| 单词 | If and only if |
| 释义 |
If and only if
中文百科
当且仅当 ↔⇔≡ 当且仅当（if and only if），中国大陆称作当且仅当，**称作若且唯若，在数学、哲学、逻辑学以及其他一些技术性领域中被用来表示“在，并且仅仅在这些条件成立的时候”之意，在英语中的对应标记为iff。虽然“A当且仅当B”是一个标准用法，但是公认的其他同样说法还有“A是B的充分及必要条件（necessary and sufficient condition）（或称为充要条件）”，或者“A成立，正当B”。 一般而言，当我们看到“A若且唯若B”，我们可以知道“如果A成立时，则B一定成立”而且“如果B成立时，则A也一定成立”（亦即，如果A不成立，则B也一定不成立）。
英语百科
If and only if 当且仅当↔⇔≡ In logic and related fields such as mathematics and philosophy, if and only if (shortened iff) is a biconditional logical connective between statements. In that it is biconditional, the connective can be likened to the standard material conditional ("only if", equal to "if ... then") combined with its reverse ("if"); hence the name. The result is that the truth of either one of the connected statements requires the truth of the other (i.e. either both statements are true, or both are false). It is controversial whether the connective thus defined is properly rendered by the English "if and only if", with its pre-existing meaning. There is nothing to stop one from stipulating that we may read this connective as "only if and if", although this may lead to confusion. |
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