I am trying to understand a line in the proof of Lemma 3.6 on p. 18 of this book by Chow, Lu, and Ni.
Say we have a Riemannian manifold $M$ with Levi-Civita connection $\nabla$. Let $\Delta$ be the Laplacian on functions. Then the authors state that (in local cordinates) we have $$\Delta\nabla_i f = \nabla_j\nabla_i\nabla_j f=\nabla_i\nabla_j\nabla_j f-R_{jijk}\nabla_k f.\quad(*)$$ It seems that $\nabla_j\nabla_i\nabla_j$ and $\nabla_i\nabla_j\nabla_j$ are short for $$\sum_{j,k}g^{jk}\nabla_j\nabla_i\nabla_k,\quad\nabla_i\sum_{j,k}g^{jk}\nabla_j\nabla_k.$$ respectively. I tried to use that $\Delta=\sum_{j,k}g^{jk}\nabla_j\nabla_k$, together with $R_{ji}=[\nabla_j,\nabla_i]$ for coordinate vectors, but somehow I'm not able to work out the details.
Question 1: How do we prove $(*)$?
There is another part of the proof of that lemma that I don't understand:
Question 2: How do we show that $-R_{jijk}\nabla_k f=R_{ij}\nabla_j f$?