## Trying to Crochet the Impossible

I’ve been crocheting hyperbolic surfaces of constant curvature. The process is time-consuming, so while I am crocheting, I wonder about the mathematics of crocheting.

Hilbert’s theorem says that I can’t embed a hyperbolic plane in 3-dimensional space. The proof is rather involved. But here, I have an explanation from the point of view of a crochet hook. My hook starts with a tiny cycle of four stitches. Then for every *x* stitches the hook makes *y* stitches in the next row, where *y* is greater than *x*. The extra stitches should be evenly distributed to guarantee that locally every small area is approximately isomorphic to other areas, meaning that the surface has a constant curvature.

The ratio of stitches in the next row to the current row is *r* = *y*/*x*. Thus, the number of stitches in each row increases exponentially. But each row is a fixed height *h*. That means after *k* rows, my thingy has to fit inside a ball of radius *kh*. But the length of the last row is 4*r*^{k-1}. It becomes huge very fast. As the last row is a physical curve made out of stitches, there is a limit of how much of it I can fit into a given volume, creating a contradiction.

That means, if I start crocheting, something should happen that won’t allow me to continue. I decided to experiment and see what actually would happen. Being lazy, I preferred the disaster to happen sooner rather than later. So I chose the ratio of three: for each stitch on my perimeter, I added three new stitches. Shortly after I started to work, the process became more and more difficult. The ball was too tight. It was challenging to hold that thing in the place where I needed to insert the hook. And the loops were getting tighter, making it more exhausting to insert the hook into the proper hole. So each new stitch was taking more and more time to complete.

To my disappointment, the thing didn’t explode, as I was secretly hoping: I just couldn’t work on it anymore.

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## Bea da Silva:

I love this post, a post like this started me crochetting and knitting. If it is mathematical and difficult I seem to like it.

28 November 2021, 2:44 am## Zarunias:

I also crotcheted some of these things some years ago. In my opinion you will get the nicest result if you use a ratio of 3/2 and about 3 balls of wool. I especially like it if they are of different colours.

Here is a picture (triangle ruler is in cm): https://zandor.popcorn.cc/bilder/IMG_1195.JPG

7 December 2021, 1:04 pm