Function sigma_coindex 

pub fn sigma_coindex(graph: &Graph) -> IgraphResult<u64>

Compute the sigma coindex.

\bar{σ}(G) = Σ_{u<v, (u,v)∉E} (d(u)-d(v))²

Uses: \bar{σ} = (n-1)·Σd² - 2m·Σd² / n ... — simpler to compute directly as total minus edge contribution.

Examples

use rust_igraph::{Graph, sigma_coindex};

// K_3: no non-adjacent pairs → 0
let g = Graph::from_edges(&[(0,1),(1,2),(0,2)], false, Some(3)).unwrap();
assert_eq!(sigma_coindex(&g).unwrap(), 0);

// Path 0-1-2: non-adj (0,2), (1-1)²=0
let p = Graph::from_edges(&[(0,1),(1,2)], false, Some(3)).unwrap();
assert_eq!(sigma_coindex(&p).unwrap(), 0);