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Published byBrice McGee Modified over 9 years ago
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Graph Pegging By Jason Counihan
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The Rules of Pegging We start with a graph, such as this graph representation a cube. –Graphs are made of vertices (dots) connected by edges; two vertices are “adjacent” if there is an edge connecting them.
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The Rules of Pegging Now we lay out some pegs on the graph. The layout is called a distribution of pegs.
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The Rules of Pegging We can move a peg by “jumping” it over another peg, and landing on an empty vertex. The peg it jumped over is then removed.
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The Rules of Pegging Notice that we can move a peg to any vertex we want. This means our distribution is solvable.
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The Goals of Pegging The pegging number of a graph G, written g(G), is the minimum number of pegs we need to guarantee a solvable distribution. g(G) = 5
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The Goals of Pegging The optimal pegging number of G, written g opt (G), is the minimum number of pegs needed for some solvable distribution. g opt (G) = 3
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Pegging on Paths One family of graphs that I have worked with is paths. These are just a string of vertices connected only to the vertices immediately before and after. P n represents a path of n vertices. On a path, k unpegged vertices in a row is called a k-gap.
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Pegging on Paths Lemma 1:On a path or cycle, no peg can be moved beyond the first hole of a gap without jumping a peg from the other side.
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Pegging on Paths Theorem:g(P n ) = n-1 for n > 3 Proof:To prove this, we break this into two inequalities. That is, we want to show 1)g(P n ) > n-2, and 2)g(P n ) n-1
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Pegging on Paths Step 1: g(P n ) > n-2 Proof:Observe the distribution of n-2 pegs shown above. By Lemma 1, the first vertex cannot be pegged, and thus the distribution is unsolvable and g(P n ) > n-2.
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Pegging on Paths Step 2: g(P n ) n-1 Proof:Observe that in a distribution of n-1 pegs on n vertices (where n > 3), only one vertex is not pegged. No matter where this unpegged vertex is, we can peg it.
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Pegging on Paths From the results of these two steps, we have n-2 < g(P n ) n-1 Thus, g(P n ) = n-1 for n > 3.
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Pegging on Paths Lemma 1:On a path or cycle, no peg can be moved beyond the first hole of a gap without jumping a peg from the other side. Lemma 2:A solvable distribution on a cycle or path cannot contain a 3-gap.
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Pegging on Paths Theorem:g opt (P 2k+r ) = k+r for k > 2 Proof: To prove this, we break this into two inequalities. That is, we want to show 1) g opt (P 2k+r ) k+r, and 2) g opt (P 2k+r ) k+r
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Pegging on Paths Step 1: g opt (P 2k+r ) k+r Proof:Observe the above distribution of pegs. Since it is solvable and contains k+r pegs, we know that the optimal pegging number is, at most, k+r.
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Pegging on Paths Step 2: g opt (P 2k+r ) k+r Proof:We will break this into two cases: r = 0 and r = 1. In each case, let us assume the theorem is false, and that k is the smallest integer for which the theorem fails.
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Pegging on Paths Step 2 (Case 1): Suppose g opt (P 2k ) < k. Also, g opt (P 2k ) g opt (P 2k-1 ) = g opt (P 2(k-1)+1 ) = (k-1)+1 = k Putting these inequalities together gives: k > g opt (P 2k ) g opt (P 2k-1 ) = k This yields a contradiction, and we must conclude that no such k can exist.
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Pegging on Paths Step 2 (Case 2):If the theorem is false for P k+1, then some solvable distribution of k pegs must exist. If no 2-gaps exist in a distribution of k pegs, then the distribution looks like above and is clearly not solvable. Also, by Lemma 2, no 3-gaps can exist. Thus we conclude that some 2-gap exists.
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Pegging on Paths Step 2 (Case 2):Since a 2-gap exists, some portion of the solvable distribution must appear as above. Now, two vertices and a peg can be removed and the distribution will remain solvable, contradicting k+1 being the smallest integer for which the theorem fails. We must again conclude that no such k can exist.
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Pegging on Paths From these two cases, we can conclude that no k can exist where g opt (P 2k+r ) < k+r, and thus g opt (P 2k+r ) k+r. Combining the results of the two steps, we see that k+r g opt (P 2k+r ) k+r, and thus, g opt (P 2k+r ) = k+r for k > 2
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Other Results g(C n ) = n-2 g opt (C 2k+r ) = k+r g(K n ) = g opt (K n ) = 2 If G is bipartite, then g(G) is greater than the cardinality of its larger subset of vertices. If d is the diameter of a graph G, then g(G) d-1.
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Where to go from here Examine other families of graphs (such as trees) Examine Graham’s Conjecture (I have found a few cases like C 5 C 5 for which the conjecture fails, but are there more?) Examine a weight function, for which a solvable distribution exists when for each vertex, the total weight sums to one (I have found a function that works for simple graphs involving the golden ratio)
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Thanks to: Dr. Wyels for the project idea and assistance throughout Dr. Fogel for keeping the project on schedule
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