def rotation_matrix(axis, theta):
    """The Euler–Rodrigues formula.

    Return the rotation matrix associated with counterclockwise rotation about
    the given axis by theta radians.

    Parameters
    ----------
    axis: vector to rotate around
    theta: rotation angle, in rad
    """
    axis = np.asarray(axis)
    axis = axis / np.linalg.norm(axis)
    a = np.cos(theta / 2)
    b, c, d = -axis * np.sin(theta / 2)
    aa, bb, cc, dd = a * a, b * b, c * c, d * d
    bc, ad, ac, ab, bd, cd = b * c, a * d, a * c, a * b, b * d, c * d
    return np.array([
        [aa + bb - cc - dd, 2 * (bc + ad), 2 * (bd - ac)],
        [2 * (bc - ad), aa + cc - bb - dd, 2 * (cd + ab)],
        [2 * (bd + ac), 2 * (cd - ab), aa + dd - bb - cc],
    ])