One of the first posts I made on this blog was Lambda calculus and Graham’s number, which set out how to express the insanely large number known as Graham’s Number precisely and concisely using lambda calculus.
A week ago, Reddit user u/KtoProd asked: if I wanted to get a Graham’s Number tattoo, how should I represent it? u/FavoriteColorFlavor linked to my lambda calculus post. But in a cool twist, they suggested that rather than writing these things in the usual way, they use a John Tromp lambda calculus diagram. I got into the discussion and started working with the diagrams a bit, and they really are a great way to work with lambda calculus expressions; it was a pleasure to understand how the diagram relates to what I originally wrote, and manipulate it a bit for clarity.
The bars at the top are lambdas, the joining horizontal lines are applications, and the vertical lines are variables. There are three groups; the rightmost group represents the number 2, and the middle one the number 3; with beta reduction the two lambdas in the leftmost group will consume these rightmost groups and use them to build other small numbers needed here, like 4 (22) and 64 (43). The three is also used to make the two 3s either side of the arrows. Tromp’s page about these diagrams has lots of examples.
I’m obviously biased, but this is my favourite of the suggestions in that discussion. If u/KtoProd does get it as a tattoo I hope I can share a picture with you all!
Update: Some more numbers as lambda calculus
Update 2020-02-24: I’ve added the ability to generate these diagrams to my Python lambda calculus toy. After installation, try ./trylambda demofiles/draw.olc.
Minds aren't magic 
This is pretty, but I’m finding it hard to understand how the diagram corresponds to the expression. It looks like a vertical line changes in what it signifies every time it intersects with an application line? So, e.g., the expression for 2 (https://tromp.github.io/img/cl/2.gif, λf.λx.f(f x)), the leftmost vertical line represents f at the top but the result of the whole expression at the bottom? I’m really not sure.
Yes that’s right, the line changes what it signifies with each application line, to represent the new combined expression.
If the number is so large it’s impossible to evaluate or write down even an approximation, isn’t the expression kind of dead?
Hello artist here, i dont fully (or even nearly) comprehend the mathematical implications of your design, but i am working on a concept that integrates your design with the fibonacci sequence and id like your consent to move forward with it for a potential tattoo. Please reach out if you have further questions
You are more than welcome, it’s an honour! But if you want to integrate something to do with the Fibonacci sequence, maybe a design related to that would be more useful to you? I’d be happy to work with you to come up with a design that meets your needs – I’d hate to think I’d caused someone to get a tattoo that didn’t make mathematical sense :)