# Second derivative of Kähler potential.

Does the following second covariant (in terms of Kähler geometry) derivative of Kähler potential vanish? $$K_{ij}\equiv\nabla_i\nabla_j K=0,$$ $$K_{i^*j^*}\equiv\nabla_{i^*}\nabla_{j^*} K=0?$$ Indices represent complex coordinates ($A_i$ and $\bar{A}_i$) on Kähler manifold.

The answer is in general No. Take e.g. the Fubini-Study Kaehler potential

$$K~=~\ln D, \qquad D~=~1+ Q,\qquad Q~=~\sum_{k=1}^n z^k \bar{z}^k, \tag{1}$$

with Hermitian metric

$$g_{\imath\bar{\jmath}} ~=~ \partial_{\imath} \bar{\partial}_{\bar{\jmath}}K~=~ \frac{\delta_{\imath\bar{\jmath}}}{D}-\frac{\bar{z}^{\imath}z^{\bar{\jmath}}}{D^2}~=~ D^{-1}\left(\delta_{\imath\bar{\jmath}}-\frac{\bar{z}^{\imath}z^{\bar{\jmath}}}{D}\right), \tag{2}$$

and inverse metric

$$g^{\bar{\imath}\jmath}~=~D(\delta^{\bar{\imath}\jmath}+\bar{z}^{\bar{\imath}}z^{\jmath} ) , \tag{3}$$

and Hermitian Christoffel symbols

$$\Gamma_{\imath\jmath}^{\ell} ~=~ \partial_{\imath}g_{\jmath\bar{k}}~g^{\bar{k}\ell}~=~-\frac{\bar{z}^{\imath}\delta_{\jmath}^{\ell}}{D}-\frac{\bar{z}^{\jmath}\delta_{\imath}^{\ell}}{D}. \tag{4}$$

The covariant derivative of the Kaehler potential is

$$\nabla_{\ell}K~=~\partial_{\ell}K ~=~\frac{\bar{z}^{\ell}}{D}.\tag{5}$$

Now calculate the sought-for quantity

$$\nabla_{\imath}\nabla_{\jmath}K ~=~ \nabla_{\imath}\partial_{\jmath}K ~=~\partial_{\imath}\partial_{\jmath}K -\Gamma_{\imath\jmath}^{\ell}~\partial_{\ell}K ~=~\frac{\bar{z}^{\imath}\bar{z}^{\jmath}}{D^2} ~\neq~0, \tag{6}$$ which does not vanish.

• Then, as I asked in physics section, in application to supersymmetric theories, $\Box z$ and $\partial z\partial\bar{z}$ terms cannot be used interchangeably (box and partials WRT ordinary spacetime). And I don't understand how to get the latter term, without introducing the $\partial z\partial z$ and $\partial\bar{z}\partial\bar{z}$ terms. – Kosm Jun 19 '16 at 21:21
• Why don't your i's and j's have dots? Are they sick? – Harry Wilson Jul 21 '16 at 9:58
• Purely for aesthetic reasons to avoid having bars and dots stacked on top of each other. – Qmechanic Jul 21 '16 at 10:54