Determine the maximal number of consecutive positive integers such that each of these integers has a common divisor with greater than .
Solution
We observe that . An integer has a common divisor greater than with if and only if it is divisible by , or .
Let be the desired maximal number. Only each th integer is divisible by . That means that if is divisible by , then are not divisible by . Analogously, divides only every rd integer. Six consecutive integers contain exactly three odd numbers. At most one of them is divisible by and at most one of them is divisible by . This shows that .
Now, we try to find five consecutive integers that have a common divisor greater than with .
We can do that in the following way:
That means that we want and with and odd. If we subtract the second equation from the first, we get
We obtain . We see that works, since we get and therefore and the five consecutive integers , which have the desired property.
Therefore, .