In graph theory, the shift graph Gn,k for n , k ∈ N , n > 2 k > 0 {\displaystyle n,k\in \mathbb {N} ,\ n>2k>0} is the graph whose vertices correspond to the ordered k {\displaystyle k} -tuples a = ( a 1 , a 2 , … , a k ) {\displaystyle a=(a_{1},a_{2},\dotsc ,a_{k})} with 1 ≤ a 1 < a 2 < ⋯ < a k ≤ n {\displaystyle 1\leq a_{1}<a_{2}<\cdots <a_{k}\leq n} and where two vertices a , b {\displaystyle a,b} are adjacent if and only if a i = b i + 1 {\displaystyle a_{i}=b_{i+1}} or a i + 1 = b i {\displaystyle a_{i+1}=b_{i}} for all 1 ≤ i ≤ k − 1 {\displaystyle 1\leq i\leq k-1} . Shift graphs are triangle-free, and for fixed k {\displaystyle k} their chromatic number tend to infinity with n {\displaystyle n} . It is natural to enhance the shift graph G n , k {\displaystyle G_{n,k}} with the orientation a → b {\displaystyle a\to b} if a i + 1 = b i {\displaystyle a_{i+1}=b_{i}} for all 1 ≤ i ≤ k − 1 {\displaystyle 1\leq i\leq k-1} . Let G → n , k {\displaystyle {\overrightarrow {G}}_{n,k}} be the resulting directed shift graph. Note that G → n , 2 {\displaystyle {\overrightarrow {G}}_{n,2}} is the directed line graph of the transitive tournament corresponding to the identity permutation. Moreover, G → n , k + 1 {\displaystyle {\overrightarrow {G}}_{n,k+1}} is the directed line graph of G → n , k {\displaystyle {\overrightarrow {G}}_{n,k}} for all k ≥ 2 {\displaystyle k\geq 2} .
Further facts about shift graphs Odd cycles of G n , k {\displaystyle G_{n,k}} have length at least 2 k + 1 {\displaystyle 2k+1} , in particular G n , 2 {\displaystyle G_{n,2}} is triangle free. For fixed k ≥ 2 {\displaystyle k\geq 2} the asymptotic behaviour of the chromatic number of G n , k {\displaystyle G_{n,k}} is given by χ ( G n , k ) = ( 1 + o ( 1 ) ) log log ⋯ log n {\displaystyle \chi (G_{n,k})=(1+o(1))\log \log \cdots \log n} where the logarithm function is iterated k − 1 {\displaystyle {\displaystyle k-1}} times. Further connections to the chromatic theory of graphs and digraphs have been established in. Shift graphs, in particular G n , 3 {\displaystyle G_{n,3}} also play a central role in the context of order dimension of interval orders.
Representation of shift graphs
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