## Complex Analysis: Poles, Residues, and Child’s Drawings

Thanks to Laurens Gunnarsen for his superb pedagogy and for this amazing explanation on the incredible depth of connections springing from the Sperner lemma. All errors are mine not his. This started with a chain of events, sitting in on number theory seminars and encountering Abel’s differentials of the first and second kind, interest in the dessin, and led up to asking Laurens:

## How do I understand poles and residues?

By understanding Riemann-Roch. But, first of all, you should know the zeroes and the poles of an analytic function, together with the residues of the function at each, pretty much tell you all you need to know about the function.

This is a little bit like saying that if you know the zeroes of a rational function — which is to say, the zeros of the polynomial that is its numerator — and the poles of a rational function — which is to say, the zeroes of the polynomial that is its denominator — then you basically know the rational function.  Continue reading Complex Analysis: Poles, Residues, and Child’s Drawings

## p-adic numbers are Taylor expansions

Look at $P(x) = x-p$.  Fixing $p$ fixes the P(x).

$P(x)$ is a prime in the ring $\mathbb{C}[x]$.  We can build any polynomial out of these irreducible components.

Fix a prime, and expand your function:

$$f(x) = f(p) + f'(p)(x-p) + \frac{f”(p)}{2!}(x-p)^2 + …$$

If first few functions in the taylor series of two functions agree, than the functions are close, i.e., two functions are close if their difference is divisible by high powers of the monomial.

Let’s look at the integers, replacing primes of polynomials with primes of integers, fix a prime, and expand your function:

$$n = n(p) + n'(p)p + \frac{n”(p)}{2!}p^2 + …$$

Where $n(p), n'(p), \frac{n”(p)}{2!}, …$ are formal symbols, corresponding to the last coefficient, second to last coefficient, third to last, and so on and so forth.

Two integers are close if their expansions are divisible by high powers of p. The ring of all such integer expansions has only one maximal ideal, $p$.

This is how Hensel originally came upon the definition of the p-adics!