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. It is a kind of product integral, or Volterra integral.
Definition Let A be an algebra over a field K, and a(t) be an element of A parameterized by the real numbers,
a : R → A . {\displaystyle a:\mathbb {R} \to 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
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