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The Erdős discrepancy problem and more

source — in memoriam

Raymond Redheffer was an American mathematician. He worked for his PhD under Norman Levinson, who was famous for his work on the Riemann Hypothesis. Redheffer spent his whole career at UCLA and was a great teacher—see this for more details. Redheffer wrote the mathematical content for the IBM-sponsored 50-foot poster “Men of Modern Mathematics,” which IBM last year released as a free I-Pad app.

Today Ken and I wish to talk about a problem that Gil Kalai spoke last month at Tech, and suggest an approach to solving the problem that draws on a matrix studied by Redheffer.

Unfortunately I was not able to hear Gil’s talk since I was at the RSA annual conference on security—see here. But the talk featured an old problem of Paul Erdős that is one of those terrible problems: easy to state, easy to think about, impossible to make progress on.

I have two things to say about this classic problem. First, it has a nice statement as a question about matrices. Second, it may be related to the Riemann Hypothesis. Erdős was known for offering monetary rewards up to $1,000 for problems he liked, and EDP itself carried a reward of $500. The Riemann Hypothesis, however, is one of the million-dollar Clay Millennium Prize Problems.

The Problem

The Erdős Discrepancy Problem (EDP) is ultimately about how strongly the class of arithmetical progressions suffices as a lens for viewing random behaviors in infinite sequences. Let be an infinite sequence where each is . The EDP asks whether, for any constant , we can always find and such that

Of course this is true for any “random” sequence with : the sums up to will often be the standard deviation, which is of order , and some sums will go a slight bit higher. We can of course build a non-random sequence in which these sums stay bounded, such as

,

but then taking creates huge discrepancies. To falsify the conjecture, one would have to create an ingenious sequence in which all the sums stay bounded for all .

There do exist sequences that keep all the sums lower than what happens for random sequences. An elementary way is to define

where is the lowest-order non-zero digit of written in base 3. This sequence is multiplicative, meaning . The EDP is open even for multiplicative sequences, for which only the case matters. For this multiplicative sequence, the sums stay within . For a proof of this and much more information about the EDP, see this original post by Timothy Gowers, a more-recent post by Gil, and this Polymath page.

A Linear Algebra Approach

We offer another way to view the famous EDP, and wonder how widely it is known. Define the 0-1 matrix by setting the entry to only if divides . Here is :

Then we have the following observation:

Theorem 1 The EDP is true—i.e., for every and infinite vector of there are and so that

(where we call the sum )—if and only if for every such and , there is an so that

Here is the vector of .

The key to the proof of this is quite simple. Note that the row of is

The sup-norm, , is the maximum over all rows .

The proof itself requires just some care with bounds and indexing. For the forward direction, given and we take . Then the th entry of the vector is exactly the value , no more and no less. Hence its sup-norm is at least . In the other direction, given such that has , let be the entry that achieves the sup-norm, i.e., . Then for we have that includes the same terms, so . This ends the proof.

Hence the norm assertion is exactly the same as the EDP. What Ken and I find interesting is that the matrix is upper triangular and has a number of interesting properties. The question is, can these properties be used to make progress? Let’s discuss this next.

Some Ideas

The above connection may be well known, but it is new to me. In any event it suggests a number of possible approaches to proving EDP. Linear algebra has solved many problems in the past that did not immediately seem to be about linear algebra. This is the hope—that we may be able to exploit linear algebra to solve the EDP. Or at least make some small progress.

The matrix has several properties that make it attractive. The matrix is almost recursive in structure. Let be even, and consider the four principal sub-matrices. The upper-left one is . At lower left we have the all-zero matrix, and at lower right we have the identity matrix. This leaves the matrix at upper right, call it . That is, we have:

The matrices record the way the integers in the “bottom half” divide integers in the top half. Especially for a power of 2, we might hope this has a quantifiable, regular pattern. Understanding this seems to be all we need to understand and thus make progress on EDP.

EDP And The Riemann Hypothesis

Ray Redheffer seems to have understood this, because his matrix is the same as except for the first column. That is, is the matrix whose entries are if divides or if ; otherwise, the entry is .

Actually what Redheffer did, in his source paper published in 1977, was fill the first column with variables ranging over complex numbers in the unit disc. He showed that the problem of maximizing the norm of the determinant of the resulting matrix is equivalent to maximizing over taking only the real values , , or , and the maximum is obtained by setting equal to the value of the Möbius function . That is:

1. if is square-free and has an even number of prime factors;
2. if is square-free and has an odd number of prime factors;
3. if is divisible by a square number, i.e., has a repeated prime factor.

Two years ago, we covered the relationship between and the Mertens function

In particular, the determinant of the Redheffer matrix—with all set to 1—is exactly —see also this. Thus the question of the growth of connects the matrix and the EDP to the prime number theorem and also the Riemann Hypothesis. The first connection, which is proved, is that grows less than linearly: the prime number theorem is equivalent to . The second, which is one of the great open problem in mathematics, is:

Does grow as if it were a “random” sum, for instance as

for any ?

This is equivalent to the Riemann Hypothesis. This connection is noted in terms of the related Liouville lambda function on the EDP PolyMath page at the end of this section. We wonder how far it can be widened.

A Further Idea

Let be the Redheffer matrix. We note that it has the following block structure:

The difference from is that now is a the matrix of all ‘s in the first column and zeroes everywhere else, . In Redheffer’s more general case, has variables in the first column, but in any event it is a rank-1 matrix. Now let

and . Then is equal to

We note that which is equal to . But by a standard block rule for determinants, we get that is equal to the determinant of

This is of the form where is a rank-one matrix.

Putting this all together yields that

But this means that

provided and is even.

The key question is, can we use this to get a bound on the Mertens function, or to get some congruence results? The obvious idea that divides is false—the obstacle is that is not an integer matrix. Its denominators are due to not being an integer matrix. But if it were an integer matrix, then we would get some non-trivial relationship. Does this lead anywhere?

Open Problems

Does the linear algebra approach to EDP seem useful? Also the Redheffer matrix is “almost” what we need to understand to solve EDP. Does this suggest a connection between EDP and the Riemann Hypothesis? Could that explain the reason EDP is so hard?

[fixed base-3 sequence to make it multiplicative rather than “anti”]