Observations on the Darboux coordinates for rigid special geometry

We exploit some relations which exist when (rigid) special geometry isformulated in real symplectic special coordinates $P^I=(p^\Lambda,q_\Lambda),I=1,...,2n$. The central role of the real $2n\times 2n$ matrix $M(\Re\mathcal{F},\Im \mathcal{F})$, where $\mathcal{F} =\partial_\Lambda\partial_\Sigma F$ and $F$ is the holomorphic prepotential, iselucidated in the real formalism. The property $M\Omega M=\Omega$ with $\Omega$being the invariant symplectic form is used to prove several identities in theDarboux formulation. In this setting the matrix $M$ coincides with the(negative of the) Hessian matrix $H(S)=\frac{\partial^2 S}{\partial P^I\partialP^J}$ of a certain hamiltonian real function $S(P)$, which also provides themetric of the special K\"ahler manifold. When $S(P)=S(U+\bar U)$ is regarded asa "K\"ahler potential'' of a complex manifold with coordinates$U^I=\frac12(P^I+iZ^I)$, then it provides a K\"ahler metric of an hyperk\"ahlermanifold which describes the hypermultiplet geometry obtained by c-map from theoriginal n-dimensional special K\"ahler structure.Comment: 15 pages; final version to appear on JHEP. Misprints corrected, references adde