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I'm trying to find an approximation for the optimal path for a material point, minimizing the integral associated with the total energy.

I managed to write the exact formula for the energy along a path covering the points $(x,y(x))$, which should be:

$$ E(y) = \int_{x_0}^{x_f} \sqrt{a-by(x)}\sqrt{1+(y^\prime(x))^2} \ \ \mathrm{d}x $$

Where $a$ and $b$ are constants related to mass, initial position and speed, and gravitational constant.

If I knew the path, numerically computing the integral would be trivial, but I don't know any methods to estimate $$ y = \arg\min_{z} J(z) $$

I suppose that $y$ should be at least a $\mathcal{C}^1$ function (possibly $\mathcal{C}^\infty$?), but the only way to approach the problem in a general way that I can think about is to use a (linear) spline function, replacing differentiation with incremental ratios.

I could start with a straight line path, split it in $n$ parts and find a greedy optimization for each segment of the spline, moving the node $(x_1,y_1)$ upwards or downwards along the $y$-axis until an approximate minimum is found, then repeating the same with the node $(x_2,y_2)$, but this is obviously not going to work: .

I could instead try to move all the nodes at once (except the starting and ending ones, fixed at $(x_0,y_0)$ and $(x_f,y_f)$), but this seems too computationally intensive to pursue when using a finer grid.

Is there an efficient method to find an optimal path?

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  • $\begingroup$ if you do not take friction into account, then the difference between initial and final energy is roughly proportional to height difference and so the energy is minimized, if the curve is shortest, i.e. a straight line, or do I get something wrong? $\endgroup$ – Manfred Weis Apr 12 '17 at 7:48
  • $\begingroup$ What is J(z)? Are you evaluating the action on the solution to the Euler-Lagrange equations? $\endgroup$ – AHusain Apr 14 '17 at 19:37
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Under the assumption that your solution is smooth enough, you can use the calculus of variations to turn your minimization problem into a differential equation. The differential equation is then given by the Euler-Lagrange equation, and can be solved by standard methods for numerical simulation of differential equations.

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It is not hard to show, assuming that $b>0$, that the smooth solutions of the Euler-Lagrange equation are given by the formula $$ y(x) = \frac{a}{b} - c_1^2 - \left(\frac{x-c_0}{2c_1}\right)^2 < \frac{a}{b} $$ where $c_0$ and $c_1\not=0$ are constants. For example, see my answer to the MO question Riemannian surfaces with an explicit distance function?. Along such a path, it is easy to compute the energy using your integration.

However, you should note that, when $4(a-b y_0)(a-b y_f) < b^2(x_0-x_f)^2$, the actual minimizing path is to go straight from $(x_0,y_0)$ to $(x_0,a/b)$, then go along the line $y=a/b$ to $(x_f,a/b)$, and then go straight to $(x_f,y_f)$. For an explanation of this phenomenon, see the above mentioned answer.

When $b<0$, one has to flip some signs, but essentially the same analysis holds. When $b=0$, the minimizing curves are straight line segments.

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The following iterative algorithm may provide an efficient way of iteratively generating estimates with increasing precision for the optimal path:

As the optimal path must be contained in a box $[x_0-c \lt x\lt x_f+c]\times[-\infty\lt y_{min} \le y\le \frac{a}{b}]$ for sufficiently large $c$ and sufficiently small $y_{min}$, we can restrict our considerations to that strip and proceed as follows:

  • estimate the values of $c$ and $y_{min}$

  • choose a $2D$-distribution that can be truncating to an arbitrary $2D$-interval.

  • Start: sample the distribution until the point-density inside $[x_0-c,\ x_f+c]\times[y_{min},\ \frac{a}{b}]$ is high enough and add points $(x_0,y_0)$ and $(x_f,y_f)$.

  • Repeat until the geometry of the calculated shortest path is precise enough:

    • construct the point set's Delaunay Triangulation and replace each edge by a pair of antiparallel arcs

    • calculate the arc weights via the formula supplied in the statement of the problem.

    • calculate the minimum-weight path in the modified Delaunay Triangulation from $(x_0,y_0)$ to $(x_f,y_f)$

    • cover the (undirected) edges of the shortest path with sufficiently small rectangular boxes that can be e.g., determined with an algorithm of raster graphics.

    • let the path-covering boxes be $\lbrace B_1,\ \dots,\ B_k\rbrace$, to which weights $\lbrace beta_1,\ \dots,\beta_k\rbrace$ are assigned, that equal the integral of the distribution's density function over the corresponding box.

    • assign to box $B_i$ the right-open range $\frac{\left[\sum_{j\lt i}\beta_j,\ \sum_{j\le i}\beta_j\right)}{\sum_{j\le k}\beta_j} $

    • repeat until the point-density in the path-covering boxes is high enough:

      • determine a random point from $\left[0,1\right)$ by sampling an equal distribution over that range; let $B_j$ be the path-covering box, that corresponds to the containing interval.

      • sample the point-generating distribution's restriction to $B_j$

One should however be cautious to use a shortest-path algorithm, that works with negative edge-weights, such as the Bellman-Ford algorithm; there is however no need to deal with negative cycles, because of the physical background of the arc-weights.

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