eng
University of Isfahan
Transactions on Combinatorics
2251-8657
2251-8665
2018-09-01
7
3
1
18
10.22108/toc.2017.104919.1507
22285
The annihilator graph of a 0-distributive lattice
Saeid Bagheri
bagheri_saeid@yahoo.com
1
Mahtab Koohi Kerahroodi
mahtabkh3@gmail.com
2
Department of Mathematics, Faculty of Mathematical Sciences, Malayer University, Malayer, Iran
Department of Mathematics, Faculty of Mathematical Sciences, Malayer University, Malayer, Iran.
In this article, for a lattice $mathcal L$, we define and investigate the annihilator graph $mathfrak {ag} (mathcal L)$ of $mathcal L$ which contains the zero-divisor graph of $mathcal L$ as a subgraph. Also, for a 0-distributive lattice $mathcal L$, we study some properties of this graph such as regularity, connectedness, the diameter, the girth and its domination number. Moreover, for a distributive lattice $mathcal L$ with $Z(mathcal L)neqlbrace 0rbrace$, we show that $mathfrak {ag} (mathcal L) = Gamma(mathcal L)$ if and only if $mathcal L$ has exactly two minimal prime ideals. Among other things, we consider the annihilator graph $mathfrak {ag} (mathcal L)$ of the lattice $mathcal L=(mathcal D(n),|)$ containing all positive divisors of a non-prime natural number $n$ and we compute some invariants such as the domination number, the clique number and the chromatic number of this graph. Also, for this lattice we investigate some special cases in which $mathfrak {ag} (mathcal D(n))$ or $Gamma(mathcal D(n))$ are planar, Eulerian or Hamiltonian.
http://toc.ui.ac.ir/article_22285_719ab505eba5ec2cd4bf741957e5ce29.pdf
Distributive lattice
Annihilator graph
Zero-divisor graph
eng
University of Isfahan
Transactions on Combinatorics
2251-8657
2251-8665
2018-09-01
7
3
19
27
10.22108/toc.2018.105873.1513
22346
A spectral excess theorem for digraphs with normal Laplacian matrices
Fateme Shafiei
fatemeh.shafiei66@gmail.com
1
Isfahan University of Technology
The spectral excess theorem, due to Fiol and Garriga in 1997, is an important result, because it gives a good characterization of distance-regularity in graphs. Up to now, some authors have given some variations of this theorem. Motivated by this, we give the corresponding result by using the Laplacian spectrum for digraphs. We also illustrate this Laplacian spectral excess theorem for digraphs with few Laplacian eigenvalues and we show that any strongly connected and regular digraph that has normal Laplacian matrix with three distinct eigenvalues, is distance-regular. Hence such a digraph is strongly regular with girth $g=2$ or $g=3$.
http://toc.ui.ac.ir/article_22346_7909f36dcf76d9cf3fc6a7dff2c384b9.pdf
A Laplacian spectral excess theorem
Distance-regular digraphs
Strongly regular digraphs