Integrals of periodic motion and periodic solutions for classical   equations of relativistic string with masses at ends. I. Integrals of   periodic motion
  Boundary equations for the relativistic string with masses at ends are formulated in terms of geometrical invariants of world trajectories of masses at the string ends. In the three--dimensional Minkowski space $E^1_2$, there are two invariants of that sort, the curvature $K$ and torsion $\kappa$. Curvatures of trajectories of the string ends with masses are always constant, $K_i = \gamma/m_i (i =1,2,)$, whereas torsions $\kappa_i(\tau)$ obey a system of differential equations with deviating arguments. For these equations with periodic $\kappa_i(\tau+n l)=\kappa(\tau)$, constants of motion are obtained (part I) and exact solutions are presented (part II) for periods $l$ and $2l$ where $l$ is the string length in the plane of parameters $\tau$ and $\sigma \ (\sigma_1 = 0, \sigma_2 =l)$. 
B. M. Barbashov (JINR, Dubna)