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Degree diameter problem : ウィキペディア英語版
Degree diameter problem
In graph theory, the degree diameter problem is the problem of finding the largest possible graph ''G'' (in terms of the size of its vertex set ''V'') of diameter ''k'' such that the largest degree of any of the vertices in ''G'' is at most ''d''. The size of ''G'' is bounded above by the Moore bound; for 1 < ''k'' and 2 < ''d'' only the Petersen graph, the Hoffman-Singleton graph, and maybe a graph of diameter ''k'' = 2 and degree ''d'' = 57 attain the Moore bound. In general the largest degree-diameter graphs are much smaller in size than the Moore bound.
== Formula ==
Let n_ be the maximum possible of vertices for a graph with degree at most ''d'' and diameter ''k'' then n_\leq M_, where M_ is the Moore bound:
:M_=\begin1+d\frac&\textd>2\\2k+1&\textd=2\end
This bound is attained for very few graphs, thus the study moves to how close there exist graphs to the Moore bound.
For asymptotic behaviour note that M_=d^k+O(d^).
Define the parameter \mu_k=\liminf_\frac. It is conjectured that \mu_k=1 for all ''k''. It is known that \mu_1=\mu_2=\mu_3=\mu_5=1 and that \mu_4\geq 1/4. For the general case it is known that \mu_k\geq 1.6^k. Thus, although is conjectured that \mu_k=1 is still open if it is actually exponential.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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