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Quadratic Embedding Constant


The quadratic embedding constant QEC(G) of a finite simple connected graph G on n vertices is defined as the maximum of the product vDv over all real n-vectors v satisfying v.v=1 and sum_(i=1)^(n)v_i=0, where D is the graph distance matrix (Obata and Zakiyyah 2018, Obata 2022, Choudhury and Nandi 2023). Obata and Zakiyyah (2018) give quadratic embedding constants for connected graphs on 5 and fewer vertices (though the value given for the 12th graph on 5 nodes, i.e., the kite graph, is incorect).

Quadratic embedding constants for the singleton graph K_1 (Obata and Zakiyyah 2018), complete graphs K_n with n>=2 (Obata and Zakiyyah 2018, Obata 2022), complete bipartite graphs K_(m,n) (Obata and Zakiyyah 2018, Obata 2022), cycle graphs C_n (Obata and Zakiyyah 2018, Obata 2022), path graphs P_n with n>=2 (Młotkowski 2022, Obata 2022), and wheel graphs (E. Weisstein, Jul. 3, 2023) are given by

QEC(K_1)=0
(1)
QEC(K_n)=-1
(2)
QEC(K_(m,n))=(2(m-1)(n-1)-2)/(m+n)
(3)
QEC(C_n)={0 for n even; -1/4sec^2(pi/n) for n odd
(4)
QEC(P_n)=-(1+cos(pi/n))^(-1)
(5)
QEC(W_n)={0 for n odd; -4sin^2(pi/(2(n-1))) for n even.
(6)

Obata (2022) gives quadratic embedding constants for complete k-partite graphs in general.

Any graph obtained by deleting two or more disjoint subsets from a complete graph has quadratic embedding constant equal to 0 (Obata and Zakiyyah 2018). This includes the square graph C_4, wheel graph W_5, octahedral graph K_(2,2,2), 2×3 queen graph K_(1,1,2,2), 16-cell graph K_(2,2,2,2), and complete k-partite graphs K_(1,2,2,2), K_(1,1,1,2,2), etc.

The quadratic embedding constant of a graph Cartesian product of graphs G_1, G_2, ... each having two or more vertices has QEC(G_1 square G_2 square ...)=0 (Obata 2022).

For a connected graph whose graph distance matrix D has equal row sums, the quadratic embedding constant is given by the second largest eigenvalue of D (Obata and Zakiyyah 2018).

A connected graph for which QEC(G)<=0 is called a quadratically embeddable graph.


See also

Graph Distance Matrix, Quadratically Embeddable Graph

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References

Choudhury, P. N. and Nandi, R. "Quadratic Embedding Constants of Graphs: Bounds and Distance Spectra." 27 Jun 2023. https://arxiv.org/abs/2306.15589.Młotkowski, W. "Quadratic Embedding Constants of Path Graphs." Linear Algebra Appl. 644, 95-107, 2022.Obata, N. "Complete Multipartite Graphs of Non-QE Class." 12 Jun 2022. https://arxiv.org/abs/2206.05848.Obata, N. and Zakiyyah, A. Y. "Distance Matrices and Quadratic Embedding of Graphs." Elec. J. Graph Th. Appl. 6, 37-60, 2018.Schoenberg, I. J. "Metric Spaces and Positive Definite Functions." Trans. Amer. Math. Soc. 44, 522-536, 1938.

Cite this as:

Weisstein, Eric W. "Quadratic Embedding Constant." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/QuadraticEmbeddingConstant.html