Monotone maps, sphericity and bounded second eigenvalue

Yonatan Bilu*, Nati Linial

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

12 Scopus citations

Abstract

We consider monotone embeddings of a finite metric space into low-dimensional normed space. That is, embeddings that respect the order among the distances in the original space. Our main interest is in embeddings into Euclidean spaces. We observe that any metric on n points can be embedded into l2n, while (in a sense to be made precise later), for almost every n-point metric space, every monotone map must be into a space of dimension Ω (n) (Lemma 3). It becomes natural, then, to seek explicit constructions of metric spaces that cannot be monotonically embedded into spaces of sublinear dimension.T o this end, we employ known results on sphericity of graphs, which suggest one example of such a metric space - that is defined by a complete bipartite graph. We prove that an δn-regular graph of order n, with bounded diameter has sphericity Ω(n/ (λ2 + 1)), where λ2 is the second largest eigenvalue of the adjacency matrix of the graph, and 0 < δ ≤ 1/2 is constant (Theorem 4). We also show that while random graphs have linear sphericity, there are quasi-random graphs of logarithmic sphericity (Lemma 7). For the above bound to be linear, λ2 must be constant. We show that if the second eigenvalue of an n/2-regular graph is bounded by a constant, then the graph is close to being complete bipartite. Namely, its adjacency matrix differs from that of a complete bipartite graph in only o(n2) entries (Theorem 5). Furthermore, for any 0 < δ < 1/2, and λ2, there are only finitely many δn-regular graphs with second eigenvalue at most λ2 (Corollary 4).

Original languageAmerican English
Pages (from-to)283-299
Number of pages17
JournalJournal of Combinatorial Theory. Series B
Volume95
Issue number2
DOIs
StatePublished - Nov 2005

Bibliographical note

Funding Information:
E-mail addresses: yonatan.bilu@weizmann.ac.il (Y. Bilu), nati@cs.huji.ac.il (N. Linial). 1This research was done while Y.B. was at the Hebrew University, and was supported by the Israeli Ministry of Science. 2This research is supported by the Israel Science Foundation.

Keywords

  • Bipartite graphs
  • Eigenvalues
  • Embedding
  • Finite metric space
  • Graphs
  • Second eigenvalue
  • Sphericity

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