§ Category where coproducts of computable things is not computable
- Modular lattices are an algebraic variety.
- Consider the category of modular latties.
- The free modular lattice on 2 elements and on 3 elements has dediable equality, by virtue of being finite.
- The free modular lattice on 5 elements does not have decidable equality.
- The coproduct of free modular lattice on 2 and 3 generators is the free modular lattice on 5 generators, because (where are two and three element sets), because free is left adjoint to forgetful, and the left adjoint preserve colimits!