def operator_si(u):
    """operator_si operator."""
    global _aux
    if np.ndim(u) == 2:
        P = _P2
    elif np.ndim(u) == 3:
        P = _P3
    else:
        raise ValueError("u has an invalid number of dimensions "
                         "(should be 2 or 3)")
    
    if u.shape != _aux.shape[1:]:
        _aux = np.zeros((len(P),) + u.shape)
    
    for _aux_i, P_i in zip(_aux, P):
        _aux_i[:] = binary_erosion(u, P_i)
    
    return _aux.max(0)
operator_si operator.