The 43rd International Mathematical Olympiad Shortlist Problems
2002年第43届国际数学奥林匹克备选题 
Number theory
 Express 2002^{2002} as the smallest possible number of (positive or negative) cubes.
 was used in the Olympiad.
 If N is the product of n distinct primes, each greater than 3, show that 2^{N} + 1 has at least 4^{n} divisors.
 Does the equation 1/a + 1/b + 1/c + 1/(abc) = m/(a + b + c) have infinitely many solutions in positive integers a, b, c for any positive integer m?
 m, n are integers > 1. a_{1}, a_{2}, ... , a_{n} are integers, and none is a multiple of m^{n1}. Show that there are integers e_{i}, not all zero, with e_{i} < m, such that e_{1}a_{1} + e_{2}a_{2} + ... + e_{n}a_{n} is a multiple of m^{n}.
 was used in the Olympiad.
Geometry
 B is a point on the circle S. A is a point (distinct from B) on the tangent to S
at B. C is a point not on S such that the line segment AC meets the circle S at
two distinct points. S' is a circle which touches AC at C and S at D, where B
and D are on opposite sides of the line AC. Show that the circumcenter of BCD
lies on the circumcircle of ABC.
 The sides of the triangle ABC subtend the same angles at the point F inside the triangle. The lines BF, CF meet the sides AC, AB at D, E respectively. Show that AB + AC ≥ 4 DE.
 was used in the Olympiad.
 S and S' are circles intersecting at P and Q. A, B are distinct variable points on the circle S not at P or Q. The lines AP, BP meet the circle S' again at A', B' respectively. The lines AB, A'B' meet at C. Show that the circumcenter of AA'C lies on a fixed circle (as A, B vary).
 S is a set of 5 coplanar points, no 3 collinear. M(S) is the largest area of a triangle with vertices in S. Similarly, m(S) is the smallest area. What is the smallest possible value of M(S)/m(S) as S varies?
 was used in the Olympiad.
 ABC is an acuteangled triangle. The incircle touches BC at K. The altitude AD has midpoint M. The line KM meets the incircle again at N. Show that the circumcircle of BCN touches the incircle of ABC at N.
 The circles S and S' meet at A and B. A line through A meets S, S' again at C, D respectively. M is a point on CD. The line through M parallel to BC meets BD at K, and the line through M parallel to BD meets BC at N. The perpendicular to BC at N meets S at the point E on the opposite side of BC to A. The perpendicular to BD at K meets S' at F on the opposite side of BD to A. Show that ∠EMF = 90^{o}.
Algebra
 Find all realvalued functions f(x) on the reals such that f(f(x) + y) = 2x + f(f(y)  x) for all x, y.
 The infinite real sequence x_{1}, x_{2}, x_{3}, ... satisfies x_{i}  x_{j} ≥ 1/(i + j) for all unequal i, j. Show that if all x_{i} lie in the interval [0, c], then c ≥ 1.
 p(x) = ax^{3} + bx^{2} + cx + d is a polynomial with integer coefficients and a nonzero. We have x p(x) = y p(y) for infinitely many pairs (x, y) of unequal integers. Show that p(x) has an integer root.
 as used in the Olympiad.
 Given a positive integer n which is not a cube, define a = n^{1/3}, b = 1/(a  [a]), c = 1/(b  [b]). Show that there are infinitely many such n for which we can find integers r, s, t, not all zero, such that ra + sb + tc = 0.
 A is a nonempty set of positive integers. There are positive integers b_{1}, b_{2}, ... , b_{n}, c_{1}, c_{2}, ... , c_{n} such that each set b_{i}A + c_{i} = {b_{i}a + c_{i} : a belongs to A} is a subset of A, and the n sets b_{i}A + c_{i} are pairwise disjoint. Prove that 1/b_{1} + 1/b_{2} + ... + 1/b_{n} ≤ 1.
Combinatorics
 was used in the Olympiad.
 n is an odd integer. The squares of an n x n chessboard are colored alternately black and white, with the four corner squares black. A tromino is an L shape formed from three squares. For which n can the black squares all be covered by nonoverlapping trominoes. What is the minimum number required ?
 A sequence of n positive integers is full if for eack k > 1, k only occurs if k1 occurs before the last occurrence of k. How many full sequences are there for each n ?
 T is the set of all triples (x, y, z) with x, y, z nonnegative integers < 10. A chooses a member (X, Y, Z) of T. B seeks to identify it. He is allowed to name a triple (a, b, c) in T. A must then reply with X + Y  a  b + Y + Z  b  c + Z + X  c  a. How many triples does B need to name to be sure of determining A's triple ?
 r > 1 is a fixed positive integers. F is an infinite family of sets, each of size r, no two of which are disjoint. Show that some set of size r  1 has nonempty intersection with every element of F.
 n is even. Show that there is a permutation x_{1}, x_{2}, ... , x_{n} of 1, 2, ... n such that for each i, x_{i+1} belongs to {2x_{i}, 2x_{i}  1, 2x_{i}  n, 2x_{i}  n  1}, where, as usual we take x_{n+1} to mean x_{1}.
 A graph has 120 points. A weak quartet is a set of four points with just one edge. What is the maximum possible number of weak quartets ?

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