Dirichlet convolution and the Möbius function

Recall from last time that the Dirichlet convolution of two functions f and g is written f \ast g and defined by:

\displaystyle (f \ast g)(n) = \sum_{ab = n} f(a) g(b)

where the sum is taken over all possible factorizations of n into a product ab of positive integers. Last time we saw that \ast is commutative and associative, and has \varepsilon(n) as an identity, where \varepsilon is the function which produces 1 for an input of 1 and 0 for all other inputs.

So what does the Möbius function have to do with this? Let’s start by considering a different function:

\displaystyle \mathbf{1}(n) = 1

is the function which ignores its input and always returns 1. Of course this is not the same thing as \varepsilon, and despite its name \mathbf{1} is indeed not an identity for Dirichlet convolution. That is, if we take some function f and find its Dirichlet convolution with \mathbf{1}, we don’t get f again—but we do get something interesting:

\displaystyle (\mathbf{1} \ast f)(n) = \sum_{ab=n} \mathbf{1}(a) f(b) = \sum_{ab=n} f(b) = \sum_{d \mid n} f(d)

The first step is the definition of \ast; the second step is the definition of \mathbf{1}; and the last step just notes that the sum of all f(b) where ab = n is the same as taking the sum of f(d) for all divisors of n.

That is, \mathbf{1} \ast f is the function which for a given n outputs not just f(n) but the sum of f over all divisors of n.

We’re now ready to see how \mu enters the picture!

Claim: \mathbf{1} \ast \mu = \varepsilon.

That is, \mu is the inverse of \mathbf{1} with respect to Dirichlet convolution. In my next post we’ll see some cool implications of this; for now, let’s just prove it.

Proof. Here’s the proof. Ready?

\displaystyle (\mathbf{1} \ast \mu)(n) = \sum_{d \mid n} \mu(d) = \varepsilon(n).

Actually, there was hardly anything left to prove! The first equality is because of what we just showed about taking the Dirichlet convolution of \mathbf{1} with something else. And the second equality is a property of \mu we just finished proving in some previous posts.


About Brent

Assistant Professor of Computer Science at Hendrix College. Functional programmer, mathematician, teacher, pianist, follower of Jesus.
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