Lattices with large minimal extensions

This paper characterizes those finite lattices which are a maximal sublattice of an infinite lattice. There are 145 minimal lattices with this property, and a finite lattice has an infinite minimal extension if and only if it contains one of these 145 as a sublattice. Received October 5, 1998; accepted in final form May 19, 1999.     

Forcing minimal extensions of Boolean algebras

We employ a forcing approach to extending Boolean algebras. A link between some forcings and some cardinal functions on Boolean algebras is found and exploited. We find the following applications:

1) We make Fedorchuk's method more flexible, obtaining, for every cardinal of uncountable cofinality, a consistent example of a Boolean algebra whose every infinite homomorphic image is of cardinality and has a countable dense subalgebra (i.e., its Stone space is a compact S-space whose every infinite closed subspace has weight ). In particular this construction shows that it is consistent that the minimal character of a nonprincipal ultrafilter in a homomorphic image of an algebra can be strictly less than the minimal size of a homomorphic image of , answering a question of J. D. Monk.

2) We prove that for every cardinal of uncountable cofinality it is consistent that and both and exist.

3) By combining these algebras we obtain many examples that answer questions of J.D. Monk.

4) We prove the consistency of MA + CH + there is a countably tight compact space without a point of countable character, complementing results of A. Dow, V. Malykhin, and I. Juhasz. Although the algebra of clopen sets of the above space has no ultrafilter which is countably generated, it is a subalgebra of an algebra all of whose ultrafilters are countably generated. This proves, answering a question of Arhangelskii, that it is consistent that there is a first countable compact space which has a continuous image without a point of countable character.

5) We prove that for any cardinal of uncountable cofinality it is consistent that there is a countably tight Boolean algebra with a distinguished ultrafilter such that for every the algebra is countable and has hereditary character .

     

Uniqueness of the Riemann minimal examples

We prove that a properly embedded minimal surface in R ³ of genus zero with infinite symmetry group is a plane, a catenoid, a helicoid or a Riemann minimal example. We introduce the language of Hurwitz schemes to understand the underlying moduli space of surfaces in our setting. Oblatum 30-V-1997 & 5-VIII-1997     

Uniqueness of form extensions and domination of semigroups

In this article, we study uniqueness of form extensions in a rather general setting. The method is based on the theory of ordered Hilbert spaces and the concept of domination of semigroups. Our main abstract result transfers uniqueness of form extension of a dominating form to that of a dominated form. This result can be applied to a multitude of examples including various magnetic Schrödinger forms on graphs and on manifolds.     

Joint consistency in extensions of the minimal logic

Analogs of Robinson’s theorem on joint consistency are found which are equivalent to the weak interpolation property (WIP) in extensions of Johansson’s minimal logic J. Although all propositional superintuitionistic logics possess this property, there are J-logics without WIP. It is proved that the problem of the validity of WIP in J-logics can be reduced to the same problem over the logic Gl obtained from J by adding the tertium non datur. Some algebraic criteria for validity of WIP over J and Gl are found.     

N-free orders and minimal interval extensions

We investigate problems related to the set of minimal interval extensions of N-free orders. This leads us to a correspondence between this set for an arbitrary order and a certain set of its maximal N-free reductions. We also get a 1-1-correspondence between the set of linear extensions of an arbitrary order and the set of minimal interval extensions of the linegraph of that order. This has an algorithmic consequence, namely the problem of counting minimal interval extensions of an N-free order is #P-complete. Finally a characterization of all N-free orders with isomorphic root graph is given in terms of their lattice of maximal antichains; the lattices are isomorphic iff the root graphs agree.This work was supported by the PROCOPE Program. The first author is supported by the DFG.     

Measure preserving extensions and minimal wandering rates

For conservative ergodic (infinite) measure preserving transformations we show that existence and asymptotics of minimal wandering rates are preserved if we pass to extensions or factors. Consequently, they are similarity invariants.     

Parametrization of minimal rank block Hankel matrix extensions and minimal partial realizations

A parametrization of allm-rowsn-columns extensions of block Hankel matrices with minimal rank is presented. As a particular case, a new parametrization of the minimal partial realizations of a sequence of blocks is given. In this way results of O.H. Bosgra [2], I. Gohberg/M.A. Kaashoek/L. Lerer [3], and H. Woerdeman [13] are extended.     

The lattice of extensions of the minimal logic

In this article, we survey the results on the lattice of extensions of the minimal logic Lj, a paraconsistent analog of the intuitionistic logic Li. Unlike the well-studied classes of explosive logics, the class of extensions of the minimal logic has an interesting global structure. This class decomposes into the disjoint union of the class Int of intermediate logics, the class Neg of negative logics with a degenerate negation, and the class Par of properly paraconsistent extensions of the minimal logic. The classes Int and Neg are well studied, whereas the study of Par can be reduced to some extent to the classes Int and Neg.