At least coefficients of a polynomial of degree with real coefficients are equal to . Find the maximal value of if has real roots.
*Note: Roots of need not be distinct.*
At least coefficients of a polynomial of degree with real coefficients are equal to . Find the maximal value of if has real roots.
*Note: Roots of need not be distinct.*
The polynomial satisfies the conditions. Let us show that for there is no polynomial satisfying given conditions.
Solution 1. Let be the roots, be the sum of all -tuple products of roots. Using Vieta theorem, we get
When the first three coefficients are , we have and , which is a contradiction. Hence at least one of the first three coefficients should not be equal to . This means that all roots are non-zero and hence we obtain:
If the last three coefficients are , we get and hence we conclude that , which is again a contradiction.
For case, as coefficients out of coefficients are , we conclude that either the first three or the last three coefficients should be . Hence, we are done.
The polynomial satisfies the conditions. Let us show that for there is no polynomial satisfying given conditions.
Solution 2. We will find a contradiction for case. We can express the polynomial as where is an integer satisfying and is a real number. Since all roots of are real numbers, all roots of the polynomial should be real numbers, too. If zero is a root of , then we get and in this case all roots of cannot be real. So, all roots of should be non-zero.
The polynomial has at most non-zero coefficients. By Descartes' rule of signs, has at most positive real roots. has also at most non-zero coefficients. If there are exactly non-zero coefficients, the coefficients of and should have the same sign. (Otherwise we would have at most non-zero coefficients.) Therefore, in both cases, using Descartes' rule of signs again, we conclude that has at most positive real roots. As zero is not a root, can have at most real roots. The degree of is and hence all roots cannot be real. This finishes the proof.