## Most useful isomorphisms you know?

For the discussion of math. Duh.

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Farabor
Posts: 157
Joined: Wed Apr 06, 2011 9:46 am UTC

### Most useful isomorphisms you know?

So, working on the Ph.D., I find myself proving various isomorphisms pretty regularly...which got me wondering as to which ones are actually really useful/powerful to have. Thinking over the ones I know, the biggest that stands out is in Algebraic Topology, the isomorphism between singular and simplicial Homology. For those who don't know, general results are very easy to prove in singular topology, but it's next to impossible to compute with for any practical purposes, while the reverse holds true for simplicial homology.

I'm sure there's lots of others out there though?

Tub
Posts: 465
Joined: Wed Jul 27, 2011 3:13 pm UTC

### Re: Most useful isomorphisms you know?

This may be too trivial for your taste, but having isomorphisms between any (discrete, finite) piece of information and a binary representation is pretty useful if you like having general-purpose computers.

MartianInvader
Posts: 807
Joined: Sat Oct 27, 2007 5:51 pm UTC

### Re: Most useful isomorphisms you know?

Since you mentioned algebraic topology I've always loved the isomorphism between an arbitrary group and the fundamental group of its presentation complex.
Let's have a fervent argument, mostly over semantics, where we all claim the burden of proof is on the other side!

equivariant
Posts: 2
Joined: Sun Mar 31, 2019 12:56 am UTC

### Re: Most useful isomorphisms you know?

Reminds me of a quote by someone - "Mathematics is the art of calling different things by the same name"
Algebraic topology by itself is a nice 'isomorphism' - though not really; in the sense that we can look at spaces as algebraic structures and algebraic structures as spaces (really an adjunction I guess -?)

cyanyoshi
Posts: 407
Joined: Thu Sep 23, 2010 3:30 am UTC

### Re: Most useful isomorphisms you know?

I like how random variables tend to be related to the same abstract probability space. Do you want to find a random point in the Mandelbrot set? Awesome! All you need is to apply the right deterministic function to a uniform random variable on the interval [0,1], and you got it! You can even do this in reverse if you are terrible at darts and have a Mandelbrot set-shaped dartboard!

Eebster the Great
Posts: 3409
Joined: Mon Nov 10, 2008 12:58 am UTC
Location: Cleveland, Ohio

### Re: Most useful isomorphisms you know?

cyanyoshi wrote:I like how random variables tend to be related to the same abstract probability space. Do you want to find a random point in the Mandelbrot set? Awesome! All you need is to apply the right deterministic function to a uniform random variable on the interval [0,1], and you got it! You can even do this in reverse if you are terrible at darts and have a Mandelbrot set-shaped dartboard!

You might have to be pretty good at darts to simulate a uniform random distribution.

DavCrav
Posts: 251
Joined: Tue Aug 12, 2008 3:04 pm UTC
Location: Oxford, UK

### Re: Most useful isomorphisms you know?

The complex numbers and the algebraic closure of the p-adic rationals are isomorphic as fields. Very useful if you like l-adic cohomology.

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