Show that the U-statistic \(\widehat{\sigma}^{2}\) in (1.24) is the sample variance of \(\sigma^{2}\), i.e., \(\widehat{\sigma}^{2}\) can be expressed as \(\widehat{\sigma}^{2}=\frac{1}{n-1} \sum_{i=1}^{n}\left(X_{i}-\bar{X}_{n}\right)^{2}\).