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]
Showing posts with label summatory function. Show all posts
Showing posts with label summatory function. Show all posts
Wednesday, April 13, 2011
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.
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.
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