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Kijung Kim

Publications and source records attributed to Kijung Kim.

21 records · Page 2Linked to original sources
arXiv2012

On $p$-schemes of order $p^3$

Let $(X,S)$ be a $p$-scheme of order $p^3$ and $T$ the thin residue of $S$. Now we assume that $T$ has valency $p^2$. It is easy to see that one of the following holds: (i) $|T|=p^2$ and $T\simeq C_{p^2}$; (ii) $|T|=p^2$ and $T\simeq C_p\times C_p$; (iii) $|T|<p^2$. It is known that $(X,S)$ is Schurian if (i) holds. If (ii) holds, we will show that $(X,S)$ induces a partial linear space on $X/T$. Moreover, the character degrees of $(X,S)$ coincide with the sizes of the lines of the partial linear space. Under the assumption (iii) we will show a construction of non-Schurian $p$-schemes which are algebraically isomorphic to a Schurian $p$-scheme of order $p^3$.

math.CO↗
arXiv2012

On a conjecture of Brouwer involving the connectivity of strongly regular graphs

In this paper, we study a conjecture of Andries E. Brouwer from 1996 regarding the minimum number of vertices of a strongly regular graph whose removal disconnects the graph into non-singleton components. We show that strongly regular graphs constructed from copolar spaces and from the more general spaces called $Δ$-spaces are counterexamples to Brouwer's Conjecture. Using J.I. Hall's characterization of finite reduced copolar spaces, we find that the triangular graphs $T(m)$, the symplectic graphs $Sp(2r,q)$ over the field $\mathbb{F}_q$ (for any $q$ prime power), and the strongly regular graphs constructed from the hyperbolic quadrics $O^{+}(2r,2)$ and from the elliptic quadrics $O^{-}(2r,2)$ over the field $\mathbb{F}_2$, respectively, are counterexamples to Brouwer's Conjecture. For each of these graphs, we determine precisely the minimum number of vertices whose removal disconnects the graph into non-singleton components. While we are not aware of an analogue of Hall's characterization theorem for $Δ$-spaces, we show that complements of the point graphs of certain finite generalized quadrangles are point graphs of $Δ$-spaces and thus, yield other counterexamples to Brouwer's Conjecture. We prove that Brouwer's Conjecture is true for many families of strongly regular graphs including the conference graphs, the generalized quadrangles $GQ(q,q)$ graphs, the lattice graphs, the Latin square graphs, the strongly regular graphs with smallest eigenvalue -2 (except the triangular graphs) and the primitive strongly regular graphs with at most 30 vertices except for few cases. We leave as an open problem determining the best general lower bound for the minimum size of a disconnecting set of vertices of a strongly regular graph, whose removal disconnects the graph into non-singleton components.

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