# The Unapologetic Mathematician

## Pointwise Convergence

When we evaluate a power series at a point we get a number if the series converges at that point. We even know that for each power series we have a disk where evaluation gives an absolutely convergent series at every point. In this view we regard a power series as the limit of a sequence of polynomials, evaluate each of the polynomials to get a sequence of numbers, and then take the limit of that sequence.

But what if we change it up. Let’s say we already know that our power series will converge with radius $R$. Then inside the disk $D_R$ of radius $R$ each polynomial defines a function, and evaluation of the power series defines another function. It makes sense to regard the latter as “the limit” of the sequence of the former. That is, we already have $s=\lim\limits_{n\rightarrow\infty}p_n$ as elements of the ring of power series $\mathbb{C}[[X]]$. But now we regard them as living in the ring $(D_R)^\mathbb{C}$ of complex-valued functions on the disk of radius $R$.

And we have a topology on the ring $D^\mathbb{C}$ of complex-valued functions on a domain $\mathbb{R}$. Instead of defining this topology in terms of open sets as we usually do, we define this topology in terms of which nets converge to which points. In fact, we’ll make do with sequences, since the extension to convergence of nets is straightforward.

The topology we have staring us in the face is the “pointwise” topology. That is, we say that a sequence $f_n$ of functions on $D$ converges to a function $f$ if and only if for every point $z\in D$ the evaluations converge to the evaluation of $f$: $\lim\limits_{n\rightarrow\infty}f_n(z)=f(z)$.

Alternately we can read this as a recipe: given a sequence of functions $f_n$, if for each point $x\in D$ the sequence $f_n(z)$ of complex numbers converges, then we declare the limiting function to be that function $f$ defined by $f(z)=\lim\limits_{n\rightarrow\infty}f_n(z)$. If at any point $z\in D$ the sequence $f_n(z)$ fails to converge, we declare the sequence of functions to fail to converge.

Putting a topology on a space of functions marks the first dipping of our toes into the ocean of functional analysis. There’s a lot out there, and we’ll only be wading into the shallowest waters for now. Still, it gives a hint of the incredible depth that lays just beyond the breakers crashing on the shores of second-semester calculus.

September 2, 2008 -