## Polynomial interpolation

#### February 14, 2020

From Axler’s *Linear Algebra Done Right*, chapter 4, exercise 5:

Suppose $m$ is a nonnegative integer, $z_1, \dots, z_{m+1}$ are distinct elements of $\mathbb{F}$, and $w_1, \dots, w_{m+1} \in \mathbb{F}$. Prove that there exists a unique polynomial $p \in \mathcal{P}_m(\mathbb{F})$ such that $p(z_j) = w_j$ for $j = 1, \dots, m+1$

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This result can be proved without using linear algebra. However, try to find the clearer, shorter proof that uses some linear algebra.]

If $p \ne 0$ then as $p$ is at most degree $m$, there are at most $m$ distinct $z$’s such that $p(z)=0$. Equivalently, if $p$ had at least $m+1$ distinct $z$’s such that $p(z)=0$, then $p=0$. Letting

$Z = \begin{bmatrix} 1 & z_1 & z_1^2 & \dots & z_1^m \\ 1 & z_2 & z_2^2 & \dots & z_2^m \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ 1 & z_{m+1} & z_{m+1}^2 & \dots & z_{m+1}^m \\ \end{bmatrix}$then the previous statement implies that $\text{null } Z = \{0\}$, so $Z$ is injective. As $Z$ is square, [Axler 3.69] implies that $Z$ is invertible. Hence, if

$\begin{aligned} Z^{-1}\left(\begin{bmatrix} w_1 \\ w_2 \\ \vdots \\ w_{m+1}\end{bmatrix}\right) = \begin{bmatrix} a_0 \\ a_1 \\ \vdots \\ a_{m}\end{bmatrix} \end{aligned}$then the polynomial $p(z)=a_0 + a_1 z + a_2 z^2 + \dots + a_m z^m$ is the unique polynomial such that $p(z_j) = w_j$ for $j = 1, \dots, m+1$.