The equation should be:
ddt[ξ1ξ2ξ3ξ4]=[ξ21m1(−b1ξ2−k1ξ1+k2ξ3−k2ξ1+b2ξ4−b2ξ2)ξ41m2(−k2ξ3+k2ξ1−b2ξ4+b4ξ2)]
Return to errata.