Given three functions
Initially, we pick a set containing two of these functions, and we perform some operations on it. Allowed operations include:
* Take two functions and add one of , or to .
* Take a function and add to for is an arbitrary positive integer.
* Take a function and choose a real number , and add to one of the function .
Show that no matter how we pick in the beginning, there is no way we can perform finitely many operations on that would eventually yield the third function not in .
Solution
First of all, from , after all of the operations, we get a polynomial variable , in which is not a polynomial, so it is not possible to have
Next, we will show that cannot be obtained. Indeed,
so obviously . The given operations will generate functions that are composite functions of so for each of those functions, if taking the derivative, they still have a solution of . However, does not satisfy this because
Finally, we will show that cannot be obtained using polynomial congruence, namely by considering modulo . Next, if is divisible by , we denote
It is easy to check that the properties of the integer congruence are still true here. From this, we can also define congruence for fractions. Back to the problem, we have
Therefore, all functions generated by , when taking congruent modulo are constant. We show that does not have that property. Indeed, notice that so
So that
The problem is completely solved. □