##
**An exponential mapping in the generalized Dido problem.**
*(English.
Russian original)*
Zbl 1079.53050

Sb. Math. 194, No. 9, 1331-1359 (2003); translation from Mat. Sb. 194, No. 9, 63-90 (2003).

The classical Dido problem can be stated as follows. Given two points in the plane connected by a curve \(\gamma_{0}\) and a number \(S\), it is required to connect these points by a shortest curve \(\gamma\) so that the domain in the plane bounded by the curves \(\gamma_{0}\) and \(\gamma\) has area \(S\). The solution of this problem is also well-known: it is an arc or circle or a segment of a straight line connecting the given points.

The paper under review deals with the following generalization of the Dido problem: Apart from two points \((x_{0},y_{0})\), \((x_{1},y_{1}) \in \mathbb R^{2}\), a curve \(\gamma_{0}\) connecting them, a number \(S \in \mathbb R\), also a fixed point \(c=(c_{x},c_{y}) \in \mathbb R^{2}\) are given. It is required to find a shortest curve \(\gamma\) connecting the points \((x_{0},y_{0})\) and \((x_{1},y_{1})\) such that the domain \(D \subset \mathbb R^{2}\) bounded by the curves \(\gamma_{0}\) and \(\gamma\) has the given area \(S\) and center of gravity \(c\). In spite of having a simple and natural statement, the generalized Dido problem is still open.

The goal of this paper is to give a description of the extremals of this problem. For this proposal, the generalized Dido problem is stated as an optimal control problem in a 5-dimensional space with a 2-dimensional control and quadratic cost functional, which is a nilpotent sub-Riemannian problem with the growth vector \((2,3,5)\). Extremals of this problem are parametrized by Jacobian elliptic functions.

The paper under review deals with the following generalization of the Dido problem: Apart from two points \((x_{0},y_{0})\), \((x_{1},y_{1}) \in \mathbb R^{2}\), a curve \(\gamma_{0}\) connecting them, a number \(S \in \mathbb R\), also a fixed point \(c=(c_{x},c_{y}) \in \mathbb R^{2}\) are given. It is required to find a shortest curve \(\gamma\) connecting the points \((x_{0},y_{0})\) and \((x_{1},y_{1})\) such that the domain \(D \subset \mathbb R^{2}\) bounded by the curves \(\gamma_{0}\) and \(\gamma\) has the given area \(S\) and center of gravity \(c\). In spite of having a simple and natural statement, the generalized Dido problem is still open.

The goal of this paper is to give a description of the extremals of this problem. For this proposal, the generalized Dido problem is stated as an optimal control problem in a 5-dimensional space with a 2-dimensional control and quadratic cost functional, which is a nilpotent sub-Riemannian problem with the growth vector \((2,3,5)\). Extremals of this problem are parametrized by Jacobian elliptic functions.

Reviewer: Adrian Sandovici (Groningen)

### MSC:

53C17 | Sub-Riemannian geometry |

37N35 | Dynamical systems in control |

70Q05 | Control of mechanical systems |

93B29 | Differential-geometric methods in systems theory (MSC2000) |