IMO 1988

Let $n$ be an even positive integer. Let $A_1, A_2, \ldots, A_{n + 1}$ be sets having $n$ elements each such that any two of them have exactly one element in common while every element of their union belongs to at least two of the given sets. For which $n$ can one assign to every element of the union one of the numbers 0 and 1 in such a manner that each of the sets has exactly $\frac {n}{2}$ zeros?

A function $f$ defined on the positive integers (and taking positive integers values) is given by:

$\begin{matrix} f(1) = 1, f(3) = 3 f(2 \cdot n) = f(n) f(4 \cdot n + 1) = 2 \cdot f(2 \cdot n + 1) - f(n) f(4 \cdot n + 3) = 3 \cdot f(2 \cdot n + 1) - 2 \cdot f(n), \end{matrix}$

for all positive integers $n.$ Determine with proof the number of positive integers $\leq 1988$ for which $f(n) = n.$

Show that the solution set of the inequality $$\sum^{70}_{k = 1} \frac {k}{x - k} \geq \frac {5}{4}$$ is a union of disjoint intervals, the sum of whose length is 1988.

In a right-angled triangle $ABC$ let $AD$ be the altitude drawn to the hypotenuse and let the straight line joining the incentres of the triangles $ABD, ACD$ intersect the sides $AB, AC$ at the points $K,L$ respectively. If $E$ and $E_1$ dnote the areas of triangles $ABC$ and $AKL$ respectively, show that $$\frac {E}{E_1} \geq 2.$$

Let $a$ and $b$ be two positive integers such that $a \cdot b + 1$ divides $a^{2} + b^{2}$ . Show that $\frac {a^{2} + b^{2}}{a \cdot b + 1}$ is a perfect square.