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In the graph shown in the following figure, the number of trees (P) and the number of cut sets (Q) are :

Four nodes at the corners of a rectangle; branch 1 joins the top two, branch 4 the bottom two, and branches 2 and 3 are crossing diagonals with no node where they cross

  • A.P = 2, Q = 2
  • B.P = 2, Q = 6
  • C.P = 4, Q = 6
  • D.P = 4, Q = 10

Answer: C

The graph has P = 4 trees and Q = 6 cut sets.

The two diagonals cross without a node, so the graph has 4 nodes and 4 branches. Tracing it: top left to top right (1), top right to bottom left (3), bottom left to bottom right (4), and bottom right back to top left (2). It is a single closed loop of four branches.

A tree joins all 4 nodes with 4 - 1 = 3 branches and no loop. Removing any one of the four branches leaves such a tree, so there are 4 trees.

A cut set is a minimal set of branches whose removal splits the graph into two parts. In a single loop, removing one branch leaves it connected, but removing any two branches separates it, so every pair is a cut set: \(\binom{4}{2} = 6\).

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