# A new type of manifold, is such a construction interesting? Is it relevant for the Euler-Lagrange equations

Recently, I've been wondering how to rewrite the standard Euler-Lagrange equations: \begin{align} \dfrac{\partial L}{\partial q^i} - \dfrac{d}{dt} \left(\dfrac{\partial L}{\partial \dot{q}^i}\right) &= 0 \end{align} (all derivatives being evaluated at appropriate points along a stationary curve of the action functional) without referencing the coordinates $$(q^1, \dots q^n, \dot{q}^1, \dots, \dot{q}^n)$$ on the tangent bundle. So, I tried to rewrite things as much as possible using only "natural operations" on the tangent bundle, like Lie-derivative, exterior derivative etc. Of course, I didn't succeed (and any articles I tried to search online were too abstract to understand), so what I did was expand the total time derivative: \begin{align} \dfrac{\partial L}{\partial q^i} - \left[\dfrac{\partial^2 L}{\partial q^j\partial \dot{q}^i}\dot{q}^j + \dfrac{\partial^2 L}{\partial \dot{q}^j\partial \dot{q}^i}\ddot{q}^j \right] &= 0. \end{align} This gave me the idea that perhaps thinking of the Lagrangian, $$L$$, as a function on the tangent bundle $$TQ \to \Bbb{R}$$ is perhaps not the most appropriate/natural setting. It seems that the tangent bundle only has coordinates $$(q, \dot{q})$$, whereas the Euler-Lagrange equations which are second order differential equations, involving $$\ddot{q}$$. So, it seemed to me that it would be nice to construct a new manifold $$M$$, from the original "configuration manifold" $$Q$$, so that on the new manifold $$M$$, we have local coordinates $$(q^i, \dot{q}^i, \ddot{q}^i)$$, for $$1 \leq i \leq n$$.

To formalize this idea of introducing a larger manifold with extra coordinates for higher derivatives, my plan was to mimic the construction of $$TQ$$ as much as possible.

Let $$k \geq 1$$ be an integer and let $$Q$$ be a smooth manifold modeled on a Banach space $$E$$. Now, we define a relation $$\sim_k$$ on the set of all smooth curves $$\gamma:I \to Q$$, where $$I$$ is an open set in $$\Bbb{R}$$ containing $$0$$, by declaring $$\gamma_1 \sim_k \gamma_2$$ if and only if there is a chart $$(U, \alpha)$$ of $$Q$$ such that (the composition below makes sense and) for every $$r \in \{0, \dots, k\}$$, we have \begin{align} (\alpha \circ \gamma_1)^{(r)}(0) &= (\alpha \circ \gamma_2)^{(r)}(0). \end{align} A tedious, but straight forward induction exercise and chain rule shows that this relation doesn't depend on the choice of chart $$(U, \alpha)$$, so we're actually justified with using the notation $$\sim_k$$ without referencing the chart. This is also an equivalence relation. For the lack of a better name, I shall denote the quotient set of equivalence classes as \begin{align} C^kQ := (\text{smooth curves in Q})/\sim_k, \end{align} and I shall call it "the $$k^{th}$$ order contact manifold of curves in $$Q$$". Given a smooth curve $$\gamma$$, we shall indicate the equivalence class either as $$C^k\gamma$$ or $$[\gamma]$$, whichever is more convenient. Next, I outline how I put a manifold structure.

I realized that this space has quite a bit of structure: we can define a projection map $$\pi_k : C^kQ \to Q$$ by sending $$C^k \gamma \mapsto \gamma(0)$$. We can even make this into a smooth manifold as follows: given a chart $$(U, \alpha)$$ on $$Q$$, we define the chart $$(C^kU, C^k \alpha)$$ by defining $$C^kU := \pi_k^{-1}[U]$$ and $$C^k \alpha : C^kU \to \alpha[U] \times E^k$$, \begin{align} C^k \alpha([\gamma]) &:= \left( (\alpha \circ \gamma)(0), (\alpha \circ \gamma)'(0), \dots (\alpha \circ \gamma)^{k}(0) \right) \end{align} This is a well-defined map because of how the equivalence relation was defined. It is also easily seen to be a bijective map, and also, if $$(V, \beta)$$ is another chart on $$Q$$ with $$U \cap V \neq \emptyset$$, then using the chain rule, it is straightforward (though tedious) to seem that $$(C^k \beta) \circ (C^k \alpha)^{-1}$$ is a smooth map between open subsets of Banach spaces.

Also, I only just recently read up about fiber bundles, but I believe that based on what I've constructed, we have that $$\pi_k :C^kQ \to Q$$ is a fiber bundle, with typical fiber $$E^k$$, whereby the maps $$C^k\alpha$$ defined above provide us with the local trivializations. Is this the correct way of using the terminology?

Here are my questions:

• Have people considered such spaces $$C^kQ$$? Are they interesting manifolds to study, and am I right in thinking that these manifolds would be a more appropriate setting to formulate Lagrangian mechanics? I believe that this is true, because if for some reason we decided to consider a Lagrangian which depends on higher derivatives of the curve, then the tangent bundle alone is insufficient to capture such information. In any case, I would appreciate some confirmation/denial along with references (if any).

• Is it possible to formulate the Euler-Lagrange equations in a coordinate-free manner, perhaps using such spaces? If yes, I'd appreciate some references (which hopefully aren't too abstract).

• For $$k=1$$, this construction yields precisely the tangent bundle, in which case, the fiber over each point $$x \in Q$$, namely $$T_xQ$$ can be given a natural vector space structure. However, for $$k>1$$, am I right in saying that we cannot endow each fiber with a vector space structure? Of course, if we choose a particular chart $$(U, \alpha)$$, we can establish a bijection between the fiber $$\pi_k^{-1}(\{x\})$$ and $$E^k$$, and hence inherit a vector space structure that way, but I believe that this is not a chart-independent construction. Is this right?

• I didn't read everything you wrote, but the idea of taking the $k$-th derivative tangent vectors is described by looking at the space of $k$-jets $\Bbb R\to M$ at $0$. The language of jets and jet bundles is natural when tracking higher derivatives than $1$. However higher derivatives are messy, and this language is (imo) unpleasant. You can find a short introduction in these notes by Ballmann or in this book in chapter 4. Jun 1 '20 at 9:20
• @s.harp I see, so what I've been calling $C^kQ$ is more commonly referred to as the $k^{th}$ jet space $J^k(\Bbb{R}, Q)$. Thanks for pointing me in the right direction regarding some "buzz words" to search, and for the references. Jun 1 '20 at 9:28