Show there is a bijective correspondence of \(A \times B\) with \(B \times A\) .
Solution
Define \[ \tau:A\times B\longrightarrow B\times A, \qquad \tau(a,b)=(b,a). \] Then \(\tau\) is its own inverse: \[ \tau(\tau(a,b))=(a,b). \] Hence \(\tau\) is bijective.