Möbius inversion
Theorem 1 (Moebius Inversion).
Let be the Moebius function, and let and be two functions on the positive integers. Then the following two conditions are equivalent:
(1) | |||||
(2) |
Proof: Fix some . Assuming (1), we have
the last step following from the identity given in the Mobius function entry.
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 , ordered by the relation between elements and , is replaced by a more general ordered set, and is replaced by a function of two variables.
Let be a locally finite ordered set, i.e. an ordered set such that is a finite set for all . Let be the set of functions such that
(3) | |||||
(4) |
becomes a monoid if we define the product of any two of its elements, say and , by
The sum makes sense because 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 of defined simply by
The function , regarded as a matrix over , has an inverse matrix, say . That means
Thus for any , the equations
(5) | |||||
(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 , ordered by the relation between elements and . In this case, is essentially a function of only one variable:
Proposition 3: With the above notation, for all such that .
The proof is fairly straightforward, by induction on the number of elements of the interval .
Now let be a function from to some additive group, and write for all pairs such that .
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 such that , we have , where 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 and define by .
Then
Title | Möbius inversion |
Canonical name | MobiusInversion |
Date of creation | 2013-03-22 11:46:58 |
Last modified on | 2013-03-22 11:46:58 |
Owner | mathcam (2727) |
Last modified by | mathcam (2727) |
Numerical id | 25 |
Author | mathcam (2727) |
Entry type | Topic |
Classification | msc 11A25 |
Synonym | Moebius inversion |
Related topic | MoebiusFunction |
Defines | Mobius inversion formula |
Defines | Mobius transform |
Defines | Mobius function |
Defines | Mobius-Rota inversion |