Source: J. Symbolic Logic
Volume 73, Issue 2
We investigate the structure of the Medvedev lattice as a partial order.
We prove that every interval in the lattice is either finite, in which
case it is isomorphic to a finite Boolean algebra, or contains
an antichain of size 22ℵ0, the size of the lattice itself.
We also prove that it is consistent with ZFC that the lattice has chains
of size 22ℵ0, and in fact that these big chains occur in
every infinite interval.
We also study embeddings of lattices and algebras.
We show that large Boolean algebras can be embedded into the
Medvedev lattice as upper semilattices, but that
a Boolean algebra can be embedded as a lattice only if it is countable.
Finally we discuss which of these results hold for the closely related
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