81 research outputs found
Beyond sum-free sets in the natural numbers
For an interval [1,N]⊆N, sets S⊆[1,N] with the property that |{(x,y)∈S2:x+y∈S}|=0, known as sum-free sets, have attracted considerable attention. In this paper, we generalize this notion by considering r(S)=|{(x,y)∈S2:x+y∈S}|, and analyze its behaviour as S ranges over the subsets of [1,N]. We obtain a comprehensive description of the spectrum of attainable r-values, constructive existence results and structural characterizations for sets attaining extremal and near-extremal values.Publisher PDFPeer reviewe
Primitive free cubics with specified norm and trace
The existence of a primitive free (normal) cubic x3 - ax2 + cx - b over a finite field F with arbitrary specified values of a (≠0) and b (primitive) is guaranteed. This is the most delicate case of a general existence theorem whose proof is thereby completed
Grid classes and the Fibonacci dichotomy for restricted permutations
We introduce and characterise grid classes, which are natural generalisations
of other well-studied permutation classes. This characterisation allows us to
give a new, short proof of the Fibonacci dichotomy: the number of permutations
of length n in a permutation class is either at least as large as the nth
Fibonacci number or is eventually polynomial
Characterising bimodal collections of sets in finite groups
A collection of disjoint subsets A = {A 1 ,A 2 ,...,A m } of a finite abelian group is said to have the bimodal property if, for any non-zero group element δ, either δ never occurs as a difference between an element of A i and an element of some other set A j , or else for every element a i in A i there is an element a j ∈ A j for some j 6= i such that a i − a j = δ. This property arises in various familiar situations, such as the cosets of a fixed subgroup or in a group partition, and has applications to the construction of optimal algebraic manipulation detection (AMD) codes. In this paper, we obtain a structural characterisation for bimodal collections of sets
Grid classes and partial well order
We prove necessary and sufficient conditions on a family of (generalised)
gridding matrices to determine when the corresponding permutation classes are
partially well-ordered. One direction requires an application of Higman's
Theorem and relies on there being only finitely many simple permutations in the
only non-monotone cell of each component of the matrix. The other direction is
proved by a more general result that allows the construction of infinite
antichains in any grid class of a matrix whose graph has a component containing
two or more non-monotone-griddable cells. The construction uses a
generalisation of pin sequences to grid classes, together with a number of
symmetry operations on the rows and columns of a gridding.Comment: 22 pages, 7 figures. To appear in J. Comb. Theory Series
Non-disjoint strong external difference families can have any number of sets
Strong external difference families (SEDFs) are much-studied combinatorial
objects motivated by an information security application. A well-known
conjecture states that only one abelian SEDF with more than 2 sets exists. We
show that if the disjointness condition is replaced by non-disjointness, then
abelian SEDFs can be constructed with more than 2 sets (indeed any number of
sets). We demonstrate that the non-disjoint analogue has striking differences
to, and connections with, the classical SEDF and arises naturally via another
coding application
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