No complete classification scheme for 3-manifolds is known so there is no list of possible manifolds that can be checked to verify that they all have different homotopy groups.
Hindi kumpleto pag-uuri pamamaraan para sa 3-manifolds ay kilala kaya walang listahan ng mga posibleng na manifolds ay maaaring checked upang mapatunayan na sila ang lahat ay may iba't-ibang homotopy grupo.