1. Undirected, unlooped, multi-edged: multigraph
b) directed, looped, multi-edged: directed pseudo-multigraph

2. ac bd
b) cd cd dd ee ab bc

3. {{paper}}

4. vertices: 6
   edges: 6
   degree: a: 2 b: 4 c: 1 f: 3 e: 2 d: 0
   isolated: d
   pendant: c
b) vertices: 5
   edges: 14
   degree: a: 6 b: 6 c: 6 d: 5 e: 3
   isolated: -
   pendant: -

5. vertices: 4
   in-a : 2
   out-a: 2
   in-b:  3
   out-b: 4
   in-c:  2
   out-c: 1
   in-d:  1
   out-d: 1

6. {ac} {bde}
b) Not bipartite: 3-loop bcf would require 3 sets

7. {{ paper }}

8. a -> abcd
   b ->    d
   c -> ab  
   d ->  bcd

9.    | a b c d
    --+--------
    a | 1 1 1 1
    b | 0 0 0 1
    c | 1 1 0 0
    d | 0 1 1 1

10. {{ paper }}

11. v1 -> u1
    v2 -> u4
	v3 -> u2
	v4 -> u5
	v5 -> u3

12. v1 -> u4
	v2 -> u3
	v3 -> u1
	v4 -> u2

13. PSCL
 a) YNN4
 b) N---
 c) N---
 d) YYY5