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這個證明不難的,不過無限集的基數確實是個有意思的東西,比如偶數(2,4,6…2n)是正整數(1,2,3,…n)的一個真子集然而通過:2←→1,4←→2,6←→3,.,2n←→n,.它們之間建立了一個一一對映,從而偶數和正整數具有了…
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