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I have seen some examples (though I am currently looking for a good rigorous explanation and a source would be much appreciated) of taking a second order linear ODE and turning it into a linear system of 2 equations. My question is, can you go the other way? That is, given some continuous vector field, can we find a differential equation corresponding to it (of any order)?

Thanks!

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    I mean that for first order and second order ODE's we can get vector fields on $\mathbb{R}^2$ (the first is just a direction field, the second by some massaging, making $y'$ the vertical axis and $y$ the horizontal axis, I believe). However, given a vector field, is it always a solution to some differential equation, of some kind? Can we find that equation?2011-05-25

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I assume you're talking about a vector field on $\mathbb{R}^n$ -- indeed, you likely mean a vector field on $\mathbb{R}^2$.

Assuming that's the case, the answer is that if the vector field $\vec{V}(x,y) = v_1(x,y)\hat{i} + v_2(x,y)\hat{j}$ obeys "The Canonical Flip on the Double Tangent Bundle" -- that is to say, if $v_1(x,y) = y$ -- then $\ddot{x} = v_2(x, \dot{x})$ is the second-order ordinary differential equation corresponding to $\vec{V}$.

(For an explanation of the correspondence between ODEs and vector fields, try Ordinary Differential Equations by Arnold. Also, many introductory texts on differential manifolds discuss how tangent vector field are [first-order] ODEs on manifolds; I prefer Differential Manifolds by Kosinski.

To understand The Canonical Flip on the Double Tangent Bundle, I recommend Transversal Mappings and Flows by Abraham and Robbin and The Geometry of Jet Bundles by Saunders.)