Ordered exponential

The ordered exponential, also called the path-ordered exponential, is a mathematical operation defined in non-commutative algebras, equivalent to the exponential of the integral in the commutative algebras. In practice the ordered exponential is used in matrix and operator algebras.
Definition.
Let "A" be an algebra over a real or complex field "K", and "a"("t") be a parameterized element of "A",
The parameter "t" in "a"("t") is often referred to as the "time parameter" in this context.
The ordered exponential of "a" is denoted
where the term "n" = 0 is equal to 1 and
where formula_3 is a higher-order operation that ensures the exponential is time-ordered: any product of "a"("t") that occurs in the expansion of the exponential must be ordered such that the value of "t" is increasing from right to left of the product; a schematic example:
This restriction is necessary as products in the algebra are not necessarily commutative.
The operation maps a parameterized element onto another parameterized element, or symbolically,
There are various ways to define this integral more rigorously.
Product of exponentials.
The ordered exponential can be defined as the left product integral of the infinitesimal exponentials, or equivalently, as an ordered product of exponentials in the limit as the number of terms grows to infinity:
where the time moments are defined as for , and .
The ordered exponential is in fact a geometric integral. 
Solution to a differential equation.
The ordered exponential is unique solution of the initial value problem:
Solution to an integral equation.
The ordered exponential is the solution to the integral equation:
This equation is equivalent to the previous initial value problem.
Infinite series expansion.
The ordered exponential can be defined as an infinite sum,
This can be derived by recursively substituting the integral equation into itself.
Example.
Given a manifold formula_10 where for a formula_11 with group transformation formula_12 it holds at a point formula_13:
Here, formula_15 denotes exterior differentiation and formula_16 is the connection operator (1-form field) acting on formula_17. When integrating above equation it holds (now, formula_16 is the connection operator expressed in a coordinate basis)
with the path-ordering operator formula_20 that orders factors in order of the path formula_21. For the special case that formula_16 is an antisymmetric operator and formula_23 is an infinitesimal rectangle with edge lengths formula_24 and corners at points formula_25 above expression simplifies as follows :
Hence, it holds the group transformation identity formula_27. If formula_28 is a smooth connection, expanding above quantity to second order in infinitesimal quantities formula_24 one obtains for the ordered exponential the identity with a correction term that is proportional to the curvature tensor.