## May 27, 2024

### Lanthanides and the Exceptional Lie Group G_{2}

#### Posted by John Baez

The lanthanides are the 14 elements defined by the fact that their electrons fill up, one by one, the 14 orbitals in the so-called f subshell. Here they are:

lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium.

They are also called ‘rare earths’, but that term is often also applied to 3 more elements. Why? That’s a fascinating puzzle in its own right. But what matters to me now is something else: an apparent connection between the lanthanides and the exceptional Lie group G_{2}!

Alas, this connection remains profoundly mysterious to me, so I’m pleading for your help.

## May 26, 2024

### Wild Knots are Wildly Difficult to Classify

#### Posted by John Baez

In the real world, the rope in a knot has some nonzero thickness. In math, knots are made of infinitely thin stuff. This allows mathematical knots to be tied in infinitely complicated ways — ways that are impossible for knots with nonzero thickness! These are called ‘wild’ knots.

Check out the wild knot in this video by Henry Segerman. There’s just one point where it needs to have zero thickness. So we say it’s wild at just one point. But some knots are wild at many points.

There are even knots that are wild at *every* point! To build these you need to recursively put in wildness at more and more places, forever. I would like to see a good picture of such an everywhere wild knot. I haven’t seen one.

Wild knots are extremely hard to classify. This is not just a feeling — it’s a theorem. Vadim Kulikov showed that wild knots are harder to classify than any sort of countable structure that you can describe using first-order classical logic with just countably many symbols!

Very roughly speaking, this means wild knots are so complicated that we can’t classify them using anything we can write down. This makes them very different from ‘tame’ knots: knots that aren’t wild. Yeah, tame knots are hard to classify, but nowhere near *that* hard.

## May 15, 2024

### 3d Rotations and the 7d Cross Product (Part 1)

#### Posted by John Baez

There’s a dot product and cross product of vectors in 3 dimensions. But there’s also a dot product and cross product in 7 dimensions obeying a lot of the same identities! There’s nothing really like this in other dimensions.

The following stuff is well-known: the group of linear transformations of $\mathbb{R}^n$ preserving the dot and cross product is called $SO(3)$. It consists of rotations. We say $SO(3)$ has an ‘irreducible representation’ on $\mathbb{R}^3$ because there’s no linear subspace of $\mathbb{R}^3$ that’s mapped to itself by every transformation in $SO(3)$, except for $\{0\}$ and the whole space.

Ho hum. But here’s something more surprising: it seems that $SO(3)$ also has an irreducible representation on $\mathbb{R}^7$ where every transformation preserves the dot product and cross product in 7 dimensions!

That’s right—no typo there. There is *not* an irreducible representation of $SO(7)$ on $\mathbb{R}^7$ that preserves the dot product and cross product. Preserving the dot product is easy. But the cross product in 7 dimensions is a strange thing that breaks rotation symmetry.

There *is*, apparently, an irreducible representation of the much smaller group $SO(3)$ on $\mathbb{R}^7$ that preserves the dot and cross product. But I only know this because people say Dynkin proved it! More technically, it seems Dynkin said there’s an $SO(3)$ subgroup of $G_2$ for which the irreducible representation of $\mathrm{G}_2$ on $\mathbb{R}^7$ remains irreducible when restricted to this subgroup. I want to see one explicitly.