2021-019

Which of the formulas below computes the average degree of an undirected network, where $N$ is the number of nodes in the network,  $k_i$ is the degree of node $i$, $N_k$ is the number of degree-$k$ nodes, and $A$ is the adjacency matrix:

1. $$\sum _{j=1}^N{A}_{j,i}$$
2. $${\displaystyle \frac{1}{2}}\sum _{i=1}^N{k}_i$$
3. $${\displaystyle \frac{1}{N}}\sum _{i=1}^N{k}_i$$
4. $${\displaystyle \frac{N_k}{N}}$$
5. None of the above

Original idea by Mauricio Schiezaro