  We consider It\^o SDE $\d X_t=\sum_{j=1}^m A_j(X_t) \d w_t^j + A_0(X_t) \d t$
on $\R^d$. The diffusion coefficients $A_1,..., A_m$ are supposed to be in the
Sobolev space $W_\text{loc}^{1,p} (\R^d)$ with $p>d$, and to have linear
growth; for the drift coefficient $A_0$, we consider two cases: (i) $A_0$ is
continuous whose distributional divergence $\delta(A_0)$ w.r.t. the Gaussian
measure $\gamma_d$ exists, (ii) $A_0$ has the Sobolev regularity
$W_\text{loc}^{1,p'}$ for some $p'>1$. Assume $\int_{\R^d}
\exp\big[\lambda_0\bigl(|\delta(A_0)| + \sum_{j=1}^m (|\delta(A_j)|^2 +|\nabla
A_j|^2)\bigr)\big] \d\gamma_d<+\infty$ for some $\lambda_0>0$, in the case (i),
if the pathwise uniqueness of solutions holds, then the push-forward $(X_t)_#
\gamma_d$ admits a density with respect to $\gamma_d$. In particular, if the
coefficients are bounded Lipschitz continuous, then $X_t$ leaves the Lebesgue
measure $\Leb_d$ quasi-invariant. In the case (ii), we develop a method used by
G. Crippa and C. De Lellis for ODE and implemented by X. Zhang for SDE, to
establish the existence and uniqueness of stochastic flow of maps.
