Thanks to Sophie Stevens for helpful discussions at the Georgia discrete analysis conference leading to this post. We show that if has small (slightly generalized) third order energy, then has a large difference set. We present an argument due to Shkredov and Schoen. This has the advantage of being drastically simpler (and slightly stronger) than the analog for sumsets, which was discussed in this previous blog post. We say if for some .

Let be finite. We define the *sumset* and *product set* via

We have the following difference–product conjecture.

**Conjecture 1:** Let . Then for any finite one has

We say a finite is convex if

The following conjecture is due to Erdős.

**Conjecture (Convex):** Suppose is convex. Then

Study of the quantity has played a crucial role in recent progress on both conjectures, which we define now. For finite, we define

where

See my recent paper on Conjecture 1 for a lengthy introduction to . We mention here that and intuitively the larger is the more additive structure has. For instance, we have

Indeed is a higher order analog of the more common additive energy of a set, where we have some flexibility in choosing (for instance, we choose below).

An application of Cauchy–Schwarz, as explained in equation 2 of this paper, reveals

Thus if , we have that . We conjecture that this can be improved.

**Conjecture (dplus):** Let be finite. Then

This would imply Conjecture (Convex) and Conjecture 1 for . The goal of the rest of this post is to prove the current state of the art progress due to Shkredov and Schoen. The proof of Theorem 1 is simple enough that we compute the constant.

**Theorem 1:** Let be a subset of any additive group. Then

We will use

as is explained using a simple geometric argument in equation 9 of this paper of Murphy, Rudnev, Shkredov, and Shteinikov. For , let

Thus

Let

be the set of popular differences. Thus

By two applications of Cauchy–Schwarz,

Combining this with (3) and (4), we find

Now we use Katz–Koelster inclusion of the form

to find

Since , we have , and so

By Cauchy–Schwarz (interpolating between and ), we have

and so by the definition of , (2), and (5), we arrive at

Simplifying gives the (slightly stronger) desired

Developing a fourth moment analog of the above result would be worthwhile as it would have applications to finite field sum-product problem (edit: Ilya Shkredov and I pursued this idea further in a recent preprint).

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