# Restricted sumset

In additive number theory and combinatorics, a restricted sumset has the form

${\displaystyle S=\{a_{1}+\cdots +a_{n}:\ a_{1}\in A_{1},\ldots ,a_{n}\in A_{n}\ \mathrm {and} \ P(a_{1},\ldots ,a_{n})\not =0\},}$

where ${\displaystyle A_{1},\ldots ,A_{n}}$ are finite nonempty subsets of a field F and ${\displaystyle P(x_{1},\ldots ,x_{n})}$ is a polynomial over F.

When ${\displaystyle P(x_{1},\ldots ,x_{n})=1}$, S is the usual sumset ${\displaystyle A_{1}+\cdots +A_{n}}$ which is denoted by nA if ${\displaystyle A_{1}=\cdots =A_{n}=A}$; when

${\displaystyle P(x_{1},\ldots ,x_{n})=\prod _{1\leq i

S is written as ${\displaystyle A_{1}\dotplus \cdots \dotplus A_{n}}$ which is denoted by ${\displaystyle n^{\wedge }A}$ if ${\displaystyle A_{1}=\cdots =A_{n}=A}$. Note that |S| > 0 if and only if there exist ${\displaystyle a_{1}\in A_{1},\ldots ,a_{n}\in A_{n}}$ with ${\displaystyle P(a_{1},\ldots ,a_{n})\not =0}$.

## Cauchy–Davenport theorem

The Cauchy–Davenport theorem named after Augustin Louis Cauchy and Harold Davenport asserts that for any prime p and nonempty subsets A and B of the prime order cyclic group Z/pZ we have the inequality[1][2]

${\displaystyle |A+B|\geq \min\{p,\ |A|+|B|-1\}.\,}$

We may use this to deduce the Erdős–Ginzburg–Ziv theorem: given any sequence of 2n−1 elements in Z/n, there are n elements that sums to zero modulo n. (Here n does not need to be prime.)[3][4]

A direct consequence of the Cauchy-Davenport theorem is: Given any set S of p−1 or more nonzero elements, not necessarily distinct, of Z/pZ, every element of Z/pZ can be written as the sum of the elements of some subset (possibly empty) of S.[5]

Kneser's theorem generalises this to general abelian groups.[6]

## Erdős–Heilbronn conjecture

The Erdős–Heilbronn conjecture posed by Paul Erdős and Hans Heilbronn in 1964 states that ${\displaystyle |2^{\wedge }A|\geq \min\{p,2|A|-3\}}$ if p is a prime and A is a nonempty subset of the field Z/pZ.[7] This was first confirmed by J. A. Dias da Silva and Y. O. Hamidoune in 1994[8] who showed that

${\displaystyle |n^{\wedge }A|\geq \min\{p(F),\ n|A|-n^{2}+1\},}$

where A is a finite nonempty subset of a field F, and p(F) is a prime p if F is of characteristic p, and p(F) = ∞ if F is of characteristic 0. Various extensions of this result were given by Noga Alon, M. B. Nathanson and I. Ruzsa in 1996,[9] Q. H. Hou and Zhi-Wei Sun in 2002,[10] and G. Karolyi in 2004.[11]

## Combinatorial Nullstellensatz

A powerful tool in the study of lower bounds for cardinalities of various restricted sumsets is the following fundamental principle: the combinatorial Nullstellensatz.[12] Let ${\displaystyle f(x_{1},\ldots ,x_{n})}$ be a polynomial over a field F. Suppose that the coefficient of the monomial ${\displaystyle x_{1}^{k_{1}}\cdots x_{n}^{k_{n}}}$ in ${\displaystyle f(x_{1},\ldots ,x_{n})}$ is nonzero and ${\displaystyle k_{1}+\cdots +k_{n}}$ is the total degree of ${\displaystyle f(x_{1},\ldots ,x_{n})}$. If ${\displaystyle A_{1},\ldots ,A_{n}}$ are finite subsets of F with ${\displaystyle |A_{i}|>k_{i}}$ for ${\displaystyle i=1,\ldots ,n}$, then there are ${\displaystyle a_{1}\in A_{1},\ldots ,a_{n}\in A_{n}}$ such that ${\displaystyle f(a_{1},\ldots ,a_{n})\not =0}$.

The method using the combinatorial Nullstellensatz is also called the polynomial method. This tool was rooted in a paper of N. Alon and M. Tarsi in 1989,[13] and developed by Alon, Nathanson and Ruzsa in 1995-1996,[9] and reformulated by Alon in 1999.[12]

## References

1. Nathanson (1996) p.44
2. Geroldinger & Ruzsa (2009) pp.141–142
3. Nathanson (1996) p.48
4. Geroldinger & Ruzsa (2009) p.53
5. Wolfram's MathWorld, Cauchy-Davenport Theorem, http://mathworld.wolfram.com/Cauchy-DavenportTheorem.html, accessed 20 June 2012.
6. Geroldinger & Ruzsa (2009) p.143
7. Nathanson (1996) p.77
8. Dias da Silva, J. A.; Hamidoune, Y. O. (1994). "Cyclic spaces for Grassmann derivatives and additive theory". Bulletin of the London Mathematical Society. 26 (2): 140–146. doi:10.1112/blms/26.2.140.
9. Alon, Noga; Nathanson, Melvyn B.; Ruzsa, Imre (1996). "The polynomial method and restricted sums of congruence classes" (PDF). Journal of Number Theory. 56 (2): 404–417. doi:10.1006/jnth.1996.0029. MR 1373563.
10. Hou, Qing-Hu; Sun, Zhi-Wei (2002). "Restricted sums in a field". Acta Arithmetica. 102 (3): 239–249. Bibcode:2002AcAri.102..239H. doi:10.4064/aa102-3-3. MR 1884717.
11. Károlyi, Gyula (2004). "The Erdős–Heilbronn problem in abelian groups". Israel Journal of Mathematics. 139: 349–359. doi:10.1007/BF02787556. MR 2041798.
12. Alon, Noga (1999). "Combinatorial Nullstellensatz" (PDF). Combinatorics, Probability and Computing. 8 (1–2): 7–29. doi:10.1017/S0963548398003411. MR 1684621.
13. Alon, Noga; Tarsi, Michael (1989). "A nowhere-zero point in linear mappings". Combinatorica. 9 (4): 393–395. doi:10.1007/BF02125351. MR 1054015.
• Geroldinger, Alfred; Ruzsa, Imre Z., eds. (2009). Combinatorial number theory and additive group theory. Advanced Courses in Mathematics CRM Barcelona. Elsholtz, C.; Freiman, G.; Hamidoune, Y. O.; Hegyvári, N.; Károlyi, G.; Nathanson, M.; Solymosi, J.; Stanchescu, Y. With a foreword by Javier Cilleruelo, Marc Noy and Oriol Serra (Coordinators of the DocCourse). Basel: Birkhäuser. ISBN 978-3-7643-8961-1. Zbl 1177.11005.
• Nathanson, Melvyn B. (1996). Additive Number Theory: Inverse Problems and the Geometry of Sumsets. Graduate Texts in Mathematics. 165. Springer-Verlag. ISBN 0-387-94655-1. Zbl 0859.11003.