  We say that a system of differential equations
d^2x(t)/dt^2=Adx(t)/dt+Bx(t)+Cu(t), in which A and B are m-by-m complex
matrices and C is an m-by-n complex matrix, is rigid if it can be reduced by
substitutions x(t)=Sy(t), u(t)=Udy(t)/dt+Vy(t)+Pv(t) with nonsingular S and P
to each system obtained from it by a small enough perturbation of its matrices
A,B,C. We prove that there exists a rigid system if and only if
m<n(1+square_root{5})/2, and describe all rigid systems.
