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Wednesday, April 13, 2011

Möbius Inversion with Liouville Function

This article concerns Möbius Inversion with the Liouville Function.  For n=pa11pa22..., the number of distinct prime factors of n is

Question
(i)  Prove that
(ii) Hence show that

Solution:-
(i) Note that μ(d)=0 for divisors d containing any prime power of index 2 or higher.  We only need to consider divisors of the form d=pi1pi2...pih, where h=0 is interpreted as d=1.
    d|nμ(d)λ(d)
= h=0kpi1,pi2,...,pihμ(pi1pi2...pih)λ(pi1pi2...pih)
= h=0kpi1,pi2,...,pih(-1)h(-1)1+1+...+1   (where 1+1+...+1 has h copies of 1)
= h=0kpi1,pi2,...,pih(-1)2h = h=0kpi1,pi2,...,pih1 = h=0k(kh)
= 2k  = 2ω(n)
(ii)  Recall that λ(nd)=λ(n)λ(d)=λ(n)λ(d)
     From the result in part (i) we multiply both sides by λ(n) to get
                  d|nμ(d)λ(n)λ(d)=λ(n)2ω(n)
                  d|nμ(d)λ(nd)=λ(n)2ω(n)
    Treating this as the "Möbius Inversed" formula, the "original" formula is
                  d|nλ(d)2ω(d)=λ(n)
Hence
     d|nλ(nd)2ω(d) =   d|nλ(n)λ(d)2ω(d) =   λ(n)d|nλ(d)2ω(d)
=   λ(n)λ(n) =   λ(n)2 =   1

[End]

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