## Cyclotomic matrices and graphs over the ring of integers of some imaginary quadratic fields.(English)Zbl 1238.05166

Let $$A$$ be an $$n \times n$$ hermitian matrix over the ring of integers of an imaginary quadratic number field and let $$\chi_A(x) \in \mathbb{Z}[x]$$ be its characteristic polynomial. Then $$A$$ is called cyclotomic if $$x^n \chi_A(x+1/x)$$ is a cyclotomic polynomial (i.e., all roots are roots of unity), or equivalently, all eigenvalues are in the interval $$[-2,2]$$. The author points out that for square-free $$d \notin \{-1,-2,-3,-7,-11,-15\}$$, all hermitian cyclotomic matrices over the ring of integers of $$\mathbb{Q}(\sqrt{d})$$ have integer entries, hence are symmetric integer cyclotomic matrices, whose classification was achieved by J. McKee and C. Smyth [“Integer symmetric matrices having all their eigenvalues in the interval $$[-2,2]$$,” J. Algebra 317, No. 1, 260–290 (2007; Zbl 1140.15007)]. The author gives a classification of all hermitian cyclotomic matrices over the ring of integers of $$\mathbb{Q}(\sqrt{d})$$ for $$d \in \{-2,-7,-11,-15\}$$. Thus the only remaining cases are $$d \in \{ -1, -3 \}$$.
Extending the work of J. H. Smith [“Some properties of the spectrum of a graph,” Combinat. Struct. Appl., Proc. Calgary Int. Conf. Comb. Struct. Appl., Calgary 1969, 403–406 (1970; Zbl 0249.05136)] (developed by McKee and Smyth [loc. cit.]), the author associates to each indecomposable hermitian cyclotomic matrix a cyclotomic $$\mathcal{L}$$-signed charged graph (or $$\mathcal{L}$$-graph), where $$\mathcal{L}$$ is the set of integers of $$\mathbb{Q}(\sqrt{d})$$ with norm in the interval $$[0,4]$$. The matrix is viewed as a generalized adjacency matrix of the graph. Maximal such cyclotomic $$\mathcal{L}$$-graphs are then classified and it is shown that any such graph embeds into a maximal one. For $$d \in \{ -11, -15\}$$, it is proved that any maximal cyclotomic $$\mathcal{L}$$-graph, not already appearing in the above classifications (i.e., arising from symmetric integer matrices), is equivalent to one of a finite number of sporadic graphs.
The main step in the classification for $$d \in \{-2, -7\}$$ is that a cyclotomic $$\mathcal{L}$$-graph is maximal if and only if it is 4-cyclotomic, i.e., each vertex has weight 4. In the latter cases, aside from a finite number of sporadic examples, there appear three new infinite families of maximal cyclotomic $$\mathcal{L}$$-graphs in the classification. The author’s introduction includes a clear exposition of the relationship between such matrix classification results and Lehmer’s conjecture.

### MSC:

 05C50 Graphs and linear algebra (matrices, eigenvalues, etc.) 11R11 Quadratic extensions 15B57 Hermitian, skew-Hermitian, and related matrices 15B36 Matrices of integers 15B33 Matrices over special rings (quaternions, finite fields, etc.) 11R06 PV-numbers and generalizations; other special algebraic numbers; Mahler measure 05C22 Signed and weighted graphs

### Citations:

Zbl 1140.15007; Zbl 0249.05136

SageMath
Full Text:

### References:

  Boyd, D.W., Small salem numbers, Duke math. J., 44, 315-328, (1977) · Zbl 0353.12003  Breusch, R., On the distribution of the roots of a polynomial with integral coefficients, Proc. amer. math. soc., 2, 939-941, (1951) · Zbl 0044.00904  Cameron, P.J.; Seidel, J.J.; Tsaranov, S.V., Signed graphs, root lattices, and Coxeter groups, J. algebra, 164, 1, 173-209, (1994) · Zbl 0802.05043  Cauchy, A.L., Sur lʼéquation à lʼaide de laquelle on détermine LES inégalités séculaires des mouvements des planètes, (), 174-195  Dobrowolski, E., A note on integer symmetric matrices and mahlerʼs measure, Canad. math. bull., 51, 57-59, (2008) · Zbl 1132.11312  Estes, D.R.; Guralnick, R.M., Minimal polynomials of integral symmetric matrices, Linear algebra appl., 192, 83-99, (1993) · Zbl 0791.15012  Fisk, S., A very short proof of cauchyʼs interlace theorem for eigenvalues of Hermitian matrices, Amer. math. monthly, 112, 118, (2005)  Horn, R.A.; Johnson, C.R., Matrix analysis, (1985), Cambridge University Press · Zbl 0576.15001  Kronecker, L., Zwei Sätze über gleichungen mit ganzzahligen coefficienten, J. reine angew. math., 53, 173-175, (1857)  Lehmer, D.H., Factorization of certain cyclotomic functions, Ann. of math. (2), 34, 461-479, (1933) · Zbl 0007.19904  McKee, J.F.; Smyth, C.J., Integer symmetric matrices having all their eigenvalues in the interval $$[- 2, 2]$$, J. algebra, 317, 260-290, (2007) · Zbl 1140.15007  McKee, J.F.; Smyth, C.J., Integer symmetric matrices of small spectral radius and small Mahler measure · Zbl 1243.15020  Mossinghoff, M., List of small salem numbers  Smyth, C.J., On the product of the conjugates outside the unit circle of an algebraic integer, Bull. London math. soc., 3, 169-175, (1971) · Zbl 0235.12003  G. Taylor, Cyclotomic matrices and graphs, PhD thesis, Edinburgh, 2010.  Zaslavsky, T., Signed graphs, Discrete appl. math., Discrete appl. math., 5, 2, 248-74, (1983), Erratum: · Zbl 0503.05060  Stein, W.A., Sage mathematics software (version 2.10.0), the sage development team, (2008)
This reference list is based on information provided by the publisher or from digital mathematics libraries. Its items are heuristically matched to zbMATH identifiers and may contain data conversion errors. It attempts to reflect the references listed in the original paper as accurately as possible without claiming the completeness or perfect precision of the matching.