Define the operator " $ * $ " on $ \mathbb{R} $ as $ x*y=x+y+xy. $ [b]a)[/b] Show that $ \mathbb{R}\setminus\{ -1\} , $ along with the operator above, is isomorphic with $ \mathbb{R}\setminus\{ 0\} , $ with the usual multiplication. [b]b)[/b] Determine all finite semigroups of $ \mathbb{R} $ under " $ *. $ " Which of them are groups? [b]c)[/b] Prove that if $ H\subset_{*}\mathbb{R} $ is a bounded semigroup, then $ H\subset [-2, 0]. $

On a set $G$ we are given an operation $*: G \times G \to G$, that for every pair $(x,y)$ of elements of $G$ gives back $x*y \in G$, and for every elements $x,y,z \in G$ the equation $(x*y)*z=x*(y*z)$ holds. $G$ is partitioned into three non-empty sets $A,B$ and $C$. Can it be that for every three elements $a \in A, b \in B, c \in C$ we have $a*b \in C, b*c \in A, c*a \in B$

The finite set $M$ of real numbers is such that among any three of its elements there are two whose sum is in $M$. What is the maximum possible cardinality of $M$? [hide=Remark about the other problems] Problem 2 is UK National Round 2022 P2, Problem 3 is UK National Round 2022 P4, Problem 4 is Balkan MO 2021 Shortlist N2 (the one with Bertrand), Problem 5 is IMO Shortlist 2021 A1 and Problem 6 is USAMO 2002/1. Hence neither of these will be posted here. [/hide]

Let $*$ denote a binary operation on a set $S$ with the property that $$(w*x)*(y*z) = w * z$$ for all $w,x,y,z\in S.$ Show (a) If $a*b=c,$ then $c*c = c.$ (b) If $a*b=c,$ then $a*x=c*x$ for all $x\in S.$

On R a binary algebraic operation ''*'' is defined which satisfies the following two conditions: i) for all $a,b \in R$, there exists a unique $x \in R$ such that $x *a=b$ (write $x=b/a$) ii) $(a*b)*c= (a*c)* (b*c)$ for all $a,b,c \in R$ a) Is this operation necesarily commutative (i.e. $a*b=b*a$ for all $a,b \in R$) ? b) Prove that $(a/b)/c = (a/c) / (b/c)$ and $(a/b)*c = (a*c) / (b*c)$ for all $a,b,c \in R$. A. Mirotin

Let ⋆ be a binary operation on a nonempty set $M$. That is, every pair $(a,b) \in M$ is assigned an element $a$ ⋆$ b$ in $M$. Suppose that ⋆ has the additional property that $(a $ ⋆ $b) $ ⋆$ b= a$ and $a$ ⋆ $(a$ ⋆$ b)= b$ for all $a,b \in M$. (a) Show that $a$ ⋆ $b = b$ ⋆ $a$ for all $a,b \in M$. (b) On which finite sets $M$ does such a binary operation exist?

Let $S$ be a set with a binary operation $\ast$ such that 1) $a \ast(a\ast b)=b$ for all $a,b\in S$. 2) $(a\ast b)\ast b=a$ for all $a,b\in S$. Show that $\ast$ is commutative and give an example where $\ast$ is not associative.

Given a set $S$ of $n$ variables, a binary operation $\times$ on $S$ is called [i]simple[/i] if it satisfies $(x \times y) \times z = x \times (y \times z)$ for all $x,y,z \in S$ and $x \times y \in \{x,y\}$ for all $x,y \in S$. Given a simple operation $\times$ on $S$, any string of elements in $S$ can be reduced to a single element, such as $xyz \to x \times (y \times z)$. A string of variables in $S$ is called[i] full [/i]if it contains each variable in $S$ at least once, and two strings are [i]equivalent[/i] if they evaluate to the same variable regardless of which simple $\times$ is chosen. For example $xxx$, $xx$, and $x$ are equivalent, but these are only full if $n=1$. Suppose $T$ is a set of strings such that any full string is equivalent to exactly one element of $T$. Determine the number of elements of $T$.

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

A [i]bypass[/i] operation on a set $S$ is a mapping $B: S\times S \rightarrow S$ with the property $B(B(w, x), B(y,z)) = B(w,z)$ for all $w, x, y, z \in S$. (a) Prove that $B(a,b)=c$ implies $B(c,c)=c$ when $B$ is a bypass. (b) Prove that $B(a,b)=c$ implies $B(a,x)=B(c,x)$ for all $x\in S$ when $B$ is a bypass. (c) Construct a bypass operation $B$ on a finite set S with the following three properties [list=i] [*] $B(x,x)=x$ for all $x\in S$. [*] There exist $d$ and $e$ in $S$ with $B(d,e)=d \ne e.$ [*] There exist $f$ and $g$ in $S$ with $B(f,g)\ne f.$ [/list]

Is it possible to define an operation $\star$ on $\mathbb Z$ such that[list=a][*] for any $a, b, c$ in $\mathbb Z, (a \star b) \star c = a \star (b \star c)$ holds; [*] for any $x, y$ in $\mathbb Z, x \star x \star y = y \star x \star x=y$?[/list]