## happenings – 31 May

Not much. I’m down with a cold, and don’t feel at all energetic or like focusing on anything.

Sure, I’m browsing “simplicial complexes” in books other than Bloch: his treatment is a little too condensed for me, and I want to play with those things. And I’m thinking about PCA / FA as Malinowski does it, but he has broken out of the box we’ve been in, in a couple of ways, and I’m struggling with the presentation (FYI, “target testing”, and “noise”).

But mostly I’m sneezing my way thru a roll of paper towels, sitting in a recliner in front of the TV. Oh, right now I’m heating up a medium pot of chicken soup, and putting this out while I stay close to the stove.

I’ll put out some math when I’m up to it.

## discussion

There was a lot going on in the example of the SN decomposition (2 of 3). First off, we found eigenvalues of a non-diagonable matrix A, and constructed a diagonal matrix D from them. Then we found 2 eigenvectors and 1 generalized eigenvector of A, and used them to construct a transition matrix P. We used that transition matrix to go from our diagonal D back to the original basis, and find S similar to D.

So S is diagonable while A is not. And A and S have the same eigenvalues; and the columns of P should be eigenvectors of S. They are. The generalized eigenvector that we found for A is an (ordinary) eigenvector of S, but we had to get a generalized eigenvector of A in order to construct S from D.

I wonder. Can I understand the distinction between eigenvectors and generalized eigenvectors by studying S and A? We’ll see.
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## let’s look at an example of the SN decomposition.

This comes from Perko p. 35, example 3. We will compute exp(A) using the SN decomposition described in the previous post (“1 of 3”).

We take the following matrix:

$A = \left(\begin{array}{lll} 1 & 0 & 0 \\ -1 & 2 & 0 \\ 1 & 1 & 2\end{array}\right)$

I ask Mathematica® to find its eigenvalues and eigenvectors. For eigenvalues, I get

$\lambda = \{2,\ 2,\ 1\}$

For the eigenvector matrix, I get

$\left(\begin{array}{lll} 0 & 0 & -1 \\ 0 & 0 & -1 \\ 1 & 0 & 2\end{array}\right)$
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## discussion

one correction 30 Aug 2008, per Brian Hall’s comment and counterexample here. see “edit” in this post.

Let me emphasize something I take for granted: i’m writing about theory as opposed to computations by computer. every time I say, for example, that two matrices A and B are similar via

$B = P^{-1}\ A\ P\$,

I am asserting that P is a transition matrix for a change-of-basis, and by definition it is invertible. In practice, the matrix P can be ill-conditioned, and computing $P^{-1}$ on a computer may be extremely hazardous.

An updated version of a classic paper may be found here . Having just looked at it, I may pepper my theoretical discussion with some vaguely computational comments.

Back to my ivory tower. Well, it’s hardly that, but it’s still far removed from numerical algorithms.

The definition of the matrix exponential is easy enough to write: as the exponential of a number x has the expansion

$e^x = 1 + x + x^2\ /\ 2+ x^3\ /\ 3!\ +\ ...$

we define

$exp(A) = I + A + A^2\ /\ 2 + A^3\ /\ 3!\ +\ ....$

But how do we actually compute it (in theory)? Well, we could just try computing the powers of A and look for a pattern, but there are easier ways. Far easier ways. (and just doing it numerically can be a bad idea: we can encounter disastrous cancellation between one power of A and the next.)
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## References for generalized eigenvectors, the matrix exponential and for the SN decomposition.

one correction 30 Aug 2008, per Brian Hall’s comment below. see “edit” in this post.

First, let’s talk about books which are already in the bibliography. It’s pretty easy to find statements that for matrix exponentials,

exp(A+B) = exp(A) exp(B)
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## Topological Surfaces: Bloch Ch 2.

Let me talk about the second chapter, “Topological Surfaces” in Bloch’s “A First Course in Geometric Topology and Differential Geometry”. I finished it quite a while ago, but I’ve had trouble deciding how to talk about it. I don’t want to just summarize it. Instead, I think I’ll try asking and answering some leading questions.

First of all, why did I choose to read Bloch? A few years ago, I was seriously shaken up by the following, from Freed & Uhlenbeck’s “Instantons and Four-Manifolds”, p. 1: “A basic problem is to ascertain when a topological manifold admits a PL [piecewise linear] structure and, if it does, whether there is also a compatible smooth [differential] structure. By the early 1950’s it was known that every topological manifold of dimension less than or equal to three admits a unique smooth structure.” They were setting us up for the fact that it isn’t generally true in dimensions greater than 3.

To be explicit about the challenge we face: topological, simplicial (which Bloch tells me generalizes to PL), and differential structures on a manifold coincide for low dimensions but not for high, and “high” means “greater than 3”.

This was all news to me. I knew, for a small value of knew, differential geometry, and I’d seen some simplicial stuff – triangulating and cutting up surfaces – and I was acquainted with topology, possibly for a large value of acquainted, but their “basic problem” was nothing I’d ever heard before.
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## Corrections

So far, there are only two that I know of. One is here in “attitude & transition matrices”; the other is here in “the SVD generalizes eigenstructure”.

I was reading yet another book on PCA / FA last night, and I came across a definition saying that two matrices A and B were similar if$B = P^T\ A\ P$. Of course, I objected that P needed to be orthogonal, and we couldn’t guarantee that in general. This morning, out with one of the cats – and, therefore, thinking instead of computing or writing! – I observed that the author had made that mistake because the matrices $X^T\ X$ and $X\ X^T$ are symmetric, and for symmetric matrices it is certainly true that P may be chosen orthogonal. He had been careless because within the realm of PCA / FA, we are only finding eigendecompositions of symmetric matrices. Within the realm of PCA / FA, we may take P to be orthogonal, because we’re looking at $X^T\ X$ and/or $X\ X^T$ when we’re not using the SVD.

It was a while after that I began to wonder if I had made the very same mistake.