Showing posts with label summatory function. Show all posts
Showing posts with label summatory function. Show all posts

Wednesday, April 13, 2011

Sum of Möbius function over Complementary Divisors of Square-Divisors

While considering the summatory function of the Liouville Function, we found that
which takes the value of 1 when n is a perfect square and 0 otherwise.  The following question was given as a challenge and we present its solution in this article.

Question
Find
Solution:-
From
     ` \sum_{d|n}\lambda(d)=\bb{1}_\text{PerfectSquare}(n) `
using the Möbius Inversion formula, we have
     ` \sum_{d|n}\mu(\frac{n}{d})\bb{1}_\text{PerfectSquare}(d)=\lambda(n) `
The summands will be zero except when ` d=D^2 `.  Hence we get
     ` \sum_{D^2|n}\mu(\frac{n}{D^2}) =\lambda(n) `
[End]

Liouville Function, Summatory of

We introduced the Liouville function here.  The summatory function of ` f(n) ` is defined as
In this post, we examine the summatory function of the Liouville function.


Question
Prove that
Solution:-
   Note that for ` n=p^a ` where p is a prime number,
   ` \sum_{d|n}\lambda(d) `  =   ` \sum_{j=0}^a \lambda(p^j) `  =   ` \sum_{j=0}^a (-1)^j `
=   ` \frac{1\cdot(1-(-1)^{a+1})}{1-(-1)}`  =   ` \frac{1-(-1)^{a+1}}{2} `
=   ` \bb{1}_\text{even}(a) `  =   ` {(1,\text{if }a\text{ is even}),(0,\text{if }a\text{ is odd}):} `       ("Even indicator function")

Hence for ` n=p_1^{a_1}p_2^{a_2}...p_k^{a_k} `
   ` \sum_{d|n}\lambda(d) `  =   ` \sum_{i_1,i_2,...,i_k}\lambda(p_1^{i_1}p_2^{i_2}...p_k^{i_k}) ` =   ` \sum_{i_1,i_2,...,i_k}\lambda(p_1^{i_1})\lambda(p_2^{i_2})...\lambda(p_k^{i_k}) `
=   ` \sum_{i_1=0}^{a_1}\lambda(p_1^{i_1})\sum_{i_2=0}^{a_2}\lambda(p_2^{i_2})...\sum_{i_1=0}^{a_k}\lambda(p_k^{i_k}) `
=   ` \bb{1}_\text{even}(a_1)\bb{1}_\text{even}(a_2)...\bb{1}_\text{even}(a_k) ` 
=   ` {(1,\text{if }\forall i\text{ }a_i\in2ZZ),(0,\text{otherwise}):} `      =   ` {(1,\text{if }n\text{ is a perfect square}),(0,\text{otherwise}):} `

Challenge: Find ` \sum_{D^2|n}\mu(\frac{n}{D^2}) `, where ` \mu() ` is the Möbius function.  Solution here.