On the grasshopper problem with signed jumps

The 6th problem of the 50th International Mathematical Olympiad (IMO), held in Germany, 2009, was the following. Let $a_1,a_2,...,a_n$ be distinct positive integers and let $M$ be a set of $n-1$ positive integers not containing $s=a_1+a_2+...+a_n$. A grasshopper is to jump along the real axis, starting at the point 0 and making $n$ jumps to the right with lengths $a_1,a_2,...,a_n$ in some order. Prove that the order can be chosen in such a way that the grasshopper never lands on any point in $M$. The problem was discussed in many on-line forums, as well by communities of students as by senior mathematicians. Though there have been attempts to solve the problem using Noga Alon's famous Combinatorial Nullstellensatz, up to now all known solutions to the IMO problem are elementary and inductive. In this paper we show that if the condition that the numbers $a_1,...a_n$ are positive is omitted, it allows us to apply the polynomial method to solve the modified problem.Comment: Submitted to AMS Monthly on 1st March, 2010; last revision in August, 201

Alon's Nullstellensatz for multisets

Alon's combinatorial Nullstellensatz (Theorem 1.1 from \cite{Alon1}) is one of the most powerful algebraic tools in combinatorics, with a diverse array of applications. Let \F be a field, $S_1,S_2,..., S_n$ be finite nonempty subsets of \F. Alon's theorem is a specialized, precise version of the Hilbertsche Nullstellensatz for the ideal of all polynomial functions vanishing on the set S=S_1\times S_2\times ... \times S_n\subseteq \F^n. From this Alon deduces a simple and amazingly widely applicable nonvanishing criterion (Theorem 1.2 in \cite{Alon1}). It provides a sufficient condition for a polynomial $f(x_1,...,x_n)$ which guarantees that $f$ is not identically zero on the set $S$. In this paper we extend these two results from sets of points to multisets. We give two different proofs of the generalized nonvanishing theorem. We extend some of the known applications of the original nonvanishing theorem to a setting allowing multiplicities, including the theorem of Alon and F\"uredi on the hyperplane coverings of discrete cubes.Comment: Submitted to the journal Combinatorica of the J\'anos Bolyai Mathematical Society on August 5, 201

Remarks to Arsovski's proof of Snevily's conjecture

Based on the recent work of Arsovski, we confirm a conjecture of Feng, Sun, and Xiang, and we give a shortened proof of Snevily's conjecture.Comment: 5 pages, LaTeX2e; v2: revised version incorporating suggestions by the referee (e.g. title was changed); to appear in Ann. Univ. Sci. Budapest. E\"otv\"os Sect. Math

Some extensions of Alon's Nullstellensatz

Alon's combinatorial Nullstellensatz, and in particular the resulting nonvanishing criterion is one of the most powerful algebraic tools in combinatorics, with many important applications. In this paper we extend the nonvanishing theorem in two directions. We prove a version allowing multiple points. Also, we establish a variant which is valid over arbitrary commutative rings, not merely over subrings of fields. As an application, we prove extensions of the theorem of Alon and F\"uredi on hyperplane coverings of discrete cubes.Comment: Inital submission: Thu, 24 Mar 201

Analízis feladatgyűjtemény II.

Ez a feladatgyűjtemény elsősorban azon egyetemi hallgatók számára készült, akik matematikát, ezen belül kalkulust és analízist tanulnak. A könyv fő feladata bevezetni az olvasót a a differenciál és integrálszámításba és ezek alkalmazásaiba