What does [math]\partial _{\lambda}{{\Gamma}^{\lambda}}_{{\mu}{\nu}} +{{\Gamma}^{\gamma}_{{\mu}{\lambda}}}{{\Gamma}^{\lambda}_{{\nu}{\gamma}}}=\frac{8{\pi}G}{c^4}(T_{{\mu}{\nu}}-\frac{1}{2}Tg_{{\mu}{\nu}})[/math] mean?