<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def operator_is(u):
 """operator_is 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_dilation(u, P_i)
    
    return _aux.min(0)