This website contains problems from math contests. Problems and corresponding tags were obtained from the Art of Problem Solving website.

Tags were heavily modified to better represent problems.

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Found problems: 79

2001 China Team Selection Test, 2.1

Let the vertex set \( V \) of a graph be partitioned into \( h \) parts \( (V = V_1 \cup V_2 \cup \cdots \cup V_h) \), with \(|V_1| = n_1, |V_2| = n_2, \ldots, |V_h| = n_h \). If there is an edge between any two vertices only when they belong to different parts, the graph is called a complete \( h \)-partite graph, denoted as \( k(n_1, n_2, \ldots, n_h) \). Let \( n \) and \( r \) be positive integers, \( n \geq 6 \), \( r \leq \frac{2}{3}n \). Consider the complete \( r + 1 \)-partite graph \( k\left(\underbrace{1, 1, \ldots, 1}_{r}, n - r\right) \). Answer the following questions: 1. Find the maximum number of disjoint circles (i.e., circles with no common vertices) in this complete \( r + 1 \)-partite graph. 2. Given \( n \), for all \( r \leq \frac{2}{3}n \), find the maximum number of edges in a complete \( r + 1 \)-partite graph \( k(1, 1, \ldots, 1, n - r) \) where no more than one circle is disjoint.

2023 Tuymaada Olympiad, 5

A graph contains $p$ vertices numbered from $1$ to $p$, and $q$ edges numbered from $p + 1$ to $p + q$. It turned out that for each edge the sum of the numbers of its ends and of the edge itself equals the same number $s$. It is also known that the numbers of edges starting in all vertices are equal. Prove that \[s = \dfrac{1}{2} (4p+q+3).\]

2019 Simurgh, 3

We call a graph symmetric, if we can put its vertices on the plane such that if the edges are segments, the graph has a reflectional symmetry with respect to a line not passing through its vertices. Find the least value of $K$ such that the edges of every graph with $100$ vertices, can be divided into $K$ symmetric subgraphs.

2011 Indonesia TST, 2

A graph $G$ with $n$ vertex is called [i]good [/i] if every vertex could be labelled with distinct positive integers which are less than or equal $\lfloor \frac{n^2}{4} \rfloor$ such that there exists a set of nonnegative integers $D$ with the following property: there exists an edge between $2$ vertices if and only if the difference of their labels is in $D$. Show that there exists a positive integer $N$ such that for every $n \ge N$, there exist a not-good graph with $n$ vertices.