题目内容
设G是一个至少含有两个数的数集,若对任意a,b∈G,都有a+b,a-b,ab,
(除数b≠0),则称G是一个数域,例如有理数集Q是数域.有下列命题:①数域必含有0,1两个数;②整数集是数域;③若有理数集Q?M,则数集M必为数域;④数域必为无限集.其中正确命题的个数是
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