I am learning about the Ricci Flow, reading: Lectures on the Ricci flow - Peter Topping (download pdf).
In Proposition 2.3.1 (i.e. page 28) the expression appears (To understand the equation, you may have to read the few lines above Proposition 2.3.1). I don't know how this is defined. Can someone tell me?
In addition, I don't know the definition of in Proposition 2.3.6.