Editing Ordered exponential (section)

== Definition ==

Let {{math|''A''}} be an [[algebra over a field|algebra]] over a [[field (mathematics)|field]] {{math|''K''}}, and {{math|''a''(''t'')}} be an element of  {{math|''A''}} [[function (mathematics)|parameterized]] by the real numbers,
:<math>a : \R \to A. </math>

The parameter {{mvar|t}} in {{math|''a''(''t'')}} is often referred to as the ''time parameter'' in this context.

The ordered exponential of {{math|''a''}} is denoted
:<math>\begin{align}
\operatorname{OE}[a](t) \equiv \mathcal{T} \left\{e^{\int_0^t a(t') \, dt'}\right\} & \equiv \sum_{n = 0}^\infty \frac{1}{n!} \int_0^t dt'_1 \cdots \int_0^t dt'_n \; \mathcal{T} \left\{a(t'_1) \cdots a(t'_n)\right\} \\
& = \sum_{n = 0}^\infty \int_0^t dt'_1 \int_0^{t'_1} dt'_2 \int_0^{t'_2}dt'_3 \cdots \int_0^{t'_{n-1}} dt'_n \; a(t'_n) \cdots a(t'_1)
\end{align}</math>

where the term {{math|1=''n'' = 0}} is equal to 1 and where <math>\mathcal{T}</math> is the [[Path-ordering#Time ordering|time-ordering operator]]. It is a higher-order operation that ensures the exponential is time-ordered, so that any product of {{math|''a''(''t'')}} that occurs in the expansion of the exponential is ordered such that the value of {{mvar|t}} is increasing from right to left of the product. For example:
:<math>\mathcal{T} \left\{a(1.2) a(9.5) a(4.1)\right\} = a(9.5) a(4.1) a(1.2).</math>
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,
:<math>\operatorname{OE} \mathrel{:} (\R \to A) \to (\R \to A). </math>

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 (mathematics)|limit]] as the number of terms grows to infinity:
:<math>\operatorname{OE}[a](t) = \prod_0^t e^{a(t') \, dt'} \equiv
    \lim_{N \to \infty} \left(
      e^{a(t_N) \, \Delta t} e^{a(t_{N-1}) \, \Delta t} \cdots
      e^{a(t_1) \, \Delta t} e^{a(t_0) \, \Delta t}
    \right)
  </math>

where the time moments {{math|{{mset|''t''<sub>0</sub>, ..., ''t''<sub>''N''</sub>}}}} are defined as {{math|''t''<sub>''i''</sub> ≡ ''i'' Δ''t''}} for {{math|1=''i'' = 0, ..., ''N''}}, and {{math|Δ''t'' ≡ ''t'' / ''N''}}.

The ordered exponential is in fact a [[Product integral#Type II|geometric integral]]{{Broken anchor|date=2024-05-27|bot=User:Cewbot/log/20201008/configuration|target_link=Product integral#Type II|reason= The anchor (Type II) [[Special:Diff/835454693|has been deleted]].}}.<ref name=nnc>Michael Grossman and Robert Katz. [https://books.google.com/books?q=%22Non-Newtonian+Calculus%22&btnG=Search+Books&as_brr=0 ''Non-Newtonian Calculus''], {{ISBN|0912938013}}, 1972.</ref><ref>A. E. Bashirov, E. M. Kurpınar, A. Özyapıcı. [http://linkinghub.elsevier.com/retrieve/pii/S0022247X07003824 ''Multiplicative calculus and its applications''], Journal of Mathematical Analysis and Applications, 2008.</ref><ref name=FvA>Luc Florack and Hans van Assen.[https://doi.org/10.1007%2Fs10851-011-0275-1 "Multiplicative calculus in biomedical image analysis"], Journal of Mathematical Imaging and Vision, 2011. </ref>

=== Solution to a differential equation ===

The ordered exponential is unique solution of the [[initial value problem]]:
:<math>\begin{align}
    \frac{d}{d t} \operatorname{OE}[a](t) &= a(t) \operatorname{OE}[a](t), \\[5pt]
    \operatorname{OE}[a](0) &= 1.
\end{align}</math>

=== Solution to an integral equation ===

The ordered exponential is the solution to the [[integral equation]]:
:<math>\operatorname{OE}[a](t) = 1 + \int_0^t a(t') \operatorname{OE}[a](t') \, dt'. </math>

This equation is equivalent to the previous initial value problem.

=== Infinite series expansion ===

The ordered exponential can be defined as an infinite sum,
:<math>\operatorname{OE}[a](t) = 1 + \int_0^t a(t_1) \, dt_1+ \int_0^t dt_1 \int_0^{t_1} dt_2 \; a(t_1) a(t_2) + \cdots.</math>

This can be derived by recursively substituting the integral equation into itself.