Function second_inverse_nirmala 

pub fn second_inverse_nirmala(graph: &Graph) -> IgraphResult<f64>

Compute the second inverse Nirmala index.

IN₂(G) = Σ_{(u,v)∈E} 1/√(d(u) · d(v))

This equals the Randić connectivity index R(G). Self-loops and edges where either endpoint has degree 0 are skipped.

Examples

use rust_igraph::{Graph, second_inverse_nirmala};

// K_3: 3 edges × 1/√(2·2) = 3×0.5 = 1.5
let g = Graph::from_edges(&[(0,1),(1,2),(0,2)], false, Some(3)).unwrap();
assert!((second_inverse_nirmala(&g).unwrap() - 1.5).abs() < 1e-10);