CPTS553- Graph Theory: Assignment 7 Solved

Questions with a (⋆) are each worth 1 bonus point for 453 students.

Some questions about drawing 𝑄𝑘 graphs on surfaces. A. Show that 𝑄3 can be drawn on the plane.Use the fact that 𝑄4 is bipartite and a total edge count argument to show that 𝑄4 cannot be drawn on the plane.
Draw 𝑄4 on the torus (use the 𝑎𝑏𝑎−1𝑏−1 square representation drawn below). HINT:  𝑄4 is isomorphic to 𝐶4 × 𝐶4.
Show that 𝑔(𝑄5) ≥ 5. Recall that for 𝑄5 we have 𝑛 = 32 and 𝑚 =
80. As a first step, use a total edge count argument to show that
𝑟 ≤ 40.  Feed this information into Euler’s formula  𝑛 − 𝑚 + 𝑟 = 2 − 2 𝑔(𝑄5).

(⋆) Generalize the strategy in part D to obtain a “meaningful” lower bound for 𝑔(𝑄𝑘).  Here, recall that 𝑛 = 2𝑘 and 𝑚 = 𝑘2𝑘−1.     
The Petersen graph 𝑃 is depicted:
Use a total edge count argument to show that 𝑃 is non-planar. You may use the fact that 𝑃 has no triangles or 4-cycles as subgraphs. B. Draw 𝑃 on a torus without the edges crossing.          
Draw 𝐾4,4 on a torus without the edges crossing. Suggestion:  Start with your two partite sets (every edge joins a red vertex to a blue vertex) arranged as shown: