def skew(xi):
    """Return the skew-symmetric matrix that can be used to calculate
    cross-products with vector xi.

    Multiplying this matrix by a vector `v` gives the same result
    as `xi x v`.

    Parameters
    ----------
    xi : :obj:`numpy.ndarray` of float
        A 3-entry vector.

    Returns
    -------
    :obj:`numpy.ndarray` of float
        The 3x3 skew-symmetric cross product matrix for the vector.
    """
    S = np.array([[0, -xi[2], xi[1]],
                  [xi[2], 0, -xi[0]],
                  [-xi[1], xi[0], 0]])
    return S
Return the skew-symmetric matrix that can be used to calculate
    cross-products with vector xi.

    Multiplying this matrix by a vector `v` gives the same result
    as `xi x v`.

    Parameters
    ----------
    xi : :obj:`numpy.ndarray` of float
        A 3-entry vector.

    Returns
    -------
    :obj:`numpy.ndarray` of float
        The 3x3 skew-symmetric cross product matrix for the vector.