Sum-free set
Set disjoint from its sumset with itself From Wikipedia, the free encyclopedia
In additive combinatorics and number theory, a subset A of an abelian group G is said to be sum-free if the sumset A + A is disjoint from A. In other words, A is sum-free if the equation has no solution with .
For example, the set of odd numbers is a sum-free subset of the integers, and the set {N + 1, ..., 2N } forms a large sum-free subset of the set {1, ..., 2N }. Fermat's Last Theorem is the statement that, for a given integer n > 2, the set of all nonzero nth powers of the integers is a sum-free set.
Some basic questions that have been asked about sum-free sets are:
- How many sum-free subsets of {1, ..., N } are there, for an integer N? Ben Green has shown[1] that the answer is , as predicted by the Cameron–Erdős conjecture.[2]
- How many sum-free sets does an abelian group G contain?[3]
- What is the size of the largest sum-free set that an abelian group G contains?[3]
A sum-free set is said to be maximal if it is not a proper subset of another sum-free set.
Let be defined by is the largest number such that any subset of with size n has a sum-free subset of size k. The function is subadditive, and by the Fekete subadditivity lemma, exists. Erdős proved that , and conjectured that equality holds.[4] This was proved by Eberhard, Green, and Manners.[5]
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References
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