






Stefan Grünewald,
Jacobus Koolen and
WooSun Lee. Quartets in maximal weakly compatible split systems. In Applied Mathematics Letters, Vol. 22(6):16041608, 2009. Note: http://dx.doi.org/10.1016/j.aml.2009.05.006.
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"Weakly compatible split systems are a generalization of unrooted evolutionary trees and are commonly used to display reticulate evolution or ambiguity in biological data. They are collections of bipartitions of a finite set X of taxa (e.g. species) with the property that, for every four taxa, at least one of the three bipartitions into two pairs (quartets) is not induced by any of the Xsplits. We characterize all split systems where exactly two quartets from every quadruple are induced by some split. On the other hand, we construct maximal weakly compatible split systems where the number of induced quartets per quadruple tends to 0 with the number of taxa going to infinity. © 2009."






Andreas W. M. Dress,
Katharina Huber,
Jacobus Koolen and
Vincent Moulton. Compatible decompositions and block realizations of finite metrics. In EJC, Vol. 29(7):16171633, 2008. Keywords: abstract network, block realization, from distances, phylogenetic network, phylogeny, realization, reconstruction. Note: http://www.ims.nus.edu.sg/preprints/200721.pdf.
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"Given a metric D defined on a finite set X, we define a finite collection D of metrics on X to be a compatible decomposition of D if any two distinct metrics in D are linearly independent (considered as vectors in RX × X), D = ∑d ∈ D d holds, and there exist points x, x′ ∈ X for any two distinct metrics d, d′ in D such that d (x, y) d′ (x′, y) = 0 holds for every y ∈ X. In this paper, we show that such decompositions are in onetoone correspondence with (isomorphism classes of) block realizations of D, that is, graph realizations G of D for which G is a block graph and for which every vertex in G not labelled by X has degree at least 3 and is a cut point of G. This generalizes a fundamental result in phylogenetic combinatorics that states that a metric D defined on X can be realized by a tree if and only if there exists a compatible decomposition D of D such that all metrics d ∈ D are split metrics, and lays the foundation for a more general theory of metric decompositions that will be explored in future papers. © 2007 Elsevier Ltd. All rights reserved."





