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Möbius inversion

(Topic)

Theorem 1 (Moebius Inversion) Let $\mu$ be the Moebius function, and let and be two functions on the positive integers. Then the following two conditions are equivalent:

$\displaystyle f(n)$	$\displaystyle =$	$\displaystyle \sum_{d\vert n}g(d)\textrm{ for all }n\in\mathbb{N}^{*}$	(1)
$\displaystyle g(n)$	$\displaystyle =$	$\displaystyle \sum_{d\vert n}\mu(d)f\left(\frac{n}{d}\right)\textrm{ for all }n\in\mathbb{N}^{*}$	(2)

Proof: Fix some $n\in\mathbb{N}^{*}$ . Assuming (1), we have

$\displaystyle \sum_{d\vert n}\mu(d)f\left(\frac{n}{d}\right)$	$\displaystyle =$	$\displaystyle \sum_{d\vert n}\mu(d)\sum_{e\vert n/d}g(e)$
	$\displaystyle =$	$\displaystyle \sum_{k\vert n}\sum_{d\vert k}\mu(d)g\left(\frac{n}{k}\right)$
	$\displaystyle =$	$\displaystyle \sum_{k\vert n}g\left(\frac{n}{k}\right)\sum_{d\vert k}\mu(d)$
	$\displaystyle =$	$\displaystyle g(n),$

the last step following from the identity given in the Mobius function entry.

Conversely, assuming (2), we get

$\displaystyle \sum_{d\vert n}g(d)$	$\displaystyle =$	$\displaystyle \sum_{d\vert n}\sum_{e\vert d}\mu(e)f\left(\frac{d}{e}\right)$
	$\displaystyle =$	$\displaystyle \sum_{k\vert n}f\left(\frac{n}{k}\right)\sum_{e\vert k}\mu\left(\frac{k}{e}\right)$
	$\displaystyle =$	$\displaystyle f(n)$ (by the same identity)

as claimed.

Definitions: In the notation above, is called the Möbius transform of , and formula (2) is called the Möbius inversion formula.

Möbius-Rota inversion

G.-C. Rota has described a generalization of the Möbius formalism. In it, the set $\mathbb{N}^{*}$ , ordered by the relation $x\vert y$ between elements

and

, is replaced by a more general ordered set, and $\mu$ is replaced by a function of two variables.

Let $(S,\le)$ be a locally finite ordered set, i.e. an ordered set such that $\{z\in S\vert x\le z\le y\}$ is a finite set for all $x,y\in S$ . Let be the set of functions $\alpha:S\times S\to\mathbb{Z}$ such that

$\displaystyle \alpha(x,x)$	$\displaystyle =$	$\displaystyle 1\textrm{ for all }x\in S$	(3)
$\displaystyle \alpha(x,y)$	$\displaystyle \neq$	$\displaystyle 0\textrm{ implies }x\leq y$	(4)

becomes a monoid if we define the product of any two of its elements, say $\alpha$ and $\beta$ , by

$\displaystyle (\alpha\beta)(x,y)=\sum_{t\in S}\alpha(x,t)\beta(t,y).$

The sum makes sense because $\alpha(x,t)\beta(t,y)$ is nonzero for only finitely many values of

. (Clearly this definition is akin to the definition of the product of two square matrices.)

Consider the element $\iota$ of defined simply by

$\begin{displaymath}\iota(x,y)= \begin{cases} 1 & \textrm{ if $x\le y$} \\ 0 & \text{otherwise.} \end{cases}\end{displaymath}$

The function $\iota$ , regarded as a matrix over $\mathbb{Z}$ , has an inverse matrix, say $\nu$ . That means

$\begin{displaymath} \sum_{t\in S}\iota(x,t)\nu(t,y)= \begin{cases} 1 & \text{if $x=y$,} \\ 0 & \text{otherwise.} \end{cases}\end{displaymath}$

Thus for any $f,g\in A$ , the equations

$\displaystyle f$	$\displaystyle =$	$\displaystyle \iota g$	(5)
$\displaystyle g$	$\displaystyle =$	$\displaystyle \nu f$	(6)

are equivalent.

Now let's sketch out how the traditional Möbius inversion is a special case of Rota's notion. Let be the set $\mathbb{N}^{*}$ , ordered by the relation $x\vert y$ between elements and . In this case, $\nu$ is essentially a function of only one variable:

Proposition 3: With the above notation, $\nu(x,y)=\mu(y/x)$ for all $x,y\in\mathbb{N}^{*}$ such that $x\vert y$ .

The proof is fairly straightforward, by induction on the number of elements of the interval $\{z\in S\vert x\le z\le y\}$ .

Now let be a function from $\mathbb{N}^{*}$ to some additive group, and write $\overline{g}(x,y)=g(y/x)$ for all pairs such that $x\vert y$ .

Example:Let be a set, and let be the set of all finite subsets of , ordered by inclusion. The ordered set is left-finite, and for any $x,y\in S$ such that $x\subset y$ , we have $\nu(x,y)=(-1)^{\vert y-x\vert}$ , where $\vert z\vert$ denotes the cardinal of the finite set .

A slightly more sophisticated example comes up in connection with the chromatic polynomial of a graph or matroid.

An Additional Generalization

A final generalization of Moebius inversion occurs when the sum is taken over all integers less than some real value rather than over the divisors of an integer. Specifically, let $f:\mathbb{R}\rightarrow\mathbb{C}$ and define $F:\mathbb{R}\rightarrow \mathbb{C}$ by $F(x)=\sum_{n\leq x} f(\frac{x}{n})$ .

Then

$\displaystyle f(x)=\sum_{n\leq x} \mu(n) F\left(\frac{x}{n}\right).$

"Möbius inversion" is owned by mathcam. [ full author list (3) | owner history (2) ]

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