domino question
by kjhgyuio, Apr 21, 2025, 10:02 PM
demonic monic polynomial problem
by iStud, Apr 21, 2025, 9:51 PM
(a) Let
be a monic polynomial so that there exists another real coefficients
that satisfy
Determine all complex roots that are possible from 
(b) For arbitrary polynomial
that satisfies (a), determine whether
should have real coefficients or not.


![\[P(x^2-2)=P(x)Q(x)\]](http://latex.artofproblemsolving.com/6/9/7/697faa929e4fda7a6e9b1cd97849bd42ffc14306.png)

(b) For arbitrary polynomial


fun set problem
by iStud, Apr 21, 2025, 9:47 PM
Given a set
with exactly 9 elements that is subset of
. Prove that there exist two subsets
and
that satisfy the following:
-
and
are non-empty subsets from
,
- the sum of all elements in each of
and
are equal, and
-
is an empty subset.




-



- the sum of all elements in each of


-

Funny easy transcendental geo
by qwerty123456asdfgzxcvb, Apr 21, 2025, 7:23 PM
Let
be a logarithmic spiral centered at the origin (ie curve satisfying for any point
on it, line
makes a fixed angle with the tangent to
at
). Let
be a rectangular hyperbola centered at the origin, scaled such that it is tangent to the logarithmic spiral at some point.
Prove that for a point
on the spiral, the polar of
wrt.
is tangent to the spiral.






Prove that for a point



This post has been edited 3 times. Last edited by qwerty123456asdfgzxcvb, 5 hours ago
Inequality with three conditions
by oVlad, Apr 21, 2025, 1:48 PM
Let
be non-negative real numbers such that
Prove that 

![\[b+c\leqslant a+1,\quad c+a\leqslant b+1,\quad a+b\leqslant c+1.\]](http://latex.artofproblemsolving.com/b/9/e/b9e86e898c0536b7323a03611d5bdbf679caa710.png)

An easy FE
by oVlad, Apr 21, 2025, 1:36 PM
Determine all functions
such that
for any real numbers
and 

![\[f(xy-1)+f(x)f(y)=2xy-1,\]](http://latex.artofproblemsolving.com/8/8/8/888ca39f2b7f8cec6d6426bee28d40eade40a66e.png)


Interesting F.E
by Jackson0423, Apr 18, 2025, 4:12 PM
Show that there does not exist a function
satisfying the condition that for all
,
![\[
f(x + y^2) \geq f(x) + y.
\]](//latex.artofproblemsolving.com/3/a/a/3aa20083835682dbd81be692eba65cab19e923e5.png)
~Korea 2017 P7
![\[
f : \mathbb{R}^+ \to \mathbb{R}
\]](http://latex.artofproblemsolving.com/e/7/b/e7bfc5b236fb6f00ef328f850b1b14632cbf8416.png)

![\[
f(x + y^2) \geq f(x) + y.
\]](http://latex.artofproblemsolving.com/3/a/a/3aa20083835682dbd81be692eba65cab19e923e5.png)
~Korea 2017 P7
This post has been edited 3 times. Last edited by Jackson0423, Yesterday at 3:23 PM
Reason: Sorry guys..
Reason: Sorry guys..
GCD Functional Equation
by pinetree1, Jun 25, 2019, 5:36 PM
Let
be a function satisfying
for all integers
and
. Show that there exist positive integers
and
such that
for all integers
.
Ankan Bhattacharya








Ankan Bhattacharya
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
p^3 divides (a + b)^p - a^p - b^p
by 62861, Feb 23, 2017, 5:14 PM
Prove that there are infinitely many triples
of positive integers with
prime,
, and
, such that
is a multiple of
.
Noam Elkies






Noam Elkies
This post has been edited 1 time. Last edited by 62861, May 18, 2018, 11:58 PM
The oldest, shortest words — "yes" and "no" — are those which require the most thought.
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