Moore graphs of are -valent and order is
Lemma: a key feature of Moore graphs with diameter 2 is that any nodes X and that are not adjacent have exactly one shared neighbour Y. Proof of lemma: at least one Y because distance XY is not 1 so must be 2, at most one because XYZY would be a 4-cycle.
Lemma: every two adjacent nodes B and C lie together on some 5-cycle. Proof of lemma: every node (other than B) is at distance 1 or 2 from B, and every node (other than C) at distance 1 or 2 from C. There are no nodes X at distance 1 from both (BXC would be a 3-cycle). Suppose the graph only has nodes at distance 1 from B and 2 from C (call them A), and nodes at distance 1 from C and 2 from B (call them D). Now no cycle can exist (the only edges are AB, BC, CD; any edge of type AA, AC, , BD, DD would create a 3-or 4-cycle). But Moore graphs have cycles by definition. So there must be at least one node E at distance 2 from both B and C. Let A be the joint neighbour of B and E, and D that of C and E (note A D, otherwise BAC would be a 3-cycle). Now CDEAB is a 5-cycle with edge BC.
Lemma: two nodes O and Q at distance 2 have the same valency. Proof of lemma: let P be the unique joint neighbour. Let O have other neighbours N and let Q have other neighbours R.
No N can be adjacent to P (3-cycle PON) so the unique joint neighbours of any N and Q must be among the R. Different N and N cannot use the same R (4-cycle ONRN) so we have . By the same argument (swapping O and Q, N and R) also so we have , both nodes have valency .
Lemma: two adjacent nodes O and S have the same valency. Proof of lemma: let OPQRS be the 5-cycle through SO. Calling the valency of Q again , both O and S have that same valency by the previous lemma.
Proof of theorem: the graph is connected. Travel from any node to any other via adjacent ones, the valency stays the same by the last lemma (let’s keep calling it ).
Now let N and R be any two adjacent nodes. N has other neighbours O and R has other neighbours Q. Call the joint neighbour of O and Q now P, these nodes are all distinct (4-cycles of type NOPO and/or RQPQ otherwise) and none of them coincide with N, R, the Os or Qs (3- or 4-cycles otherwise). On the other hand, there are no further nodes (distance from N or R otherwise). Tally: , for valency .
|Title||Moore graphs of are -valent and order is|
|Date of creation||2013-03-22 15:11:30|
|Last modified on||2013-03-22 15:11:30|
|Last modified by||marijke (8873)|