# Homework Help: Path-ordered product in parallel transport

1. Feb 11, 2006

### SmithWillSuffice

I'm reading about bundes and connections but I cannot get past a little problem involving path-ordered exponentials. I hope someone can help me out. I'll try to state the problem as well as possible with plain text LaTeX. My question is just this: How does the the integral (of a general non-commuting matrix function A, such as a connection) like this,

\int_{t\ge t_1\ge \ldots \ge t_n \ge 0} A(t_1)\ldotsA(t_n) dt_n\ldots dt_1

become,

\frac{1}{n!} \int{t_i\in [0,1]} \mathcal{P}A(t_1)\ldotsA(t_n) dt_n\ldots dt_1

where \mathcalP} is the path-ordered product operator, and how does this path-ordered integral become,

\frac{1}{n!} \mathcal{P} \left( \int_0^t A(s) ds \right)

???
Background: the first integral arises when one constructs for example an iterated solution to a simple first-order matrix DE, du/dt = -A(t)u(t), whereby A(t1) need not commute with A(t2).

I can see how the latter conversion can be made if the multiple integral $\int{t_i\in [0,1]}...$ can be taken as a product of $n$ decoupled integrals. But I do not quite see how the $1/n!$ arises in the first conversion from the coupled multiple integral to the decoupled path-ordered multiple integral. I've tried integration by parts, Taylor expansion, and a few other tricks, including staring at the integrals for a long time! I'm missing a basic clue though.

I think I can take the final result and differentiate to prove that it solves the matrix DE that arises in the parallel transport equation for a vector say $u$ on a bundle fibre,

d u(t) / dt + A(t) u(t) = 0

where $A$ is the connection for the covariant derivative along a path. The solution is obviously,

u(t) = \mathcal{P} \exp^{-\int_0^t A(s) ds} u(0)

It's just bugging the heck out of me that I cannot figure out how to construct the formal path-ordered exponential solution without resorting to "guess the answer and back-substitute".

muchos gracias in advance for any hints/help.

---
NotAStudentButAPerpetualSeeker

2. Feb 11, 2006

### Hurkyl

Staff Emeritus
If you put the tags:

[ tex ] [ /tex ]

around your LaTeX, the forum software will produce the corresponding LaTeX images. (Omit the spaces, of course) If you want to inline them in a paragraph, use [ itex ] and [ /itex ] instead.

3. Feb 11, 2006

### Hurkyl

Staff Emeritus
There is an obvious map from the set of all n-long sequences onto the set of all nondecreasing n-long sequenes, right? How many times is each such nondecreasing n-long sequence counted?