18 research outputs found
Even Orientations and Pfaffian graphs
We give a characterization of Pfaffian graphs in terms of even orientations,
extending the characterization of near bipartite non--pfaffian graphs by
Fischer and Little \cite{FL}. Our graph theoretical characterization is
equivalent to the one proved by Little in \cite{L73} (cf. \cite{LR}) using
linear algebra arguments
On the Gibson Bounds over Finite Fields
2010 Mathematics Subject Classification: 15A15, 15A04.We investigate the Pólya problem on the sign conversion between the permanent and the determinant over finite fields. The main attention is given to the sufficient conditions which guarantee non-existence of sing-conversion. In addition we show that F3 is the only field with the property that any matrix with the entries from the field is convertible. As a result we obtain that over finite fields there are no analogs of the upper Gibson barrier for the conversion and establish the lower convertibility barrier.* The work on this project is partially supported by the grants MD-2502.2012.1 and RFBR 12-01-00140
Computing the Girth of a Planar Graph in Linear Time
The girth of a graph is the minimum weight of all simple cycles of the graph.
We study the problem of determining the girth of an n-node unweighted
undirected planar graph. The first non-trivial algorithm for the problem, given
by Djidjev, runs in O(n^{5/4} log n) time. Chalermsook, Fakcharoenphol, and
Nanongkai reduced the running time to O(n log^2 n). Weimann and Yuster further
reduced the running time to O(n log n). In this paper, we solve the problem in
O(n) time.Comment: 20 pages, 7 figures, accepted to SIAM Journal on Computin