Evènement pour le groupe GT Graphes et Applications

Date 2011-04-08  14:00-15:00
TitreNowhere-zero flows on signed eulerian graphs  
RésuméA signed graph is a graph in which every edge is labelled with a sign, + or -. In the talk we introduce flows on signed graphs as a natural analogue of flows on unsigned (that is, all-positive) graphs, based on the use of bidirected rather than directed edges. After giving a brief survey of known results about nowhere-zero flows on signed graphs we concentrate on flows on signed eulerian graphs. We show that every signed eulerian graph that has a nowhere-zero flow has a nowhere-zero 4-flow. This indicates a significant difference from the unsigned case where every eulerian graph is known to have a nowhere-zero 2-flow. We also characterise those signed eulerian graphs whose flow number equals 2, 3, and 4, respectively. This is a joint work with Edita Macajova.  
LieuSalle 178 
OrateurMartin Skoviera 

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