A polynomial is completely determined by any part of it

complex-analysisinterpolationpolynomials

I was watching this Mathologer video (https://youtu.be/YuIIjLr6vUA?t=1652) and he says at 27:32

First, suppose that our initial chunk is part of a parabola, or if you like a cubic, or any polynomial. If I then tell you that my mystery function is a polynomial, there's always going to be exactly one polynomial that continues our initial chunk. In other words, a polynomial is completely determined by any part of it. […] Again, just relax if all this seems a little bit too much.

So he didn't give a proof of the theorem in bold text – I think this is very important.

I understand that there always exists a polynomial of degree $n$ that passes through a set of $n+1$ points (i.e. there are finitely many custom points to be passed by, the chunk has to be discrete, like $(1,1),(2,2),(3,3),(4,5)$). But there also exists some polynomial of degree $m$ ($m\ne n$) that passes through the same set of points.

But how do I prove that there exists one and only one polynomial that passes through a set of infinitely many points?

Best Answer

If $p$ and $q$ are polynomials agreeing on infinitely many points, then $p-q$ is a polynomial that’s 0 on infinitely many points.

But if a polynomial $f$ of degree $n$ is $0$ on more than $n$ points, then it’s zero everywhere. (If it has zeroes $a_1, \ldots a_n$, then by repeated division it’s of the form $c(x-a_1)\cdots(x-a_n)$; if it’s zero at some other point as well, then we get $c=0$.)

So a polynomial that’s 0 on infinitely many points is 0 everywhere. So, going back to the beginning, if $p$ and $q$ are polynomials agreeing on infinitely many points, then $p-q$ is zero everywhere, i.e. $p=q$.

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