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On the Möbius function

April 11, 2015 by Sabyasachi Mukherjee. 7 comments

The Möbius function is a rather useful one, especially when dealing with multiplicative functions. But first of all, a few definitions are in order.

Definition 1: Let $$\omega(n)$$ be the number of distinct prime divisors of $$n$$.

Definition 2: The Möbius function, $$\mu(n)$$ is defined as $$(-1)^{\omega(n)}$$ if $$n$$ is square-free and $$0$$ otherwise. (A number $$n$$ is square-free if there is no prime $$p$$ such that $$p^2$$ divides $$n$$.)

It may be a good idea to compute the following: $$\omega(n)$$ and $$\mu(n)$$ for $$n=64, 12, 17, 1$$ to get an idea of what’s going on.

A Möbius function is an arithmetic function, i.e. a function from $$\mathbb{N}$$ to $$\mathbb{C}$$. Can you think of some other arithmetic functions?

Definition 3:  If $$f(n)$$ is an arithmetic function not identically zero such that $$f(m)f(n)=f(mn)$$ for every pair of positive integers $$m, n$$ satisfying $$(m,n)=1$$, then $$f(n)$$ is multiplicative.

Theorem 0: $$\mu(n)$$ is a multiplicative function.

Proof: The proof is left to the reader.

Theorem 1: If $$f(n)$$ is a multiplicative function, so is $$\displaystyle \sum_{d|n} f(d)$$.

Before we jump to the proof, let us be clear on the notation. For $$n=12$$, $$\displaystyle \sum_{d|n} f(d)=f(1)+f(2)+f(3)+f(4)+f(6)+f(12)$$ i.e. the sum taken over the divisors of $$n$$.

Proof: Consider the sets $$A= \{d : 0<d, d|n \}$$ and $$B= \{d_1d_2 : d_1|m_1, d_2|m_2, (m_1,m_2)=1,m_1m_2=n\}$$.

(Note that such $$m_1, m_2$$ exist. Take $$m_1=1,m_2=n$$).

Now take $$d \in A$$. Then $$1\cdot d$$ divides $$1\cdot n$$ which means that $$d \in B$$ and so $$A \subset B$$.

Now take $$s \in B$$. So $$s$$ is of the form $$d_1d_2$$ where $$d_1|m_1$$ and $$d_2 |m_2$$ such that $$(m_1,m_2)=1$$ with their product $$n$$. So, $$s$$ divides $$m$$ and hence $$B \subset A$$. From these, we infer that $$A=B$$.

Now, suppose that $$(m,n)=1$$. Then $$\displaystyle F(mn)=\sum_{d|mn} f(d)=\sum_{d_1|m,d_2|n,d_1d_2=mn}f(d_1d_2)=\sum_{d_1|m}\sum_{d_2|n} f(d_1d_2)$$

So, $$F(mn)=\sum_{d_1|m}\sum_{d_2|n}f(d_1)f(d_2)(\text{since f is multiplicative})=\sum_{d_1|m}f(d_1)\sum_{d_2|n}f(d_2).$$

Now with this theorem in hand, let us a prove a few more theorems.

Theorem 2: $$\displaystyle \sum_{d|n} \mu(d)$$ is $$0$$ if $$n>1$$ and $$1$$ if $$n=1$$.

Proof: The case $$n=1$$ is trivial.

Suppose that $$n>1$$.

Since $$\mu(n)$$ is multiplicative, so is $$\displaystyle F(n)= \sum_{d|n} \mu(d)$$. Now let us recall that $$n>1$$ can be written as a product of primes, say $$\displaystyle n=p_i^{a_i}\dots p_k^{a_k}$$. So, we can write $$F(n)=F(p_1^{a^1})\dots F(p_k^{a_k})$$.

As $$a_i\ge 1$$, $$F(p_i^{a_i})= 0$$ (using the definition of $$\mu$$.)That means $$F(n)=0$$. The desired conclusion now follows from this discussion.

Before we now move on to a theorem which shows a connection between the Euler’s $$\varphi-$$function and the Möbius function, let us state a really important theorem, also known as the Möbius Inversion formula.

Theorem 3: If $$\displaystyle F(n)=\sum_{d|n}f(d)$$ for every positive integer $$n$$, then $$\displaystyle f(n)=\sum_{d|n}\mu(d)F(\frac{n}{d})$$.

Proof:  We will flesh this proof in somewhat lesser detail because we have developed most of the techniques related to his proof.

Note that $$\displaystyle \sum_{d|n}\mu(d)F(\frac{n}{d})$$

$$\displaystyle =\sum_{d|n}\mu(d)\sum_{k|(n/d)} f(k)= \sum_{dk|n}\mu(d)f(k)=\sum_{dk|n}\mu(k)f(d)$$.

Can the reader now complete the proof? (Hint: Use Theorem 2).

Theorem 4: $$\displaystyle \varphi(n)= n\sum_{d|n} \frac{\mu(d)}{d}.$$

Proof:  To write the proof, we use a lemma.

Lemma: $$\displaystyle \sum_{d|n} \varphi(d)=n$$.

Proof of the lemma: Note that for $$n=1$$, this is true. Suppose that $$n>1$$. Then $$\displaystyle n=p_1^{a_1}\dots p_k^{a_k}$$ for some primes $$p_1,\dots, p_k$$. As $$\varphi(n)$$ is multiplicative, so is $$\displaystyle F(n)= \sum_{d|n} \varphi(d)$$

That means $$\displaystyle F(n)=\prod_{i=1}^k F(p_i^{a_i})$$. A quick calculation reveals that $$F(p_i^{a_i})=p_i^{a_i}$$ which gives $$F(n)=n$$.

We can now use this lemma and  the Möbius Inversion formula to finish off the proof.

Here as some other problems on multiplicative functions.

Problem 1:  $$\displaystyle \frac{1}{\varphi(n)}=\frac{1}{n}\sum_{d|n}\frac{\mu(d)^2}{\varphi(d)}$$.

Problem 2:  For each positive integer $$n$$, $$\displaystyle \mu(n)\mu(n+1)\mu(n+2)\mu(n+3) = 0$$.

Problem 3: $$\displaystyle \sum_{d|n}|\mu(d)| = 2^{\omega(n)}$$.

When do n and 2n have the same decimal digits?

June 11, 2014 by MJD. 3 comments

A recent question on math.stackexchange asks for the smallest positive number $$A$$ for which the number $$2A$$ has the same decimal digits in some other order.

Math geeks may immediately realize that $$142857$$ has this property, because it is the first 6 digits of the decimal expansion of $$\frac 17$$, and the cyclic behavior of the decimal expansion of $$\frac n7$$ is well-known. But is this the minimal solution? It is not. Brute-force enumeration of the solutions quickly reveals that there are 12 solutions of 6 digits each, all permutations of $$142857$$, and that larger solutions, such as $$1025874$$ and $$1257489$$, seem to follow a similar pattern. What is happening here?

Stuck in Dallas-Fort Worth airport last month, I did some work on the problem, and although I wasn’t able to solve it completely, I made significant progress. I found a method that allows one to hand-calculate that there is no solution with fewer than six digits, and to enumerate all the solutions with 6 digits, including the minimal one. I found an explanation for the surprising behavior that solutions tend to be permutations of one another. The short form of the explanation is that there are fairly strict conditions on which sets of digits can appear in a solution of the problem. But once the set of digits is chosen, the conditions on that order of the digits in the solution are fairly lax.

So one typically sees, not only in base 10 but in other bases, that the solutions to this problem fall into a few classes that are all permutations of one another; this is exactly what happens in base 10 where all the 6-digit solutions are permutations of $$124578$$. As the number of digits is allowed to increase, the strict first set of conditions relaxes a little, and other digit groups can (and do) appear as solutions.

Notation

The property of interest, $$P_R(A)$$, is that the numbers $$A$$ and $$B=2A$$ have exactly the same base-$$R$$ digits. We would like to find numbers $$A$$ having property $$P_R$$ for various $$R$$, and we are most interested in $$R=10$$. Suppose $$A$$ is an $$n$$-digit numeral having property $$P_R$$; let the (base-$$R$$) digits of $$A$$ be $$a_{n-1}\ldots a_1a_0$$ and similarly the digits of $$B = 2A$$ are $$b_{n-1}\ldots b_1b_0$$. The reader is encouraged to keep in mind the simple example of $$R=8, n=4, A=\mathtt{1042}, B=\mathtt{2104}$$ which we will bring up from time to time.

Since the digits of $$B$$ and $$A$$ are the same, in a different order, we may say that $$b_i = a_{P(i)}$$ for some permutation $$P$$. In general $$P$$ might have more than one cycle, but we will suppose that $$P$$ is a single cycle. All the following discussion of $$P$$ will apply to the individual cycles of $$P$$ in the case that $$P$$ is a product of two or more cycles. For our example of $$A=\mathtt{1042}, B=\mathtt{2104}$$, we have $$P = (0\,1\,2\,3)$$ in cycle notation. We won’t need to worry about the details of $$P$$, except to note that $$i, P(i), P(P(i)), \ldots, P^{n-1}(i)$$ completely exhaust the indices $$0. \ldots n-1$$, and that $$P^n(i) = i$$ because $$P$$ is an $$n$$-cycle.

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