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I'm trying to read papers on numerical ODE algorithms and I always seem to stumble upon huge amounts of exponentials multiplied by each other. For example in New families of symplectic splitting methods for numerical integration in dynamical astronomy there are a few of them. Is it the ordinary exponential that is meant or is it just like a way of writing something else (just like with complex numbers)?

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This is a way of writing the flow of a vector field. In other words, if you have an ordinary differential equation (ODE) given by $\dot{x}(t) = v(x(t))$ with initial value $x(0) = x_0$, you could formally write its solution as $x(t) = \mathrm{e}^{t v} x_0$. The formal solution coincides with the actual solution when $v$ is a linear mapping (think of the case in which $v(x) = A x$ for some matrix $A$).

Similarly, if we have another ODE $\dot{y}(t) = w(y(t))$ and an initial value $y(0) = y_0$, then we could write $\mathrm{e}^{t v} \mathrm{e}^{s w} y_0$ to represent the curve resulting from solving the second initial value problem up to time $s$, then using $y(s; y_0)$ as the initial value of the first ODE $\dot{x} = v(x)$ and solving that initial value problem up to time $t$. To reiterate, you can also think about the product of exponentials as meaning that you must first obtain the integral curve of the vector field $w$ that passes through $y_0$, follow said curve until it reaches $y(s)$, and then follow the integral curve of the vector field $v$ that starts at $y(s)$ and goes on for another $t$ units of time. I hope this helps clarify things a little bit more.

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  • $\begingroup$ What does multiplying these exponentials represent? $\endgroup$ – Hugo Gransträm Mar 23 at 19:03
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    $\begingroup$ I've edited my answer to explain it. $\endgroup$ – Juan M. Bello-Rivas Mar 23 at 20:11
  • $\begingroup$ Could that be thought of as composition? $\endgroup$ – Hugo Gransträm Mar 23 at 20:14
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    $\begingroup$ That's exactly what it is: composing the flow of one vector field with the flow of another. $\endgroup$ – Juan M. Bello-Rivas Mar 23 at 20:18
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    $\begingroup$ Think I'm starting to get the hang of this now :-) thank you very much Juan 😊 have a great day $\endgroup$ – Hugo Gransträm Mar 23 at 20:22

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