91Ó°ÊÓ

a) How many subgraphs \(H=(V, E)\) of \(K_{6}\) satisfy \(|V|=3 ?\) (If two subgraphs are isomorphic but have different vertex sets, consider them distinct.) b) How many subgraphs \(H=(V, E)\) of \(K_{6}\) satisfy \(|V|=4 ?\) c) How many subgraphs does \(K_{6}\) have?

Prove that in any directed graph or multigraph \(G=(V, E), \sum_{\text {uev }} o d(v)=\sum_{\text {revid }}(v)\).