\[
\boxed{
 \left\{ {\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}x \left( t \right) +{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}y \left( t \right) -x \left( t \right) =0,{\frac {{\rm d}^{2}}{{\rm d}{t}^{2}}}x \left( t \right) -{\frac {\rm d}{{\rm d}t}}x \left( t \right) +{\frac {\rm d}{{\rm d}t}}y \left( t \right) =0 \right\} }
\]