An NT for a break
by reni_wee, May 28, 2025, 7:24 AM
Prove that there are no positive integers
and
such that
.



Combi Algorithm/PHP/..
by CatalanThinker, May 28, 2025, 5:47 AM
5. [Czech and Slovak Republics 1997]
Each side and diagonal of a regular n-gon (n ≥ 3) is colored blue or green. A move consists of choosing a vertex and
switching the color of each segment incident to that vertex (from blue to green or vice versa). Prove that regardless of the initial coloring, it is possible to make the number of blue segments incident to each vertex even by following a sequence of moves. Also show that the final configuration obtained is uniquely determined by the initial coloring.
Each side and diagonal of a regular n-gon (n ≥ 3) is colored blue or green. A move consists of choosing a vertex and
switching the color of each segment incident to that vertex (from blue to green or vice versa). Prove that regardless of the initial coloring, it is possible to make the number of blue segments incident to each vertex even by following a sequence of moves. Also show that the final configuration obtained is uniquely determined by the initial coloring.
Unexpecredly Quick-Solve Inequality
by Primeniyazidayi, May 28, 2025, 5:18 AM
Nice inequality
by TUAN2k8, May 28, 2025, 2:03 AM
Let
be an even integer and let
be real numbers satisfying
.
Prove that




Prove that

exponential diophantine in integers
by skellyrah, May 27, 2025, 7:04 PM
find all integers x,y,z such that 

Turkish JMO 2025?
by bitrak, May 27, 2025, 2:04 PM
Let p and q be prime numbers. Prove that if pq(p+ 1)(q + 1)+ 1 is a perfect square, then pq + 1 is also a perfect square.
x^2+y^2+z^2+xy+yz+zx=6xyz diophantine
by parmenides51, Mar 2, 2024, 7:46 PM
Prove that there are infinite triples of positive integers
such that



This post has been edited 1 time. Last edited by parmenides51, Mar 2, 2024, 7:46 PM
p divides x^x-c
by mistakesinsolutions, Jun 13, 2023, 7:37 PM
Show that for integer c and a prime p,
has a solution

This post has been edited 1 time. Last edited by mistakesinsolutions, Jun 13, 2023, 7:38 PM
1. Algebra and sigma algebra on a set
by adityaguharoy, Feb 28, 2018, 3:28 PM
Definitions of algebra on a set and sigma algebra on a set
Let
be an infinite set. And let
be the collection of all subsets
of
such that either
or
is finite. Prove that
is an algebra on
, but not a sigma algebra on
.
Proof
A related exercise :
Let
be an uncountable set
be the collection of all subsets
of
such that either
is countable or
is countable.
Is
a
algebra on
?
Answer
Hint to sketch
Definition (of Algebra on a set )
Let
be an arbitrary set. Then a collection
of subsets of
is called an algebra on
if and only if all the following are true :

for each
the set 
for every finite sequence
of elements each
the union 
Definition (of sigma algebra on a set )
Let
be an arbitrary set. Then a collection
of subsets of
is called an
algebra on
if and only if all the following are true :

for each
the set 
for every infinite sequence
of elements each
the union 
Note that : since
, so we conclude that every
algebra on a set
is also an algebra on
.
Let













Definition (of sigma algebra on a set )
Let














Note that : since




Let









Proof
Note that given a set
then
is infinite if and only if there is a one one function from
to
.
Thus, since given that
is an infinite set so, there must be a one one function from
to
. Let
be such a function.
Then, note that since each of the sets
(
) is finite so they all belong to
.
But, if
be the union
then
is an infinite set and also
is an infinite set. Thus,
is not an element of
.
So, (as per definitions),
is not a
algebra on
.
However, since finite union of finite sets is finite and by de Moivre’s law, we get that,
has to be an algebra on
.
This completes the proof.




Thus, since given that




Then, note that since each of the sets



But, if






So, (as per definitions),



However, since finite union of finite sets is finite and by de Moivre’s law, we get that,


This completes the proof.
A related exercise :
Let

a set
is called to be uncountable if and only if there is no one one function from
to 
. Let 







Is



Answer
Yes.
Hint to sketch
Countable union of countable sets is countable. Show this ! and then use this to conclude the result.
This post has been edited 2 times. Last edited by adityaguharoy, Feb 28, 2018, 4:16 PM
IMO 2017 Problem 4
by Amir Hossein, Jul 19, 2017, 4:30 PM
Let
and
be different points on a circle
such that
is not a diameter. Let
be the tangent line to
at
. Point
is such that
is the midpoint of the line segment
. Point
is chosen on the shorter arc
of
so that the circumcircle
of triangle
intersects
at two distinct points. Let
be the common point of
and
that is closer to
. Line
meets
again at
. Prove that the line
is tangent to
.
Proposed by Charles Leytem, Luxembourg

























Proposed by Charles Leytem, Luxembourg
This post has been edited 1 time. Last edited by djmathman, Jun 16, 2020, 4:13 AM
The oldest, shortest words — "yes" and "no" — are those which require the most thought.
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