Show that there exist infinitely many positive integers such that
is an integer.
Show that there exist infinitely many positive integers such that
is an integer.
Let . Note that
This means that for every positive integer . By induction on , one can easily see that for every positive integers and . Note that the required condition is equivalent to . From the discussion above, if there exists a positive integer so that can be written as , for some positive integer , then . If we choose and for some positive integer , then and since there are infinitely many positive integers of the form , we have the desired result.