# I What does this symbol mean??

1. Oct 27, 2016

### sa1988

The below image shows the end part of a formal proof of the inertia tensor. There's a weird symbol in there that I've never seen before, and google isn't bringing much in the way of explanation.

Anybody know what it is? At a guess I'd say it has quite a strong connection to the Kronecker delta in the step above, but a definitive answer would be great. Thanks

2. Oct 27, 2016

### Alex163

http://anorien.csc.warwick.ac.uk/mirrors/CTAN/info/symbols/comprehensive/symbols-letter.pdf

Page 52 gives the LaTex (AMS) description "binary negative entail", naively I'd say that does sound like it'd fit with being a more formal definition of whatever category contains the Kronecker delta. Further attempts at squeezing meaningful links out this symbol lead to vague hints about it being to do with model theory. Which immediately filled me with existential dread. Sorry I can't be of any more help to you, I'd like to know the answer to this as well.

3. Oct 27, 2016

### sa1988

That PDF is gigantic! How did you manage to find it specifically in there?!

Either way, yeah it's an odd one. I can fathom out what it means, based on the step above, but it would be interested to see it properly defined.

4. Oct 27, 2016

### Staff: Mentor

You could compare the latex mnemonic for that symbol with the kronecker delta you expect to see if a simple typo caused it.

5. Oct 27, 2016

### sa1988

There are over 300 with the string "dash" in them, haha

6. Oct 27, 2016

### TeethWhitener

It almost looks like the rendering of \mathbb{1} (for the identity matrix) went horribly wrong.
Edit: \mathbb{1} doesn't render properly (irony!), but \mathbb{I} gets pretty close: $\mathbb{I}$
2nd edit: Better yet, with the double underline: $\underline{\underline{\mathbb{I}}}$

7. Nov 14, 2016

### sa1988

Sorry for the late reply. I don't come on here much.

I just came back to this thread to say that I've figured it out, but it looks like I was beaten to it!

It certainly must be the identity matrix.

I came across it again in the same lecture notes, at a part covering oscillations using a Lagrangian formulation and finding normal modes using eigenvalues of the resulting matrix construction.

After a bit of careful checking, it now seems obvious that the weird symbol is supposed to be the identity matrix. The image below demonstrates it pretty well (from the original tensor matrix notes I was dealing with before).

That would obviously only work if the symbol there is actually the identity matrix.