The ordered exponential (also called the path-ordered exponential) is a mathematical object, defined in non-commutative algebras, which is equivalent to the exponential function of the integral in the commutative algebras. Therefore it is a function, defined by means of a function from real numbers to a real or complex associative algebra. In practice the values lie in matrix and operator algebras.
For the element A(t) from the algebra (set g with the non-commutative product *), where t is the "time parameter", the ordered exponential
of A can be defined via one of several equivalent approaches:
- As the limit of the ordered product of the infinitesimal exponentials:
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- where the time moments are defined as for, and .
- Via the initial value problem, where the OE(t) is the unique solution of the system of equations:
- Via an integral equation:
- Via Taylor series expansion:
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