* Addendum squared or cubed, depending on what you count.
@ 2000-01-18 15:14 Peter Freyd
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From: Peter Freyd @ 2000-01-18 15:14 UTC (permalink / raw)
To: categories
Vaughan writes:
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.
Indeed, but there's no _final_ cubical coalgebra. What I don't have is
a category with an associative bifunctor with a final cubical
coalgebra but no final square coalgebra.
On a _discrete_ category being the unique coalgebra is necessary and
sufficient for being a final coalgebra. It's easy to see that in a
semigroup if x is a unique solution to xxx = x then x is the
unique solution to xx = x.
After writing all that, I note that in Vaughan's example, * is not a
functor. If it were, there would have to map from 1*1 to 1*3 and
1*1 would be a square coalgebra.
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2000-01-18 15:14 Addendum squared or cubed, depending on what you count Peter Freyd
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