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De Bruijn Graph





DEFINITION

In Graph Theory , an n-dimensional de Bruijn graph of m symbols is a Directed Graph with m^n vertices and the edges defined as below.

The vertices consist of all possible n way combinations of the m symbols. If we have the symbols s_1,\cdots,s_m then the set of vertices is:

: V=\{(s_1,\cdots,s_1,s_1),(s_1,\cdots,s_1,s_2),\cdots,(s_m,\cdots,s_m,s_m)\}

If one of the vertices can be expressed by shifting all symbols by one place to the left and adding a new symbol at the end of another vertex, then the latter has a directed edge to the former vertex. Thus the set of (directed) edges is:

: E=\{((v_1,v_2,\cdots,v_n),(w_1,w_2,\cdots,w_n)) : v_2=w_1,v_3=w_2,\cdots,v_n=w_{n-1}\}


Properties

  • If n=1 then the condition for any two vertices forming an edge holds vacuously, and hence all the vertices are connected forming a total of m^2 edges.

  • Each vertex has exactly m incoming and m outgoing edges.



EXAMPLES

For simplified notation, let our symbols be the Whole Numbers 1,2,3,etc. Then, following are some examples of de Bruijn graphs.


de Bruijn (2,1) : 2 symbols and 1 dimension



de Bruijn (2,2) : 2 symbols and 2 dimensions



de Bruijn (3,2) : 3 symbols and 2 dimensions