Let $u: V \rightarrow M$, where $(V,g)$ is a Lorentzian manifold and $(M,h)$ is a Riemannian manifold. Also $V=S \times R$, where $S$ is an $n$-dimensional orientable smooth manifold. Now i $\partial_i u^A$ and $\partial_t u^A$ are mappings from $S \times R$ into $T^∗ V\otimes TM$.
Now in a paper, there is an expression for
$D_i \nabla^{\lambda} \nabla_{\lambda} u^A$
I don't understand what $\nabla^{\lambda} \nabla_{\lambda}$ means, I've never seen the term $\nabla^{\lambda}$ before.