# Definitions of Hessian in Riemannian Geometry

I am wondering if there is any quick way to see the following two definitions of Hessian coincide with each other without using local coordinates.

• $$Hess(f)(X,Y)=⟨∇Xgradf,Y ⟩\operatorname{Hess}(f)(X,Y)= \langle \nabla_X \operatorname{grad}f,Y \ \rangle$$; and
• $$Hess(f)(X,Y)=X(Yf)−(∇XY)f\operatorname{Hess}(f)(X,Y)=X (Yf) - (\nabla_XY) f$$.

This question arises from the last two equation in the Wikipedia article. This is a common problem in differential geometry, namely too many different notations mixed up in one place.

The definition is clearly stated in the aforementioned article. The Hessian of a smooth function $f:M\rightarrow \mathbb{R}$ on an arbitrary smooth manifold with an arbitrary connection $\nabla$ is the iterated covariant derivative of function $f$, that is

so that $\operatorname{Hess}(f) \in \Gamma(T^*M \otimes T^*M)$, in other terms it is a $(0,2)$-tensor field on $M$. For any two vector fields $X,Y$ on $M$ we have a smooth real-valued function $\operatorname{Hess}(f)(X,Y)=\nabla{\nabla{f}}\,(X,Y)$ on manifold $M$. All we know about this function is that it is bilinear.

Given a connection $\nabla$ we know how to compute $\nabla{f}\;$ for any smooth function, and $\nabla_X{Y}\;$ for any two (smooth) vector fields $X,Y$. This rule is linear in slot $X$ and satisfies a product rule in slot $Y$. For tensors of all other types the covariant derivatives are computed using the requirement that it commutes with contractions and satisfies the product rule with respect to the tensor product.

In particular, $\nabla{f}\equiv \operatorname{d}f\;$ by definition and

This can also be seen as

where $\mathcal{C}$ denotes the contraction operator (as well as symbol $\cdot$ there).

Now using the declared properties of covariant derivative we can write down the following calculation

Rewriting this using conventions (1) we get

Now let us recall that in Riemannian geometry we have canonical isomorphisms between tangent and cotangent spaces (so called musical isomorhphisms), so we can identify

However, one must keep in mind that this really means

which by definition is a unique vector such that

Another feature of Riemannian geometry is that we use the Levi-Civita connection by default.

Now using (2) it is easy to complete what @Jason suggested:

quod erat demonstrandum (QED).