決定兩個有限生成的Mn(Z)子半群是否有相同的元素。 Word problem for groups 共軛問題 群同構問題(Group isomorphism problem) 決定兩個有限單形(simplicial complex)是否表現同胚空間。 決定兩個有限單形(simplicial complex)是否(同胚至)流形。
can be defined as being the least fixpoint (up to isomorphism) of some type equation; the isomorphism is then given by an initial algebra. Dually, final