Find all real-valued continuous function f on the reals such that f(x)=f(cos x) for every real number x.
The best solution was submitted by Hojin Kim (김호진), 2009학번. Congratulations!
Here is his Solution of Problem 2009-10.
Alternative solutions were submitted by 백형렬(수리과학과 2003학번, +3), 김치헌(수리과학과 2006학번, +3), 이재송(전산학과 2006학번, +3), 권상훈(수리과학과 2006학번, +3), 조용화(수리과학과 2006학번, +2), 김일희 & 오성진 (Princeton Univ., Graduate students).