Balancing pairs and the cross product conjecture G. R. Brightwell S. Felsner W. T. Trotter Article Received: 20 June 1995 Accepted: 31 August 1995 DOI :
10.1007/BF01110378

Cite this article as: Brightwell, G.R., Felsner, S. & Trotter, W.T. Order (1995) 12: 327. doi:10.1007/BF01110378
Abstract In a finite partially ordered set, Prob (x >y ) denotes the proportion of linear extensions in which elementx appears above elementy . In 1969, S. S. Kislitsyn conjectured that in every finite poset which is not a chain, there exists a pair (x,y ) for which 1/3⩽Prob(x >y )⩽2/3. In 1984, J. Kahn and M. Saks showed that there exists a pair (x,y ) with 3/11<Prob(x >y )<8/11, but the full 1/3–2/3 conjecture remains open and has been listed among ORDER's featured unsolved problems for more than 10 years.

In this paper, we show that there exists a pair (x,y ) for which (5−√5)/10⩽Prob(x >y )⩽(5+√5)/10. The proof depends on an application of the Ahlswede-Daykin inequality to prove a special case of a conjecture which we call the Cross Product Conjecture. Our proof also requires the full force of the Kahn-Saks approach — in particular, it requires the Alexandrov-Fenchel inequalities for mixed volumes.

We extend our result on balancing pairs to a class of countably infinite partially ordered sets where the 1/3–2/3 conjecture isfalse , and our bound is best possible. Finally, we obtain improved bounds for the time required to sort using comparisons in the presence of partial information.

Mathematics Subject Classifications (1991) 06A07 06A10

Key words Partially ordered set linear extension balancing pairs cross-product conjecture Ahlswede-Daykin inequality sorting An extended abstract of an earlier version of this paper appears as [6]. The results here are much stronger than in [6], and this paper has been written so as to overlap as little as possible with that version.

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Authors and Affiliations G. R. Brightwell S. Felsner W. T. Trotter 1. Department of Mathematics London School of Economics London U.K. 2. Fachbereich Mathematik, Institut für Informatik Freie Universität Berlin Berlin Germany 3. Department of Mathematics Arizona State University Tempe U.S.A.