additive combinatorics

One major area of study in additive combinatorics is that of inverse problems: for instance, given the sumset A#43;B is small in size, what can we say about the structures of A and B? In the case of integer sumsets, Freiman's theorem provides a partial answer.

We now discuss what appears at first glance to be an unrelated topic, namely that of additive combinatorics (and its noncommutative counterpart, multiplicative combinatorics). One of the main objects of study in either additive or multiplicative combinatorics are approximate groups — sets A (typically finite) contained in an additive or multiplicative ambient group G that are "almost groups" in the sense that they are "almost" closed under either addition or multiplication.

2014, Terence Tao, Hilbert's Fifth Problem and Related Topics, American Mathematical Society, page 14

This book deals with additive combinatorics, a vibrant area of current mathematical research. Additive combinatorics—an offspring of combinatorial number theory and additive number theory—can be described as the study of combinatorial properties of sumsets (collections of sums with terms from given subsets) in additive structures.

2016, Bela Bajnok, Additive Combinatorics: A Menu of Research Problems, Taylor & Francis (CRC Press), page xi