@article {2013, title = {On conjugate times of LQ optimal control problems}, number = {Journal of Dynamical and Control Systems}, year = {2014}, note = {14 pages, 1 figure}, publisher = {Springer}, abstract = {Motivated by the study of linear quadratic optimal control problems, we consider a dynamical system with a constant, quadratic Hamiltonian, and we characterize the number of conjugate times in terms of the spectrum of the Hamiltonian vector field $\vec{H}$. We prove the following dichotomy: the number of conjugate times is identically zero or grows to infinity. The latter case occurs if and only if $\vec{H}$ has at least one Jordan block of odd dimension corresponding to a purely imaginary eigenvalue. As a byproduct, we obtain bounds from below on the number of conjugate times contained in an interval in terms of the spectrum of $\vec{H}$.}, keywords = {Optimal control, Lagrange Grassmannian, Conjugate point}, doi = {10.1007/s10883-014-9251-6}, url = {http://hdl.handle.net/1963/7227}, author = {Andrei A. Agrachev and Luca Rizzi and Pavel Silveira} }