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For a prime number $p$ and a sequence of integers $a_0,dots,a_kin {0,1,dots,p}$, let $s(a_0,dots,a_k)$ be the minimum number of $(k+1)$-tuples $(x_0,dots,x_k)in A_0timesdotstimes A_k$ with $x_0=x_1+dots + x_k$, over subsets $A_0,dots,A_ksubseteqmathbb{Z}_p$ of sizes $a_0,dots,a_k$ respectively. An elegant argument of Lev (independently rediscovered by Samotij and Sudakov) shows that there exists an extremal configuration with all sets $A_i$ being intervals of appropriate length, and that the same conclusion also holds for the related problem, reposed by Bajnok, when $a_0=dots=a_k=:a$ and $A_0=dots=A_k$, provided $k$ is not equal 1 modulo $p$. By applying basic Fourier analysis, we show for Bajnoks problem that if $pge 13$ and $ain{3,dots,p-3}$ are fixed while $kequiv 1pmod p$ tends to infinity, then the extremal configuration alternates between at least two affine non-equivalent sets.
What is the minimum number of triangles in a graph of given order and size? Motivated by earlier results of Mantel and Turan, Rademacher solved the first non-trivial case of this problem in 1941. The problem was revived by ErdH{o}s in 1955; it is now
We show that the number of partial triangulations of a set of $n$ points on the plane is at least the $(n-2)$-nd Catalan number. This is tight for convex $n$-gons. We also describe all the equality cases.
We explore graph theoretical properties of minimal prime graphs of finite solvable groups. In finite group theory studying the prime graph of a group has been an important topic for the past almost half century. Recently prime graphs of solvable grou
Let $p > 155$ be a prime and let $G$ be a cyclic group of order $p$. Let $S$ be a minimal zero-sum sequence with elements over $G$, i.e., the sum of elements in $S$ is zero, but no proper nontrivial subsequence of $S$ has sum zero. We call $S$ is uns
Let $T$ be a rooted tree, and $V(T)$ its set of vertices. A subset $X$ of $V(T)$ is called an infima closed set of $T$ if for any two vertices $u,vin X$, the first common ancestor of $u$ and $v$ is also in $X$. This paper determines the trees with mi