Abstract
We show how the results of Dowling and Wilson on Whitney numbers in ‘The slimmest geometric lattices’ imply minimum values for the numbers of k-dimensional flats and d-dimensional cells of a projective d-arrangement of hyperplanes and for the number of d-cells missed by an extra hyperplane. Their theorems also characterize the extremal arrangements. We extend their lattice results to doubly indexed Whitney numbers; thence we obtain minima for the number of k-dimensional cells and the number of pairs of flats with x \(\subseteq\) y and dim x=k, dim y=l. The lower bounds are in terms of the rank and number of points of the geometric lattice, or the dimension d and the number of hyperplanes of the arrangement. The minima for k-cells were conjectured by Grünbaum; R. W. Shannon proved the minima for k-dimensional flats and cells and characterized attainment for the latter by a more strictly geometric, non-latticial technique.
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Research supported by SGPNR Grant No. 74ZZ*08 and NSF Grant No. MCS-8003109.
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Zaslavsky, T. The slimmest arrangements of hyperplanes. Geom Dedicata 14, 243–259 (1983). https://doi.org/10.1007/BF00146905
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DOI: https://doi.org/10.1007/BF00146905