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Oppa Putnam style

Blogs > Geiko
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Geiko
Profile Blog Joined June 2010
France1939 Posts
Last Edited: 2013-09-21 00:04:41
September 20 2013 23:27 GMT
#1
I've recently heard about a math test in american universities called the Putnam Test (http://en.wikipedia.org/wiki/William_Lowell_Putnam_Mathematical_Competition). Math is a bit of a hobby for me so I thought it would be fun to try some problems. This is a blog so I'm using it to write down some of my thoughts, things about me and maybe discuss these problems with people who like math problem solving.

About me
+ Show Spoiler +

I'm French, 26, currently working as a structural engineer in Geneva.
As far as I can remember math has always been what I love to do. In my teens I would spend hours and days and months of my spare time on problems trying to invent new math theorems. I was naive enough to think that would be possible at my level but it was still fun and I did "invent" a couple of things (that had already been invented but still ...).
I kept on doing as much math as I could after high school, which in France means "classes préparatoires" and then "Grandes Ecoles", and successfully passed the exam for one of the hardest school in France (hardest to get in to that is). We always have shitty rating in international rankings because we are a "small" school (500 students per year) but lately we ranked 4th world in the Times Higher Education Alma Mater Index (http://www.timeshighereducation.co.uk/news/alma-mater-index-global-executives-2013/2007032.article).
At that time (20 years old) I still had no clue what I wanted to do with my life. I knew I didn't want to be a teacher, nor a researcher, and I hate finance in general so I saw little hope for me of doing something I like related to pure mathematics. A shame as I was pretty decently good in math and not so good in physics, mechanics etc... So I sold out and learnt some "practical" skills, and here I am today, working as a civil engineer. It's a great deal of fun mind you, but maths will always be in my heart.


Putnam test
So when I heard of these problems, I thought to myself "these are for american kids fresh out of high school, they can't be that hard, can they ?". In France it's common to joke on how bad high schoolers in america are at problem solving. This is because you don't learn any sort of reasoning or problem solving before college. On the other hand, american kids learn 5 times more of the calculus stuff in high school than we do. So anyways I lazily started reading some problems and realized I couldn't solve any of them that easily.

So I decided to take some time and try to solve these problems. I'll post whatever answers I can in this blog and maybe we can have a discussion for those interested. I'd rather not talk about problems I haven't solved because I hate getting help but most of the time I feel that my solutions aren't the simplest and it's interesting to share some solutions.

I started with the 2003 problems and took 8-9 hours to solve these first 6 problems (which is roughly 3 times more than the time you have during the exam ).

[image loading]
+ Show Spoiler +

The answer is obviously n (testing on a couple numbers)
I first proved that each solution for a given n that had the same number k of integers was unique. First terms are equal otherwise one is stricly bigger than the other, etc...
Then I found a way to write n as the sum of k integers for each k between 1 and n with an euclidean division. n=pk+r=(k-r)*p + (r)*(p+1).


[image loading]
+ Show Spoiler +

My answer kinda sucks for this one, I start by simplifying the problem by dividing everything by the a_i so I have a problem only in (1+c_i) etc. (where c_i is b_i/a_i). Then I just developed everything and found that solving the problem was equivalent to proving that an nth geometrical average of n terms is always smaller than an arithmetic average of the same n terms. Which is a known fact. I had forgotten how to prove this so I proved it by replacing one of the terms with x and deriving. The problem is then solved by mathematical induction. (I think you can also prove that with concave functions).


[image loading]

+ Show Spoiler +
I spent A LOT of time on this one. I'm still convinced that there is an easy solution I didn't see... So in desperation, I wrote out A(x) = cos(x) + 1/cos(x) + sin(x) + 1/sin(x) + tan(x)+ 1/tan(x) as a function of t=tan(x/2). After developing I find A(x)=B(t)=-2+1/t+(t-1)/(t^4-1). I then try to solve B'(t) = 0 which leads me to t^4-4t^3-1 = 0. I use Ferrari's method (first time this has ever been useful to me) to factor. This leads me to solving the equation (I forget what the solution is exactly). Then by cleverly rewriting B(t) with t^4-1 = -4t^3 and using the second degree equation I found to write 1/t as at+b, I find that the answer is 1-2*sqrt(2).


[image loading]
+ Show Spoiler +

This on was surprinsingly easy.
I started by proving that if abs(P(x)) <= abs(Q(x)) than
Q(x)=0 had no real answers (unless they were the same as P(x) in which case the problem is obvious.
Then I solved the problem if Q(x)=0 has no real answers. Easily enough, the minimum of P is smaller than the minimum of Q and the minimum of a second degree polynom is Delta/(2a). aP<aQ
(if we consider both polynoms to be positive (convention) and the answer is obvious.)
You just have to realize then that solving the problem is equivalent to proving that P(x)-Q(x)=0 has no answer and P(x)+Q(x)=0 has no answers. Which is easily done by looking at the Delta for P-Q and P+Q which is always negative (one or the other).


[image loading]

+ Show Spoiler +
The convention I use is that a path is written as follows (1,-1,1,1,1,-1,-1,-1) for example.
All the Dyck n-path can be written as a unique series of smaller Dyck k-path.
is s is a Dyck n-path, s=a1,a2,...,ap with ai irreductible Dyck paths (irreducticle meaning with only one return).
The function F(s) I used to link Dyck n-1 path ---> Dyck n patch with only odd returns is
For all s in Dyck(n-1), s=a0,a1,...,ak-1,b,ak+1,...,ap where b is the first (starting from the right) Dyck path with an even return. b can be () the null Dyck path. Then F(s)=(1,a0,...,ak-1,-1,ak+1,...,ap. I then had to prove that this was indeed a bijection (which was a bit tedious) but relatively easy.


[image loading]

+ Show Spoiler +
For this one, I started proving that if it existed, it was unique. I proved this by mathematical induction by building the partitions from 0.
1 and 0 can't be in the same partition otherwise 1=a+b would have 1 solution in that partition and zero in the other. By building up, at step n you realize that you need to place n in one or the other partition based on how many solution a+b=n already has in each partition.

Then I proved that the group existed by noticing that the solution pairs for n=x+y used all the numbers from 0 to n-1 (one and only once) if n is odd, and all the numbers from 0 to n-1 except n/2 if n is even. You can then notice that for each n, whatever the partitions of equal sizes, of (1,2,...,n-1) if n is even, then if you put n in the partition with n/2 they will have the same number of solutions. If n is odd, the partitions will have the same number of solutions if you place n in the partitions in the smallest partition.



If any of you want to give these problems a thought, I'd be glad to discuss your solutions and mine. In the next weeks, if I can find time, I'll be doing problems 7-12.
http://www.artofproblemsolving.com/Forum/resources/files/undergraduate_competitions/Undergraduate_Competitions-Putnam-2003-23



****
geiko.813 (EU)
DarkPlasmaBall
Profile Blog Joined March 2010
United States44322 Posts
September 20 2013 23:56 GMT
#2
These problems are awesome

The Putnam Test is pretty serious stuff, and generally not for those with no background in math (or even the STEM fields, in general).

As an aside, over the past decade or so, there's been a pretty strong push for problem solving questions and strategies to be used in American classrooms. Plus the fact that many of the historical standardized tests we use (SAT, ACT, etc.) also contain many problem solving problems.
"There is nothing more satisfying than looking at a crowd of people and helping them get what I love." ~Day[9] Daily #100
Geiko
Profile Blog Joined June 2010
France1939 Posts
September 21 2013 00:06 GMT
#3
Yeah I'm having an incredible amount of fun doing these

It's pretty awesome that there are so many of these problems, I think I have enough to keep me busy for some time.
geiko.813 (EU)
]343[
Profile Blog Joined May 2008
United States10328 Posts
September 21 2013 00:08 GMT
#4
Urgh, the actual Putnam exam isn't very fun because it's basically a speed test
Writer
Chocolate
Profile Blog Joined December 2010
United States2350 Posts
September 21 2013 02:00 GMT
#5
You probably already know about them, but the USAMO and IMO tests are challenging too. They are made for highschoolers though Also, while the average American is not too high performing in math, the top 5% of us are pretty decent. Really, for proof-based mathematics, it's only like the top .5% of the population in mathematical ability that you need to worry about anyway.
endy
Profile Blog Joined May 2009
Switzerland8970 Posts
September 21 2013 02:52 GMT
#6
Those look pretty hard for undergrad kids :o
Also only 3 hours for 6 problems ?
ॐ
munchmunch
Profile Joined October 2010
Canada789 Posts
September 21 2013 07:56 GMT
#7
The Putnam isn't designed for American students fresh out of high school, it's designed for math prodigies fresh out of high school. This is typical of the US; the average person may be mediocre, but people who stand out are encouraged and rewarded. That's why the country can have Michael Phelps, when the national sports are TV watching, couch-sitting, and potato-munching.
]343[
Profile Blog Joined May 2008
United States10328 Posts
September 21 2013 08:10 GMT
#8
Well, a lot of relatively "average" students take it (of course, this "average" is already like the top 5%, since it's self-selecting; why would you take a math contest if you don't like math?)

In particular, the median score is usually somewhere in [0,3], and the max score is 120.
Writer
munchmunch
Profile Joined October 2010
Canada789 Posts
September 21 2013 08:43 GMT
#9
Sure, but the test does not in any way distinguish between "average" students of different abilities, so you can't say that it is aimed at the "average" student. It's designed so that you have to be really fucking good to get a top score.
CoughingHydra
Profile Blog Joined May 2012
177 Posts
September 21 2013 11:26 GMT
#10
...I knew I didn't want to be a teacher, nor a researcher...

I'm curious why especially the researcher part
Eriksen
Profile Joined December 2012
Micronesia720 Posts
September 21 2013 13:07 GMT
#11
Someone teach me mathemathics

In my country, I would never be able to learn this Putnam test lol. But I find it interesting after reading this blog, maybe anyone can introduce me to a link or some to start learning this? (FYI I'm 17, and not a very intelligent person).
Whether it has ended with a happy ending or sad, I never was an important thing to you.
XDJuicebox
Profile Blog Joined January 2011
United States593 Posts
Last Edited: 2014-10-22 06:47:06
October 22 2014 06:45 GMT
#12
For number 2, isn't that just a special case of Holder's Inequaity?

And that's a pretty elegant solution for 1 :D
And then you know what happened all of a sudden?
Empyrean
Profile Blog Joined September 2004
16987 Posts
October 22 2014 18:20 GMT
#13
We had some math problem solving questions in undergrad specifically for preparation for this exam. My school's won a few times in the past. The people who do it are crazy good.
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