def advance_surf_cov(surf, Csurf, x):
    """
    Explicit integration of the covariance term, using scipy's sparse matrix for Csurf
    surf - density function along the surface
    Csurf - transition matrix
    x - grid
    """
    surf = (Csurf * surf.reshape(len(x) ** 2)).reshape(len(x), len(x))
    return surf
