Found problems: 8
2006 Lithuania National Olympiad, 4
Find the maximal cardinality $|S|$ of the subset $S \subset A=\{1, 2, 3, \dots, 9\}$ given that no two sums $a+b | a, b \in S, a \neq b$ are equal.
2014 Danube Mathematical Competition, 4
Let $n$ be a positive integer and let $\triangle$ be the closed triangular domain with vertices at the lattice points $(0, 0), (n, 0)$ and $(0, n)$. Determine the maximal cardinality a set $S$ of lattice points in $\triangle$ may have, if the line through every pair of distinct points in $S$ is parallel to no side of $\triangle$.
1987 Bundeswettbewerb Mathematik, 2
Let $n$ be a positive integer and $M=\{1,2,\ldots, n\}.$ A subset $T\subset M$ is called [i]heavy[/i] if each of its elements is greater or equal than $|T|.$ Let $f(n)$ denote the number of heavy subsets of $M.$ Describe a method for finding $f(n)$ and use it to calculate $f(32).$
2016 Mathematical Talent Reward Programme, MCQ: P 10
Let $A=\{1,2,\cdots ,100\}$. Let $S$ be a subset of power set of $A$ such that any two elements of $S$ has nonzero intersection (Note that elements of $S$ are actually some subsets of $A$). Then the maximum possible cardinality of $S$ is
[list=1]
[*] $2^{99}$
[*] $2^{99}+1$
[*] $2^{99}+2^{98}$
[*] None of these
[/list]
2017 Romanian Master of Mathematics Shortlist, A1
A set $A$ is endowed with a binary operation $*$ satisfying the following four conditions:
(1) If $a, b, c$ are elements of $A$, then $a * (b * c) = (a * b) * c$ ,
(2) If $a, b, c$ are elements of $A$ such that $a * c = b *c$, then $a = b$ ,
(3) There exists an element $e$ of $A$ such that $a * e = a$ for all $a$ in $A$, and
(4) If a and b are distinct elements of $A-\{e\}$, then $a^3 * b = b^3 * a^2$, where $x^k = x * x^{k-1}$ for all integers $k \ge 2$ and all $x$ in $A$.
Determine the largest cardinality $A$ may have.
proposed by Bojan Basic, Serbia
1980 VTRMC, 7
Let $S$ be the set of all ordered pairs of integers $(m,n)$ satisfying $m>0$ and $n<0.$ Let $<$ be a partial ordering on $S$ defined by the statement $(m,n)<(m',n')$ if and only if $m\le m'$ and $n\le n'.$ An example is $(5,-10)<(8,-2).$ Now let $O$ be a completely ordered subset of $S,$ in other words if $(a,b)\in O$ and $(c,d) \in O,$ then $(a,b)<(c,d)$ or $(c,d)<(a,b).$ Also let $O'$ denote the collection of all such completely ordered sets.
(a) Determine whether and arbitrary $O\in O'$ is finite.
(b) Determine whether the carnality $|O|$ of $O$ is bounded for $O\in O'.$
(c) Determine whether $|O|$ can be countable infinite for any $O\in O'.$
1998 Miklós Schweitzer, 1
Can there be a continuum set of continuum sets such that
(i) the intersection of any two is finite, and
(ii) every set that intersects all sets intersects any in an infinite set?
note: a continuum set is a set that can be put into a 1-to-1 bijection with the reals.
2018 JBMO Shortlist, A7
Let $A$ be a set of positive integers satisfying the following :
$a.)$ If $n \in A$ , then $n \le 2018$.
$b.)$ If $S \subset A$ such that $|S|=3$, then there exists $m,n \in S$ such that $|n-m| \ge \sqrt{n}+\sqrt{m}$
What is the maximum cardinality of $A$ ?