A={1,2,3},B={4,5,6}
f:A->B
f(1) =4
这样的映射有 = 2^2 = 4个"这样的映射有 = 2^2 = 4个"这个怎么来的呢？请分析一下，谢谢！应该是3^2 = 9个f(1) =4, f(2)= 4 f(3)= 4 or 5 or 63个f(1) =4, f(2)= 5 f(3)= 4 or 5 or 63个f(1) =4, f(2)= 6 f(3)= 4 or 5 or 63个