Abstract

The geometry conjecture, which was posed nearly a quarter of a century ago, states that the fixed point set of the composition of projectors onto nonempty closed convex sets in Hilbert space is actually equal to the intersection of certain translations of the underlying sets.

Highlights

  • 1.1 Fixed points of compositions of projectors Throughout,X is a real Hilbert space (1) andC1, . . . , Cm are nonempty closed convex subsets of X, (2)© Salihah Alwadani & Heinz H

  • Which turns out to be independent of the cycle chosen, makes (4) true and yields “one half” of (5), namely: Fm ⊆ Cm ∩(Cm−1 +vm−1)∩· · ·∩(C1 +v1 +· · ·+vm−1) and analogously for Fm−1, . . . , F1. This description is not fully satisfying — it is only implicit in the sense it was not known what the difference vectors are when the fixed point sets Fi are empty

  • The resolution depends on key results from monotone operator theory and yields a formula for the difference vectors

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Summary

Fixed points of compositions of projectors

This description is not fully satisfying — it is only implicit in the sense it was not known what the difference vectors are when the fixed point sets Fi are empty. Note that this description is not based on the fixed point sets F1, F2. (See [6, 7, 8, 17] for much more on the case when m = 2.) A referee pointed out that when m = 2 and F1 = F2 = ∅ one cannot expect uniqueness of the difference vectors as one may separate the sets even further. For results on underrelaxed projectors, see [5, 19].) Even when cycles exist, the “meaning” of the distance vector was not understood

Aim and outline of this paper
Product space and Attouch–Théra duality
The proof of the geometry conjecture
Revisiting known results
Two lines
The lines are parallel
The lines are not parallel
All three lines are parallel
Not all three lines are parallel
An example featuring the epigraph of the exponential function
An ordering with cycles
An ordering without cycles
Finding the difference vectors by forward-backward
Conclusion and future work
Full Text
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