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.
Let A be an algebra over a field K, and a(t) be an element of A parameterized by the real numbers, 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
It is a higher-order operation that ensures the exponential is time-ordered, so that any product of a(t) that occurs in the expansion of the exponential is ordered such that the value of t is increasing from right to left of the product.
For example: Time ordering is required, 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.
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 {t0, ..., tN} are defined as ti ≡ i Δt for i = 0, ..., N, and Δt ≡ t / N. The ordered exponential is in fact a geometric integral[broken anchor].
[1][2][3] The ordered exponential is unique solution of the initial value problem: The ordered exponential is the solution to the integral equation: This equation is equivalent to the previous initial value problem.
The ordered exponential can be defined as an infinite sum, This can be derived by recursively substituting the integral equation into itself.
denotes exterior differentiation and
is the connection operator (1-form field) acting on
When integrating above equation it holds (now,
is an infinitesimal rectangle with edge lengths
above expression simplifies as follows : Hence, it holds the group transformation identity
one obtains for the ordered exponential the identity with a correction term that is proportional to the curvature tensor.