Mobius反演的套路

标签：mob   函数   line   math   floor   min   复杂   线性筛   class   

T1

\(\sum_{i=1}^N \sum_{j=1}^M [(i,j)=1]\)

\(f(d)=\sum_{i=1}^N \sum_{j=1}^M [(i,j)=d]\)

\(g(d)=\sum_{i=1}^N \sum_{i=1}^M [d|(i,j)]=\lfloor \frac{N}{d} \rfloor \lfloor \frac{M}{d} \rfloor\)

\(g(n)=\sum_{n|d} f(d)\)

\(f(n)=\sum_{n|d} \mu(\frac{d}{n})g(d)\)

\(f(1)=\sum_{i=1}^{\min(N,M)} \mu(i)\lfloor \frac{N}{i} \rfloor \lfloor \frac{M}{i} \rfloor\)

T2

\(\sum_{i=1}^N \sum_{j=1}^M (i,j)\)

\(f(d)=\sum_{i=1}^N \sum_{j=1}^M d[(i,j)=d]=d\sum_{i=1}^N \sum_{j=1}^M [(i,j)=d]=d\sum_{i=1}^{\lfloor \frac{\min(N,M)}{d} \rfloor} \mu(i) \lfloor \frac{N}{id} \rfloor \lfloor \frac{M}{id} \rfloor\)

\(Ans=\sum_{d=1}^{\min(N,M)} f(d)=\sum_{d=1}^{\min(N,M)} d\sum_{i=1}^{\lfloor \frac{\min(N,M)}{d} \rfloor} \mu(i) \lfloor \frac{N}{id} \rfloor \lfloor \frac{M}{id} \rfloor\)

设\(w=id\)

\(Ans=\sum_{w=1}^{\min(N,M)} \sum_{d|w} \mu(\frac{w}{d}) \lfloor \frac{N}{w} \rfloor \lfloor \frac{M}{w} \rfloor=\sum_{w=1}^{\min(N,M)} \lfloor \frac{N}{w} \rfloor \lfloor \frac{M}{w} \rfloor \sum_{d|w} \mu(d)\)

\(\sum_{d|w} \mu(d)\)显然是积性函数，线性筛后做下前缀和，离线\(\Theta(\max(N,M))\)

\(\sum_{w=1}^{\min(N,M)} \lfloor \frac{N}{w} \rfloor \lfloor \frac{M}{w} \rfloor\) 整除分块可以做到在线\(\Theta(\sqrt{N}+\sqrt{M})\)

多组询问下总复杂度\(\Theta(\max(N,M)+T(\sqrt{N}+\sqrt{M}))\)

T3

\(\sum_{i=1}^N \sum_{j=1}^M \frac{ij}{(i,j)}\)

\(f(d)=\sum_{i=1}^{\lfloor \frac{N}{d} \rfloor} \sum_{j=1}^{\lfloor \frac{M}{d} \rfloor} ijd[(i,j)=1]=d \sum_{i=1}^{\lfloor \frac{N}{d} \rfloor} \sum_{j=1}^{\lfloor \frac{M}{d} \rfloor} ij[(i,j)=1]\)

\(Ans=\sum_{d=1}^{\min(N,M)} f(d)\)

\(Ans=\sum_{d=1}^{\min(N,M)} d \sum_{i=1}^{\lfloor \frac{N}{d} \rfloor} \sum_{j=1}^{\lfloor \frac{M}{d} \rfloor} ij\sum_{n|(i,j)} \mu(n)\)

\(Ans=\sum_{d=1}^{\min(N,M)} d \sum_{n=1}^{\lfloor \frac{\min(N,M)}{d} \rfloor} n (\sum_{i=1}^{\lfloor \frac{N}{dn} \rfloor} i)n(\sum_{j=1}^{\lfloor \frac{M}{dn} \rfloor} j)\)

设\(w=dn\)

\(Ans=\sum_{w=1}^{\min(N,M)} (\sum_{i=1}^{\lfloor \frac{N}{w} \rfloor} i)(\sum_{j=1}^{\lfloor \frac{M}{w} \rfloor} j) w\sum_{n|w} n \mu(n)\)

线筛前缀和+整除分块

复杂度与上题相同

Mobius反演的套路

标签：mob   函数   line   math   floor   min   复杂   线性筛   class   

原文地址：https://www.cnblogs.com/AH2002/p/10029079.html