* Can we construct free semi-lattice from free dist. lattice?
@ 2003-05-27 8:52 Christopher Townsend
2003-05-27 20:52 ` Prof. Peter Johnstone
0 siblings, 1 reply; 2+ messages in thread
From: Christopher Townsend @ 2003-05-27 8:52 UTC (permalink / raw)
To: categories
Dear All,
I have a problem which I had thought very specialised, but actually can be
stated very generally: -
I have a category C with finite limits, and so I also have a category
DLat(C) of distributive lattices which, lets say, has coequalizers. If free
distibrutive lattices can be constructed (i.e. if there exists F:C->DLat(C)
left adjoint to the forgetful functor) then do free semilattices exist?
Thanks, Christopher Townsend (OU)
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Can we construct free semi-lattice from free dist. lattice?
2003-05-27 8:52 Can we construct free semi-lattice from free dist. lattice? Christopher Townsend
@ 2003-05-27 20:52 ` Prof. Peter Johnstone
0 siblings, 0 replies; 2+ messages in thread
From: Prof. Peter Johnstone @ 2003-05-27 20:52 UTC (permalink / raw)
To: categories
On Tue, 27 May 2003, Christopher Townsend wrote:
> Dear All,
> I have a problem which I had thought very specialised, but actually can be
> stated very generally: -
>
> I have a category C with finite limits, and so I also have a category
> DLat(C) of distributive lattices which, lets say, has coequalizers. If free
> distibrutive lattices can be constructed (i.e. if there exists F:C->DLat(C)
> left adjoint to the forgetful functor) then do free semilattices exist?
>
I suspect the answer is no, for irritatingly trivial reasons.
If C is a pointed category (i.e. has a zero object), then any internal
distributive lattice in C has its top and bottom elements equal, and
so is degenerate (i.e. isomorphic to the terminal object 1). Hence
the free-distributive-lattice functor exists, and is the constant
functor with value 1. But I'm sure there must be examples of pointed,
finitely complete categories which don't have a free-semilattice
functor (though I have to admit I don't have one at my fingertips).
Peter Johnstone
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2003-05-27 8:52 Can we construct free semi-lattice from free dist. lattice? Christopher Townsend
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