Function reduced_first_zagreb 

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

Compute the reduced first Zagreb index.

M₁_red(G) = Σ_v (d(v)-1)²

For isolated vertices (degree 0), the term is (-1)² = 1, but conventionally these are treated as having reduced degree 0, so we skip vertices with degree 0.

Examples

use rust_igraph::{Graph, reduced_first_zagreb};

// K_3: d=(2,2,2), each (2-1)²=1 → 3
let g = Graph::from_edges(&[(0,1),(1,2),(0,2)], false, Some(3)).unwrap();
assert_eq!(reduced_first_zagreb(&g).unwrap(), 3);

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