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Some remarks on modular regular types

Published online by Cambridge University Press:  12 March 2014

Anand Pillay*
Affiliation:
Department of Mathematics, University of Notre Dame, Notre Dame, Indiana 46556

Extract

Here we consider some problems concerning regular types. In the first place we consider a strongly minimal set D. One can ask what is the strength of the assumption that D has (full) elimination of imaginaries (namely, every definable set X over D has as canonical parameter some tuple from D). We show that D cannot be locally modular. Nontriviality of D is immediate. However, to exclude the locally modular nontrivial case one has to understand structures of the form G/E, where G is a modular strongly minimal group and E is a definable equivalence relation on G with finite classes. We show that the quotient structure G/E can be obtained in two steps. First quotient by a finite subgroup K of G to obtain a strongly minimal group H. Now let Γ be a finite subgroup of the group Aff(H) of definable affine automorphisms of H (namely maps of the form x → αx + a, where α is a definable automorphism of H and aH), and quotient H by Γ (namely form the orbit space of H under Γ). It can clearly be arranged that Γ contains no nontrivial subgroup of translations.

In the second place we look at a nontrivial modular regular type p whose pregeometry is actually a geometry. The geometry is then known to be (infinite-dimensional) projective geometry over a division ring F. We ask whether F is definable (internally to p). If F is finite, this is clear. In fact in this case p must have U-rank 1. So we assume F to be infinite. We are only able to show definability of F in the case where F is a field, using some results on 2-transitive subgroups of PGL [V]. Moreover in the superstable case we also observe that p is isolated.

Type
Research Article
Copyright
Copyright © Association for Symbolic Logic 1991

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References

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