The 28th International Mathematical Olympiad Shortlist Problems
1987年第28届国际数学奥林匹克备选题 
 f is a realvalued function on the reals such that:
(1) if x ≥ y and f(y)  y ≥ v ≥ f(x)  x, then f(z) = v + z for some z between x and y;
(2) for some k, f(k) = 0 and if f(h) = 0, then h <= k;
(3) f(0) = 1;
(4) f(1987) ≤ 1988;
(5) f(x) f(y) = f(x f(y) + y f(x)  xy) for all x, y.
Find f(1987).
(Australia 6)
 S = {a_{1}, a_{2}, ... , a_{n}, b_{1}, b_{2}, ... , b_{n}}. There are subsets C_{1}, C_{2}, ... , C_{k}, such that (1) for no i, j do both a_{i} and b_{i} both belong to C_{j}, (2) for any pair of distinct elements of S, not of the form a_{i}, b_{i}, there is just one C_{j} containing both elements. Show that if n > 3, then k ≥ 2n. (USA 3)
 Does there exist a polynomial p(x, y) of degree 2 such that, for each nonnegative integer n, we have n = p(a, b) for just one pair (a, b) of nonnegative integers? (Finland 3)
 ABCDA'B'C'D' is any parallelepiped (with ABCD, A'B'C'D' faces and AA', BB', CC', DD' edges). Show that AC + AB'+ AD' <= AB + AD + AA' + AC' (the sum of the thre short diagonals from A is less than the sum of the three edges from A plus the long diagonal from A). (France 5)
 Find the smallest real c such that x_{1}^{1/2} + x_{2}^{1/2} + ... + x_{n}^{1/2} ≤ c (x_{1} + x_{2} + ... + x_{n})^{1/2} for all n and all real sequences x_{1}, x_{2}, x_{3}, ... which satisfy x_{1} + x_{2} + ... + x_{n} ≤ x_{n+1}. (United Kingdom 6)
 Show that a^{n}/(b+c) + b^{n}/(c+a) + c^{n}/(a+b) ≥ (2/3)^{n2} s^{n1} for all n ≥ 1, where a, b, c are the sides of a triangle and s = (a + b + c)/2. (Greece 4)
 Given any 5 real numbers u_{0}, u_{1}, u_{2}, u_{3}, u_{4}, show that we can always find 5 real numbers v_{0}, v_{1}, v_{2}, v_{3}, v_{4} such that each u_{i}  v_{i} is integral and ∑_{i<j}(v_{i}  v_{j})^{2} < 4. (Netherlands 1)
 Does there exist a subset M of Euclidean space such that any plane meets M in a finite nonempty set? (Hungary 1)
 Show that for any relatively prime positive integers m, n we can find integers a_{1}, a_{2}, ... , a_{m} and b_{1}, b_{2}, ... , b_{n} such that each product a_{i}b_{j} gives a different residue mod mn. (Hungary 2)
 Two spheres S and S' touch externally and lie inside a cone C. Each sphere touches the cone in a full circle. n solid spheres are arranged in the cone in a ring so that each touches S and S' externally, touches the cone, and touches its two neighbouring solid spheres. What are the possible values of n? (Iceland 3)
 Find the number of ways of partitioning {1, 2, 3, ... , n} into three (possibly empty) subsets A, B, C such that (1) for each subset, if the elements are written in ascending order, then they alternate in parity, and (2) if all three subsets are nonempty, then just one of them has its smallest element even. (Poland 1)
 ABC is a nonequilateral triangle. Find the locus of the centroid of all equilateral triangles A'B'C' such that A, B', C' are collinear, A', B, C' are collinear, A', B', C are collinear, and both ABC and A'B'C' have their vertices anticlockwise. (Poland 5)
 (German Democratic Republic 2), has been used in the Olympiad.
 How many ndigit words can be formed from the alphabet {0, 1, 2, 3, 4} if neighboring digits must differ by exactly 1? (German Federal Republic 1)
 (German Federal Republic 2), has been used in the Olympiad.
 (German Federal Republic 3),has been used in the Olympiad.
 Show that we can color the elements of the set {1, 2, ... , 1987} with 4 colors so that any arithmetic progression with 10 terms in the set is not monochromatic. (Romania 1)
 For any positive integer r find the smallest positive integer h(r) such that for any partition of {1, 2, ... , h(r) } into r parts, there are integers a ≥ 0 and 1 ≤ x ≤ y such that a + x, a + y and a + x + y all belong to the same part. (Romania 4)
 Given angles A, B, C such that A + B + C < 180^{o}, show that there is a triangle with sides sin A, sin B, sin C and that its area is less than (sin 2A + sin 2B + sin 2C)/8. (USSR 2)
 Show that for any integer k > 1, there is an irrational number r such that [r^{m}] = 1 mod k for every natural number m. (Yugoslavia 2)
 (USSR 4), has been used in the Olympiad.
 (Vietnam 4),has been used in the Olympiad.

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