Hi there, I'm trying to understand the derivation of geodesic deviation given here: http://wps.aw.com/wps/media/objects/500/512494/supplements/Ch21.pdf but I can't figure out why x(t)+\chi(t) obeys the geodesic equation (eq.(7)). Of course x(t) does, since it is per definition a geodesic. Could it be that \chi(t) is so small that it is negligible and doesn't change the geodesic character of x(t)? I'd really appreciate some help. Thanks!