Function reduced_sombor_index 

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

Compute the reduced Sombor index.

SO_red(G) = Σ_{(u,v)∈E} √((d(u)-1)² + (d(v)-1)²)

Uses reduced degrees d(v)-1. Self-loops are skipped. Pendant edges (where one endpoint has degree 1) contribute √(0² + (d-1)²) = d-1.

Examples

use rust_igraph::{Graph, reduced_sombor_index};

// K_3: each edge √((2-1)²+(2-1)²) = √2, 3 edges → 3√2
let g = Graph::from_edges(&[(0,1),(1,2),(0,2)], false, Some(3)).unwrap();
assert!((reduced_sombor_index(&g).unwrap() - 3.0 * std::f64::consts::SQRT_2).abs() < 1e-10);