain conditions. These are not equivalent. We adopt the convention the countable chain condition means the downwards countable chain condition, in other words no two elements have a common lower bound.