首先安利两篇Blog:
http://blog.csdn.net/skywalkert/article/details/50500009
http://www.cnblogs.com/joyouth/p/5517734.html
自己对于杜教筛的理解就是下面这个式子:
\(\left\{ {\begin{array}{*{20}{l}}
{f(n) = \sum\limits_{i = 1}^n {\varphi (i)} }\\
{(\varphi \times I)(x) = id}\\
{g(n) = \sum\limits_{i = 1}^n {id(i)} }
\end{array}} \right. \Rightarrow f(n) = g(n) – \sum\limits_{i = {\rm{2}}}^n {f(\frac{n}{i})} \)