Cospectral graphs on 12 vertices
Citation for published version (APA):Brouwer, A. E., & Spence, E. (2009). Cospectral graphs on 12 vertices. The Electronic Journal of Combinatorics, 16(1), N20-1/3.
Document status and date: Published: 01/01/2009 Document Version:
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Cospectral graphs on 12 vertices
A. E. Brouwer
Dept. of Mathematics Techn. Univ. Eindhoven P.O. Box 513, 5600MB Eindhoven
Netherlands aeb@cwi.nl
E. Spence
Dept. of Mathematics University of Glasgow Glasgow G12 8QQ Scotland ted@maths.gla.ac.uk Submitted: Jun 1, 2009; Accepted: Jun 2, 2009; Published: Jun 12, 2009Mathematics Subject Classification: 05C50, 05E99
Abstract
We found the characteristic polynomials for all graphs on 12 vertices, and report statistics related to the number of cospectral graphs.
1
Introduction
Let the spectrum of a graph be the spectrum of its 0-1 adjacency matrix. In connec-tion with the graph isomorphism problem, it is of interest what fracconnec-tion of all graphs is uniquely determined by its spectrum. Haemers conjectures that the fraction of graphs on n vertices with a cospectral mate tends to zero as n tends to infinity. Numerical data for n ≤ 9 was given in [2], and for n = 10, 11 in [3]. Here we do n= 12, and also take the opportunity to correct a few earlier values.
Both authors did the computations independently and found the same results.
2
Totals
The table below lists for n ≤ 12 the total number of graphs on n vertices, the total number of distinct characteristic polynomials of such graphs, the number of such graphs with a cospectral mate, and the size of the largest family of cospectral graphs.
n #graphs #char. pols #with mate max. family 0 1 1 0 1 1 1 1 0 1 2 2 2 0 1 3 4 4 0 1 4 11 11 0 1 5 34 33 2 2 6 156 151 10 2 7 1044 988 110 3 8 12346 11453 1722 4 9 274668 247357 51039* 10 10 12005168 10608128 2560606* 21 11 1018997864 901029366 215331676* 46 12 165091172592 148187993520 31067572481 128
The three starred entries are 1 more, 90 more, and 1 less than the corresponding values in [3]. (The first of these was given correctly in [2].)
3
Trends
In the table above we see that the fraction of graphs with a cospectral mate increases at first and starts decreasing at n = 11. Graphically:
Somewhat more illuminating are the below plots for n = 9, 10, 11, 12 where the percentage of graphs with cospectral mate is given as function of the number of edges. One sees that the central part of the graph is pressed down as we go from n = 9 to n = 12, but the parts for low or high edge density might show some increase. For some more details, see [1].
There is a clear odd-even effect.
References
[1] http://www.win.tue.nl/~aeb/graphs/cospectral/cospectralA.html [2] C. Godsil & B. McKay, Some computational results on the spectra of graphs,
in: Combinatorial Mathematics IV, Springer LNM 560 (1976) 73–92.
[3] W. H. Haemers & E. Spence, Enumeration of cospectral graphs, Europ. J. Combin. 25 (2004) 199–211.
