2013 Canadian MO Problems/Problem 1

Problem

Determine all polynomials $P(x)$ with real coefficients such that $(x+1)P(x-1)-(x-1)P(x)$ is a constant polynomial.

Solution

Let $F(x)=(x+1)P(x-1)-(x-1)P(x)$

$P(x)=\sum_{i=0}^{n}c_ix^i$

$F(x)=(x+1)\sum_{i=0}^{n}(x-1)^ic_i-(x-1)\sum_{i=0}^{n}c_ix^i$

$F(x)=\sum_{i=0}^{n}x(x-1)^ic_i+\sum_{i=0}^{n}(x-1)^ic_i-\sum_{i=0}^{n}c_ix^{i+1}+\sum_{i=0}^{n}c_ix^i$

$\sum_{i=0}^{n}(x-1)^ic_i=\sum_{j=0}^{n}\left( \sum_{i=j}^{n}(-1)^{i-j}\binom{i}{i-j}c_i \right)x^j$

$\sum_{i=0}^{n}x(x-1)^ic_i=\sum_{j=0}^{n}\left( \sum_{i=j}^{n}(-1)^{i-j}\binom{i}{i-j}c_i \right)x^{j+1}$

$F(x)=\sum_{j=0}^{n}\left( \sum_{i=j}^{n}(-1)^{i-j}\binom{i}{i-j}c_i \right)x^j+\sum_{j=0}^{n}\left( \sum_{i=j}^{n}(-1)^{i-j}\binom{i}{i-j}c_i \right)x^{j+1}-\sum_{i=0}^{n}c_ix^{i+1}+\sum_{i=0}^{n}c_ix^i$

In order for the new polynomial $F(x)$ to be a constant, all the coefficients in front of $x^i$ for $i>1$ need to be zero.

So we start by looking at the coefficient in front of $x^2$ :

$\left( c_2-c_1+\sum_{i=2}^{n}(-1)^{i-2}\binom{i}{i-2}c_i+\sum_{i=1}^{n}(-1)^{i-1}\binom{i}{i-1}c_i \right)x^2$

Since $\sum_{i=1}^{n}(-1)^{i-1}\binom{i}{i-1}c_i=c_1+\sum_{i=2}^{n}(-1)^{i-1}\binom{i}{i-1}c_i$ ,

$\left( c_2-c_1+c_1+\sum_{i=2}^{n}(-1)^{i-2}\binom{i}{i-2}c_i+\sum_{i=2}^{n}(-1)^{i-1}\binom{i}{i-1}c_i \right)x^2$

$\left( c_2+\sum_{i=2}^{n}\left((-1)^{i-2}\binom{i}{i-2}+(-1)^{i-1}\binom{i}{i-1} \right)c_i\right)x^2$

We then evaluate the term of the sum when $i=2$ :

$\left( c_2+(1-2)c_2+\sum_{i=3}^{n}\left((-1)^{i-2}\binom{i}{i-2}+(-1)^{i-1}\binom{i}{i-1} \right)c_i\right)x^2$

$\left(\sum_{i=3}^{n}\left((-1)^{i-2}\binom{i}{i-2}+(-1)^{i-1}\binom{i}{i-1} \right)c_i\right)x^2=0$

Note that since $c_0$ , $c_1$ , and $c_2$ are not present in the expression before $x^2$ , they can be anything and the coefficient in front of $x^2$ is still zero because the expression before $x^3$ also starts with $c_3$ , and the expression before $x^4$ also starts with $c_4$ and so on...

In fact in matrix form when solving for $c_i$ for for all $i$ , it will look something like this:

$\begin{bmatrix} 2 & -1 & 1 & -1 & 1 & -1 &\cdots & K_{0n}\\ 0 & 1 & -1 & 2 & -3 & 4 & \cdots & K_{1n}\\ 0 & 0 & 0 & 0 & 2 & -5 & \cdots & K_{2n}\\ 0 & 0 & 0 & -1 & 2 & 0 & \cdots & K_{3n}\\ 0 & 0 & 0 & 0 & -2 & 5 & \cdots & K_{4n}\\ 0 & 0 & 0 & 0 & 0 & -3 & \cdots & K_{5n}\\ \vdots &\vdots &\vdots &\vdots &\vdots &\vdots &\ddots &\vdots\\ 0 & 0 & 0& 0& 0& 0& \cdots & -n+2 \end{bmatrix} \begin{bmatrix}c_0 \\c_1 \\c_2\\c_3\\c_4\\c_5\\ \vdots \\ c_n \end{bmatrix} =\begin{bmatrix}0 \\0 \\0\\0\\0\\0\\ \vdots \\ 0 \end{bmatrix}$

This matrix is singular because the 3nd row and the 4rd row also start at the 3rd column, then all coefficients $c_i$ for $i \ge 3$ need to be zero so that the coefficient in front of $x^2$ is zero.

That is, $c_3=c_4=c_5=\cdots =c_n=0$ .

So now we just need to find $c_1$ and $c_2$ , for that we look at the coefficient in front of $x$ in $F(x)$ :

$\left( c_1-c_0+\sum_{i=1}^{n}(-1)^{i-1}\binom{i}{i-1}c_i+\sum_{i=0}^{n}(-1)^{i}\binom{i}{i}c_i \right)x$

Since $c_i$ =0 for $i \ge 3$ :

$\left( c_1-c_0+\sum_{i=1}^{2}(-1)^{i-1}\binom{i}{i-1}c_i+\sum_{i=0}^{2}(-1)^{i}\binom{i}{i}c_i \right)x$

$\left( c_1-c_0+\binom{1}{0}c_1-\binom{2}{1}c_2+c_0-c_1+c_2\right)x$

$\left( c_1-c_0+c_1-2c_2+c_0-c_1+c_2\right)x$

$\left(c_1-c_2\right)x$

Therefore $c_1-c_2=0$ , thus $c_1=c_2$ satisfies the condition for $F(x)$ to be a constant polynomial.

So we can set $c_1=c_2=A$ and $c_0=B$ , and all the polynomials $P(x)$ will be in the form:

$P(x)=Ax^2+Ax+B$ where $A,B \in \mathbb{R}$

~Tomas Diaz. orders@tomasdiaz.com

Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.

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2013 Canadian MO Problems/Problem 1

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