There exists a surjection from {4,5,6} to {√2,√3,2,√5}. why this statement is wrong?

This statement is wrong because the cardinality of {4,5,6} is 3, whereas the cardinality of {√2,√3,2,√5} is 4. In order for a surjection to exist between two sets, the domain must have at least as many elements as the range. Therefore, it is impossible for a surjection to exist from {4,5,6} to {√2,√3,2,√5}.

相关问题

There exists a surjection from {4,5,6} to {根号2，根号三，2，根号五}这句话为啥错的

This sentence is incorrect because it is written in a mix of Chinese and English. A correct translation of the sentence would be "There exists a surjection from {4,5,6} to {√2,√3,2,√5}."

select id from a not in (1,2,3,4,5)优化一下这个sql

可以尝试用`not exists`或`left join`的方式代替`not in`，因为`not in`的查询效率比较低，而且当被比较的列表比较大时，效率会更低。以下是两种优化方式的示例代码：

使用not exists：

select id from a where not exists (select 1 from b where a.id = b.id and b.id in (1,2,3,4,5));

使用left join：

select a.id from a left join b on a.id = b.id and b.id in (1,2,3,4,5) where b.id is null;

这里假设表b中存储了需要排除的id，如果需要排除的id比较少，那么使用`not in`也可以，但是如果需要排除的id比较多，那么使用上述两种方式会更有效率。