在看一篇论文，看到这句话，请帮忙解释一下   怎么进行的回归   核回归是什么意思   和三阶多项式什么关系   总之都不懂。。。。

In the RD tests, we examine differences in the treatment effect , denoted as τ , by fitting a local third-order polynomial estimate using a triangular kernel to the left and right of the index cutoff