# RMO 2015

I have taken inspiration from my friend Swapnil's note and have decided to post this note.I have decided that I will post one or two problems every now and then that are related to the topics of RMO.They can either be proof problems, like the proof of an integral theorem e.t.c or ones like finding values.The questions will be either from preparation books or from Mathematical olympiads.I will keep on adding them one-by-one in the space below.The main rule is that there should be just one solution to one problem,unless,of course,there are more than one way of doing it.If the solution you have is the same as the one which has already been posted,kindly refrain from posting it,but if you have another method of solving the same problem,please do post it!I will post the solution only if one hasn't been posted in $3$ days.So,$\text{Happy problem solving!}$ 1.If p is a prime number,then prove that,$(a+b)^p\equiv(a^p+b^p)\pmod{p}$ Generalize it too!(Awesome part!) 2.For $m>2$,prove that,$\phi(m)$is even where$\phi$ is the Euler's Totient Function 3.Prove that there are infinitely many squares in the sequence $1,3,6,10,15,21,28,......$. 4.Find all the pairs of positive integers (m,n) such that $2^m+3^n$ is a perfect square.$\text{INMO}$ 5.Prove that $2^p+3^p$ is not a perfect square for a prime $p$. 6.If $a\equiv b\pmod{m^n}$,then prove that $a^m\equiv b^m\pmod{m^{n+1}}$(Given by Svatejas Shivakumar).  7.Find a polynomial with integer co-efficients such that $P(a)=b,P(b)=c,P(c)=a$ where $a,b,c$ are distinct.(given by Anik Mandal).We need a solution for this.Surya Prakash has posted a solution. 8.Find the number of prime $n$ satisfying the equation $3n+1=k^2,k\in \mathbb{Z}^+$.(Given by Mehul Arora). 9.If $x^3=x+1$ then find integers $a,b,c$ such that $x^7=ax^2+bx+c$.(Given by Dev Sharma). Note by Adarsh Kumar
5 years, 9 months ago

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## Comments

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5) We consider case when $p=2$. Clearly $2^2+3^2 = 4+9 = 13$ which is not a perfect square.So now consider $p>2$. Let if possible $2^p+3^q=q^2$ for some integer $q$.We note that $2^p \equiv 0 \pmod{4}$ , $3^p \equiv 3 \pmod{4}$ , thus $2^p+3^p \equiv 3 \pmod{4}$ but $q^2 \equiv 0,1 \pmod{4}$ which is a contradiction.

- 5 years, 8 months ago

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Thanx for your solution.

- 5 years, 8 months ago

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7) If $P(x)$ is a polynomial with integer coefficients then for distinct integers $a$, $b$, we have $a-b|P(a)-P(b)$.

So, it implies that $a-b|P(a)-P(b) = b-c$. Similarly we get, $b-c|c-a$ and $c-a|a-b$. So finally what we get is $a-b|b-c$, $b-c|c-a$ and $c-a | a-b$, this is possible iff $a-b = b-c =c-a$. But this implies that $a=b=c$. This is a contradiction as $a$, $b$ and $c$ are distinct. Therefore no such polynomial exists.

- 5 years, 8 months ago

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Thanks for the solution!

- 5 years, 8 months ago

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Thanx for the solution.

- 5 years, 8 months ago

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Is the first statement of your solution a lemma?

- 5 years, 8 months ago

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No,it can be proved like this,$Since\ a-b|b-c,a-b\leq b-c\\ Since\ b-c|c-a,b-c\leq c-a\\ Since\ c-a|a-b,c-a\leq a-b$.Combining these you get that the equality holds.

- 5 years, 8 months ago

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Yes. It is a lemma. it is easy to prove

- 5 years, 8 months ago

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Can you give me hint to prove it? Is it necessary to define a degree to the polynomials?

- 5 years, 8 months ago

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Take $P(x) = a_{n} x^n + a_{n-1} x^{n-1} + \ldots + a_{0}$ where $a_{n} , a_{n-1} , \ldots a_{0}$ are integers. Now, subtract $P(a)$ from $P(b)$ and observe what happens.

- 5 years, 8 months ago

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9) $x^3=x+1 \Rightarrow x^7=x^4(x+1) = x^5+x^4 = x^2(x+1)+x(x+1) = x^3+x^2+x^2+x = 2x^2+2x+1 \\ \Rightarrow (a,b,c)=(2,2,1)$

- 5 years, 8 months ago

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No need of all that.See my solution.

- 5 years, 8 months ago

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9)$x^3=x+1\Rightarrow x^6=(x^2+2x+1)\Rightarrow x^7=x^3+2x^2+x=2x^2+2x+1$.

- 5 years, 8 months ago

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How u got this......explain

- 5 years, 5 months ago

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For the $3rd$ question,

The sequence is, $S=1+3+6+10+15........+{t}_{n}$

To get the ${t}_{n}$ we can write it as,

$S=0+1+3+6+10+15........+{t}_{n}$

$S=1+3+6+10+15+.....$

Now, Subtracting both the sums,

$0=-1-2-3-4-5-.......{t}_{n}$

Therefore we can get,

${t}_{n}=\frac {n(n+1)}{2}$

Now to prove that there are infinitely many squares, Assume that ${t}_{n}$ is a square. From here we see that if ${t}_{n}$ is a square,

${t}_{4n(n+1)}=4\frac{n(n+1)}{2}.{(2n+1)}^{2}$ is also a square.

Hence if ${t}_{1}=1$ is a square then ${t}_{8}=36$ is a square. And if ${t}_{8}$ is a square then ${t}_{4*8(8+1)}={t}_{288}=144*289$ is a square.

Therefore,we get a sequence of infinite squares from here.

- 5 years, 8 months ago

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Brilliant solution!

- 5 years, 8 months ago

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Thank you. See my solution for 2nd question

- 5 years, 8 months ago

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6) $a^{m}-b^{m}=(a-b)(a^{m-1}+a^{m-2}b+\ldots+ab^{m-2}+b^{m-1})$.

Since $a \equiv b \pmod {m^{n}},a \equiv b \pmod {m}$.

Hence, $a^{m-i}b^{i} \equiv b^{m} \pmod {m}$ and $a^{m-1}+a^{m-2}b+\ldots+ab^{m-2}+b^{m-1} \equiv mb^{m} \equiv 0 \pmod {m}$

Hence, $a^{m}-b^{m}$ is divisible by $m^{n+1}$.

Note: This solution is not original.

Credit: An Excursion in Mathematics.

- 5 years, 8 months ago

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Really elegant one! Thanx for posting it!

- 5 years, 8 months ago

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Your welcome :)

- 5 years, 8 months ago

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would it help?

The following results are by Fermet Little Theorm:

$a^p = a modp$

$b^p = b modp$

then $a^p + b^p = a + b modp$

now $(a + b)^p = a + b modp$

so $(a + b)^p = a^p + b^p modp$

- 5 years, 9 months ago

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Yeah you are right it was pretty simple.Sorry!

- 5 years, 9 months ago

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let us try 2nd question :

we can make following case and i am writing totient function as E,

Case 1 - when $m$ is prime then E(m) = m - 1, which is even.

Case 2 - when $m$ is even, clearly it would contain 2 so it even.

- 5 years, 9 months ago

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Case 3 - when $m$ is odd (composite), then m would be divisible by any prime (3,5,7etc) and we know that totient function is multiplicative so, E(m) = E(any prime)E(left out)

also from case 1 we know E(prime) is even, so its even.

- 5 years, 9 months ago

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"Clearly it would contain 2" please elaborate.

- 5 years, 9 months ago

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I am sorry. I explained wrong.

Case 2. If m is even then m would be in form m = even . odd then we know that euler of odd number is even. So it would be even. Well, there would be one more, that is, even = even. Even

- 5 years, 8 months ago

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I believe i am not correctly understanding your method.Do you mean that $m=2^{k}*(2a+1)$ where $2^k$ is the even part of the number and $2a+1$ the odd part?

- 5 years, 8 months ago

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yes but i am not confident about case 2. And is case 1 and 3 correct?

- 5 years, 8 months ago

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Could you explain after this in your case 2?Your case 1 is correct,i am not so sure about case 3,sorry.You could ask someone else.Actually there is a very simple way of proving this.If you want me to post i will.

- 5 years, 8 months ago

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Please post solution.

- 5 years, 8 months ago

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is it correct? If not, whats the mistake?

- 5 years, 8 months ago

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So, what are today's problems?

- 5 years, 9 months ago

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Thanks!

- 5 years, 9 months ago

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@Adarsh Kumar I believe that you can add a comment to the board, whenever you add a question. otherwise, it would be difficult to keep track.

You can add a comment like "Q3. is up" or "Next question!" so that people who are interested or have commented on this board get a notification, and they can check it out. Thanks.

- 5 years, 8 months ago

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Oh nice idea thanx!

- 5 years, 8 months ago

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Looks like giving a good idea gets me a downvote :3

Anyway, Glad to help :)

- 5 years, 8 months ago

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@Adarsh Kumar In question 7, please mention that a,b and c are distinct. :)

- 5 years, 8 months ago

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done!

- 5 years, 8 months ago

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6) $3n + 1 = a^2$

$3n = a^2 - 1$

$3n = (a + 1)(a - 1)$

so $a + 1 = 3 so a = 2$ OR

$a - 1 = 3 so a = 4$

then $n = 1 or n = 5$

so there are two n...

- 5 years, 8 months ago

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Is this solution correct? I don't think so...

- 5 years, 8 months ago

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You assumed that 'n' is prime, which is not given in the question.....

- 5 years, 8 months ago

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Yeah , correct.

- 5 years, 8 months ago

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@Harsh Shrivastava @Nihar Mahajan @Dev Sharma @Mehul Arora I am sorry but question 6 is wrong.It is satisfied fro more than one value of $n$.It is my fault as i should have checked the problem before posting it.Sorry for the inconvenience caused.I am going to delete it.

- 5 years, 8 months ago

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@Adarsh Kumar P has to be a prime I told you that.

- 5 years, 8 months ago

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Ni you didn't,go and see the conversation.

- 5 years, 8 months ago

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Oh , sorry if I didn't. Please repost the problem and mention that p is prime.

- 5 years, 8 months ago

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Ohk.

- 5 years, 8 months ago

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I have a question. How can I give you? (not on slack)

- 5 years, 8 months ago

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Give it here.

- 5 years, 8 months ago

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If $x^3 = x + 1$ then determine integer a,b,c $x^7 = ax^2 + bx + c$

- 5 years, 8 months ago

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Thanx for the question!Kindly delete this comment as i have posted it giving credit to you.

- 5 years, 8 months ago

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For the $2nd$ question, Case $1$: If there is atleast one odd prime factor of $m$,

Then by Euler's Totient function corolary, We can say that if prime $p$ divides a number $m$ then $p-1$ divides $\phi(m)$ . As $p$ is odd, $p-1$ is even, Therefore,$2|\phi(m)$.

Case $2$: If there is no odd prime factor of $m$,

Then $m$ can be written as $m={2}^{k}$ We can easily say that $\phi(m)={2}^{k}-{2}^{k-1}$ Hence,$2|\phi(m)$

- 5 years, 8 months ago

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All questions are done!

- 5 years, 8 months ago

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Could you please provide the solution for the $6^{th}$ one?

- 5 years, 8 months ago

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Since no one has posted the solution for the 6th one yet, should I post it (solution given in the book)?

- 5 years, 8 months ago

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Yes,please!

- 5 years, 8 months ago

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First problem can be solved easily by using binomial therom

- 5 years, 8 months ago

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no need

- 5 years, 8 months ago

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fermat's theorem, isn't it?

- 5 years, 6 months ago

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@Calvin Lin Sir, is it possible for you to close this note since we already have a part 2 for this note.

- 5 years, 8 months ago

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I don't see why this note should be locked. It is still valid for discussion, and adding of more comments / problems.

I might consider doing so if OP requests for it.

Staff - 5 years, 8 months ago

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can anybody solve the rmo 2008 Maharashtra region problem 5

- 5 years, 8 months ago

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For the $8th$ question,

$3n=(k-1)(k+1)$

By the Prime factorisation principle, There is a unique prime factorisation for every number.

From here we can say that,

$3=k-1$ and $k+1=n$

Therefore,$n=5$ is the only prime of this form.

- 5 years, 8 months ago

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