  By conformal welding, there is a pair of univalent functions $(f,g)$
associated to every point of the complex K\"ahler manifold
$\Mob(S^1)\bk\Diff_+(S^1)$. For every integer $n\geq 1$, we generalize the
definition of Faber polynomials to define some canonical bases of holomorphic
$1-n$ and $n$ differentials associated to the pair $(f,g)$. Using these bases,
we generalize the definition of Grunsky matrices to define matrices whose
columns are the coefficients of the differentials with respect to standard
bases of differentials on the unit disc and the exterior unit disc. We derive
some identities among these matrices which are reminiscent of the Grunsky
equality. By using these identities, we showed that we can define the Fredholm
determinants of the period matrices of holomorphic $n$ differentials $N_n$,
which are the Gram matrices of the canonical bases of holomorphic
$n$-differentials with respect to the inner product given by the hyperbolic
metric. Finally we proved that $\det N_n =(\det N_1)^{6n^2-6n+1}$ and
$\pa\bar{\pa}\log\det N_n$ is $-(6n^2-6n+1)/(6\pi i)$ of the Weil-Petersson
symplectic form.
