D1 June 2011 Q2
2.

(a) Define the terms
(i) tree,
(ii) minimum spanning tree.
(3)(b) Use Kruskal’s algorithm to find a minimum spanning tree for the network shown in Figure 1. You should list the arcs in the order in which you consider them. In each case, state whether you are adding the arc to your minimum spanning tree. (3)
(c) Draw your minimum spanning tree using the vertices given in Diagram 1 in the answer book. (1)
(d) State whether your minimum spanning tree is unique. Justify your answer. (1)
| Scheme | Marks |
|---|---|
| (i) A tree is a connected graph with no cycles/circuit | B1 |
| (ii) A minimum spanning tree is a tree that contains all vertices and | B1 |
| the total length of its arcs (weight of tree) is as small as possible. | B1 |
| (3) |
Notes
(a)1B1 Connected + no cycles
2B1 Contains all vertices
3B1 Total length of arcs used minimised or minimum weight. (Not shortest/smallest etc.)
| Scheme | Marks |
|---|---|
| AB, DE, BC; \(\left\{\begin{matrix}\text{reject AC}\\ \text{BD}\end{matrix}\right\}\) reject BE, reject CE, use either EF or CF | M1; A1 A1 |
| (3) |
Notes
(b)M1 First four arcs selected correctly in correct order.
1A1 Arcs selected correctly at correct time
2A1 Rejections correct and at correct time
| Scheme | Marks |
|---|---|
![]() | B1 |
| (1) |
Notes
(c)B1 CAO
| Scheme | Marks |
|---|---|
| No, there are two solutions since either EF or CF should be used. | B1 |
| (1) | |
| (8 marks) |
Notes
(d)B1 CAO - mark explanation must specify two arcs of 18 or two 18’s or ref to EF and CF
