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Ernie Croot and Olof Sisask introduced a sampling technique for approximate translation invariance in additive combinatorics. Their Croot-Sisask almost-periodicity theorem controls how much a convolution changes under many translations.^[ref]

The useful shift is from exact algebraic symmetry to small error in a function norm. Random samples approximate averages; comparing repeated approximations exposes almost-periods. The original paper carefully specifies counting convolution and counting $L^p$ norms, which determine the constants and scale of the assertion.^[ref]

Kevin Henriot works on additive combinatorics. His authored Notes on the Croot-Sisask lemma by Kevin Henriot explain a sampling method for finding approximate translation symmetries of convolutions. The author publication page identifies these as expository notes.^[ref]

The Croot-Sisask almost-periodicity theorem finds many translations changing a convolution by only a small amount in an $L^p$ norm. Replacing a large average by a random sample makes the argument probabilistic; moment estimates control its error. Henriot's account develops this functional formulation with explicit counting-norm conventions.^[ref]

Olof Sisask co-developed the Croot-Sisask almost-periodicity theorem with Ernie Croot, introducing a probabilistic method for finding almost-periods of convolutions in additive combinatorics.^[ref]

Their proof approximates a convolution by sampling and then looks for translations preserving the approximation. Norm estimates turn that probabilistic approximation into many approximate symmetries. The method provides a way to detect additive structure without requiring exact periodicity.^[ref]

Apply the Croot-Sisask almost-periodicity theorem with the sampling set $A$, $S=G$, $f=1_{-A}$, and error $ϵ/k$. Since $A+G=G$, the doubling parameter is $K={\alpha }^{-1}$, and $\Vert 1_{-A}{\Vert }_p\le 1$. We obtain $X\subseteq G$ withsuch thatfor every $t\in X-X$. Every $x\in kX-kX$ is a sum of $k$ elements of $X-X$. Telescoping these $k$ shifts and using translation invariance and the triangle inequality for the Lp norm gives

One normalized form of the Croot-Sisask almost-periodicity theorem is as follows. If finite sets $A,S$ in a group satisfy $|A+S|\le K|A|$, $p\ge 2$, and $0<ϵ<1$, then for every function $f$ there is $T\subseteq S$ withsuch that

One standard normalized form of the Croot-Sisask almost-periodicity theorem is this. Let $A,S$ be finite subsets of an abelian group with $|A+S|\le K|A|$, let $q\ge 2$, let $0<ϵ<1$, and let $f$ be a complex function. There is $T\subseteq S$ withsuch that every $t\in T-T$ satisfies

For the proof, sample $k=O(q/{ϵ}^2)$ independent points of $A$ and approximate $1_A\ast f$ by the empirical average of the corresponding translates of $f$. A moment inequality bounds the expected $L^q$ error, so many samples are good. The small size of $A+S$ lets a translation and pigeonhole argument find many shifts in $S$ producing the same good approximation. Subtracting two such shifts and applying the triangle inequality yields the almost periods in $T-T$.