17 Jan
2000
17 Jan
'00
5:37 p.m.
* | A B --+------ A | B A B | A A
The unique cubical coalgebra is A but there is no square coalgebra.
What I don't have is a counterexample with an _associative_ bifunctor.
How about the positive integers with * as sum, with 1->3 as the only nonidentity arrow? The unique cubical coalgebra is 1 but there is no square coalgebra. Vaughan