Controllability and stabilizability of a system of coupled strings with control applied at the coupled points is studied. By investigating the properties of certain exponential series, it is shown that the system is approximate controllable if and only if related systems of uncoupled strings do not share a common eigenvalue. A sufficient condition for exact controllability is also obtained in terms of the Riesz basis properties of those exponential series.
(1993). Controllability and Stabilizability of Coupled Strings with Control Applied at the Coupled Points. SIAM Journal on Control and Optimization, 31 (6), 1416-1437.