# Solution: 2013-13 Functional equation

Find all continuous functions $$f : \mathbb{R} \to \mathbb{R}$$ satisfying
$f(x) = f(x^2 + \frac{x}{3} + \frac{1}{9} )$
for all $$x \in \mathbb{R}$$.

The best solution was submitted by 강동엽. Congratulations!

Similar solutions were submitted by 김기현(+3), 김범수(+3), 김정섭(+3), 김호진(+3), 김홍규(+3), 박민재(+3), 박지민(+3), 박훈민(+3), 어수강(+3), 엄문용(+3), 윤성철(+3), 이명재(+3), 이성회(+3), 이시우(+3), 이주호(+3), 장경석(+3), 전한솔(+3), 정동욱(+3), 정성진(+3), 정종헌(+3), 조정휘(+3), 진우영(+3), 안가람(+2), 박경호(+2), 정우석(+2). Thank you for your participation.

Remark 1. As written in the rules, please submit the solution by 12PM on Wednesday. Any solution submitted after 12PM will not be graded.
Remark 2. Please write your name in the solution (not just in the email).

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# Solution: 2011-24 (n-k) choose k

Evaluate the sum $\sum_{k=0}^{[n/2]} (-4)^{n-k} \binom{n-k}{k} ,$ where [x] denotes the greatest integer less than or equal to x.

The best solution was submitted by Kang, Dongyub (강동엽), 전산학과 2009학번. Congratulations!

Here is his Solution of Problem 2011-24.

Alternative solutions were submitted by 장경석 (2011학번, +3), 서기원 (수리과학과 2009학번, +3), 김범수 (수리과학과 2010학번, +3), 박준하 (하나고등학교 2학년, +3).

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# Solution: 2011-22 Seoul Subway Line 2

In Seoul Subway Line 2,  subway stations are placed around a circular subway line. Assume that each segment of Seoul Subway Line 2 has a fixed price. Suppose that you hid money at each subway station so that the sum of the money is only enough for one roundtrip around Seoul Subway Line 2.

Prove that there is a station that you can start and take a roundtrip tour of Seoul Subway Line 2 while paying each segment by the money collected at visited stations.

The best solution was submitted by Kang, Dongyub (강동엽), 전산학과 2009학번. Congratulations!

Here is his Solution of Problem 2011-22. (typo in the lemma: replace an+i=an with an+i=ai.)

Alternative solutions were submitted by 서기원 (수리과학과 2009학번, +3 Alternative Solution), 장경석 (2011학번, +3), 김태호 (2011학번, +3), 김범수 (수리과학과 2010학번, +3), 박준하 (하나고등학교 2학년, +3).

GD Star Rating # Concluding 2011 Spring

Thanks all for participating POW actively. Here’s the list of winners:

1st prize: Park, Minjae (박민재) – 2011학번

2nd prize: Kang, Dongyub (강동엽) – 전산학과 2009학번

3rd prize: Suh, Gee Won (서기원) – 수리과학과 2009학번
3rd prize: Lee, Jaeseok (이재석) – 수리과학과 2007학번

Congratulations!

In addition to these three people, I selected one more student to receive one notebook.

Kim, Ji Won (김지원) -수리과학과 2010학번 박민재 (2011학번) 31pts
강동엽 (2009학번) 24pts
서기원 (2009학번) 16pts
이재석 (2007학번) 16pts
김지원 (2010학번) 12pts
김치헌 (2006학번) 5pts
김인환 (2010학번) 3pts
김태호 (2011학번) 3pts
양해훈 (2008학번) 3pts
이동민 (2009학번) 2pts

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# Solution: 2011-10 Multivariable polynomial

Let $$t_1,t_2,\ldots,t_n$$ be positive integers. Let $$p(x_1,x_2,\dots,x_n)$$ be a polynomial with n variables such that $$\deg(p)\le t_1+t_2+\cdots+t_n$$. Prove that $$\left(\frac{\partial}{\partial x_1}\right)^{t_1} \left(\frac{\partial}{\partial x_2}\right)^{t_2}\cdots \left(\frac{\partial}{\partial x_n}\right)^{t_n} p$$ is equal to $\sum_{a_1=0}^{t_1} \sum_{a_2=0}^{t_2}\cdots \sum_{a_n=0}^{t_n} (-1)^{t_1+t_2+\cdots+t_n+a_1+a_2+\cdots+a_n}\left( \prod_{i=1}^n \binom{t_i}{a_i} \right)p(a_1,a_2,\ldots,a_n).$

The best solution was submitted by Kang, Dongyub (강동엽), 전산학과 2009학번. Congratulations!

Here is his Solution of Problem 2011-10

An alternative solution was submitted by 박민재 (2011학번, +3).

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# Solution: 2011-8 Geometric Mean

Let f be a continuous function on [0,1]. Prove that $\lim_{n\to \infty}\int_0^1 \cdots \int_0^1 f(\sqrt[n]{x_1 x_2 \cdots x_n } ) dx_1 dx_2 \cdots dx_n = f(1/e).$

The best solution was submitted by Kang, Dongyub (강동엽), 전산학과 2009학번. Congratulations!

Here is his Solution of Problem 2011-8.

Alternative solutions were submitted by 김치헌 (수리과학과 2006학번, +3), 어수강 (홍익대학교 수학교육과 2004학번, +3).

(Here is a Solution by Chiheon Kim for Problem 2011-8.)

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# Solution: 2011-3 Counting functions

Let us write $$[n]=\{1,2,\ldots,n\}$$. Let $$a_n$$ be the number of all functions $$f:[n]\to [n]$$ such that $$f([n])=[k]$$ for some positive integer $$k$$. Prove that $a_n=\sum_{k=0}^{\infty} \frac{k^n}{2^{k+1}}.$

The best solution was submitted by Kang, Dongyub (강동엽), 전산학과 2009학번. Congratulations!

Here is his Solution of Problem 2011-3.

Alternative solutions were submitted by 서기원 (수리과학과 2009학번, +3), 박민재 (2011학번, +3), 김치헌 (수리과학과 2006학번, +2), 이동민 (수리과학과 2009학번, +2), 구도완 (해운대고등학교 3학년, +2).

P.S. A common mistake is to assume that $$\sum_{i}\sum_{j}$$ can be swapped without showing that a sequence converges absolutely.

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