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en:group:seminars:20111005 [2011/09/15 13:26]
maatouk created
en:group:seminars:20111005 [2011/09/30 11:12]
maatouk
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-page+---- dataentry seminar ---- 
 +date_dt : 2011-10-05 
 +title : Coloring random graphs online without creating monochromatic subgraphs 
 +speaker : Dr. Reto Spöhel  
 +affiliation : Max Planck Institute for Informatics 
 +time : 16h15 
 +room : BC 229 
 +table : seminars 
 +=================== 
 +template:​datatemplates:​seminar 
 +----------------------- 
 + 
 + 
 +==== abstract ==== 
 +Consider the following generalized notion of graph coloring: a coloring of 
 +the vertices of a graph G is \emph{valid} w.r.t. some given graph F if 
 +there is no copy of F in G whose vertices all receive the same color. We 
 +study the problem of computing valid colorings of the binomial random 
 +graph G_{n,p} on n vertices with edge probability p=p(n) in the following 
 +online setting: the vertices of an initially hidden instance of G_{n,p} 
 +are revealed one by one (together with all edges leading to previously 
 +revealed vertices) and have to be colored immediately and irrevocably with 
 +one of r available colors. 
 +It is known that for any fixed graph F and any fixed integer r\geq 2 this 
 +problem has a threshold p_0(F,r,n) in the following sense: For any 
 +function p(n) = o(p_0) there is a strategy that a.a.s. (asymptotically 
 +almost surely, i.e., with probability tending to 1 as n tends to infinity) 
 +finds an r-coloring of G_{n,p} that is valid w.r.t. F online, and for any 
 +function p(n)=\omega(p_0) \emph{any} online strategy will a.a.s. fail to 
 +do so. 
 + 
 +We establish a general correspondence between this probabilistic problem 
 +and a deterministic two-player game in which the random process is 
 +replaced by an adversary that is subject to certain restrictions inherited 
 +from the random setting. This characterization allows us to compute, for 
 +any F and r, a value \gamma=\gamma(F,​r) such that the threshold of the 
 +probabilistic problem is given by p_0(F,​r,​n)=n^{-\gamma}. Our approach 
 +yields polynomial-time coloring algorithms that a.a.s. find valid 
 +colorings of G_{n,p} online in the entire regime below the respective 
 +thresholds, i.e., for any p(n) = o(n^{-\gamma}). 
 + 
 +Joint work with Torsten Mütze und Thomas Rast (both ETH Zurich); appeared 
 +at  SODA '11. 
 +