diff --git a/releasenotes/notes/core-number-dense-state-60fb188fd560af1d.yaml b/releasenotes/notes/core-number-dense-state-60fb188fd560af1d.yaml
new file mode 100644
index 0000000000..a8cc2f6c07
--- /dev/null
+++ b/releasenotes/notes/core-number-dense-state-60fb188fd560af1d.yaml
@@ -0,0 +1,8 @@
+other:
+  - |
+    Reduced hashing and allocation during :func:`~rustworkx.core_number` and
+    ``rustworkx_core::connectivity::core_number`` by using dense working storage
+    for core decomposition. Adjacency uses a contiguous buffer and per-call
+    markers to remove duplicate neighbors without sorting. Public signatures,
+    result ordering, and node IDs are unchanged, including when nodes have
+    been removed from a graph.
diff --git a/rustworkx-core/src/connectivity/core_number.rs b/rustworkx-core/src/connectivity/core_number.rs
index da336f54fa..70f598043f 100644
--- a/rustworkx-core/src/connectivity/core_number.rs
+++ b/rustworkx-core/src/connectivity/core_number.rs
@@ -12,12 +12,10 @@
 
 use std::hash::Hash;
 
-use hashbrown::{HashMap, HashSet};
 use petgraph::Direction::{Incoming, Outgoing};
 use petgraph::visit::{GraphBase, IntoNeighborsDirected, IntoNodeIdentifiers, NodeCount};
-use rayon::prelude::*;
 
-use crate::dictmap::*;
+use crate::dictmap::{DictMap, InitWithHasher};
 
 /// Return the core number for each node in the graph.
 ///
@@ -55,56 +53,78 @@ where
         return DictMap::new();
     }
 
-    let mut cores: DictMap<G::NodeId, usize> = DictMap::with_capacity(node_num);
-    let mut node_vec: Vec<G::NodeId> = graph.node_identifiers().collect();
-    let mut degree_map: HashMap<G::NodeId, usize> = HashMap::with_capacity(node_num);
-    let mut nbrs: HashMap<G::NodeId, HashSet<G::NodeId>> = HashMap::with_capacity(node_num);
-    let mut node_pos: HashMap<G::NodeId, usize> = HashMap::with_capacity(node_num);
-
-    for k in node_vec.iter() {
-        let k_nbrs: HashSet<G::NodeId> = graph
-            .neighbors_directed(*k, Incoming)
-            .chain(graph.neighbors_directed(*k, Outgoing))
-            .collect();
-        let k_deg = k_nbrs.len();
-
-        nbrs.insert(*k, k_nbrs);
-        cores.insert(*k, k_deg);
-        degree_map.insert(*k, k_deg);
-    }
-    node_vec.par_sort_by_key(|k| degree_map.get(k));
-
-    let mut bin_boundaries: Vec<usize> =
-        Vec::with_capacity(degree_map[&node_vec[node_num - 1]] + 1);
-    bin_boundaries.push(0);
-    let mut curr_degree = 0;
-    for (i, v) in node_vec.iter().enumerate() {
-        node_pos.insert(*v, i);
-        let v_degree = degree_map[v];
-        if v_degree > curr_degree {
-            for _ in 0..v_degree - curr_degree {
-                bin_boundaries.push(i);
+    // DictMap provides compact working indices without requiring
+    // NodeIndexable or allocating up to the largest node ID. Keep its insertion
+    // order unchanged so the returned map still follows node_identifiers().
+    let mut cores: DictMap<G::NodeId, usize> =
+        graph.node_identifiers().map(|node| (node, 0)).collect();
+    // Store adjacency in one buffer. Each row's dense index is a fresh marker,
+    // so incoming/outgoing duplicates are removed without sorting or hashing
+    // another set. The sentinel cannot equal a live row index, including zero.
+    let mut neighbors = Vec::new();
+    let mut offsets = Vec::with_capacity(node_num + 1);
+    let mut seen = vec![usize::MAX; node_num];
+    offsets.push(0);
+    for (row, &node) in cores.keys().enumerate() {
+        for neighbor in graph
+            .neighbors_directed(node, Incoming)
+            .chain(graph.neighbors_directed(node, Outgoing))
+        {
+            let index = cores.get_index_of(&neighbor).unwrap();
+            // Match the existing neighbor set, including reciprocal edges.
+            if seen[index] != row {
+                seen[index] = row;
+                neighbors.push(index);
             }
-            curr_degree = v_degree;
         }
+        offsets.push(neighbors.len());
+    }
+    drop(seen);
+    let mut degree: Vec<_> = offsets.windows(2).map(|row| row[1] - row[0]).collect();
+    let mut bins = vec![0; degree.iter().copied().max().unwrap() + 1];
+    for &value in &degree {
+        bins[value] += 1;
+    }
+    let mut start = 0;
+    for count in &mut bins {
+        let next = start + *count;
+        *count = start;
+        start = next;
     }
 
-    for v_ind in 0..node_vec.len() {
-        let v = node_vec[v_ind];
-        let v_nbrs = nbrs[&v].clone();
-        for u in v_nbrs {
-            if cores[&u] > cores[&v] {
-                nbrs.get_mut(&u).unwrap().remove(&v);
-                let pos = node_pos[&u];
-                let bin_start = bin_boundaries[cores[&u]];
-                *node_pos.get_mut(&u).unwrap() = bin_start;
-                *node_pos.get_mut(&node_vec[bin_start]).unwrap() = pos;
-                node_vec.swap(bin_start, pos);
-                bin_boundaries[cores[&u]] += 1;
-                *cores.get_mut(&u).unwrap() -= 1;
+    // Counting sort puts every vertex in its degree bucket. Placement advances
+    // bins to bucket ends; shifting them restores starts without a second buffer.
+    let mut positions = vec![0; node_num];
+    let mut vertices = vec![0; node_num];
+    for (node, &value) in degree.iter().enumerate() {
+        positions[node] = bins[value];
+        vertices[bins[value]] = node;
+        bins[value] += 1;
+    }
+    for value in (1..bins.len()).rev() {
+        bins[value] = bins[value - 1];
+    }
+    bins[0] = 0;
+
+    for order in 0..node_num {
+        let node = vertices[order];
+        for &neighbor in &neighbors[offsets[node]..offsets[node + 1]] {
+            if degree[neighbor] > degree[node] {
+                let neighbor_degree = degree[neighbor];
+                let position = positions[neighbor];
+                let bucket_start = bins[neighbor_degree];
+                let bucket_node = vertices[bucket_start];
+                vertices.swap(position, bucket_start);
+                positions[neighbor] = bucket_start;
+                positions[bucket_node] = position;
+                bins[neighbor_degree] += 1;
+                degree[neighbor] -= 1;
             }
         }
     }
+    for (value, core) in cores.values_mut().zip(degree) {
+        *value = core;
+    }
     cores
 }
 
@@ -113,6 +133,199 @@ mod tests {
     use crate::connectivity::core_number;
     use petgraph::prelude::*;
 
+    // Exercise IDs that cannot be used as dense vector indices.
+    #[test]
+    fn test_graph_map_node_ids_and_order() {
+        let mut graph = DiGraphMap::<&str, ()>::new();
+        let nodes = ["isolated", "tail", "z", "a", "middle"];
+        for node in nodes {
+            graph.add_node(node);
+        }
+        for (a, b) in [
+            ("z", "a"),
+            ("a", "z"),
+            ("a", "middle"),
+            ("middle", "z"),
+            ("tail", "middle"),
+        ] {
+            graph.add_edge(a, b, ());
+        }
+        let result = core_number(&graph);
+        assert_eq!(result.keys().copied().collect::<Vec<_>>(), nodes);
+        assert_eq!(
+            result.values().copied().collect::<Vec<_>>(),
+            [0, 1, 2, 2, 2]
+        );
+    }
+
+    #[test]
+    fn test_deleted_and_reused_node_indices() {
+        fn check<Ty: petgraph::EdgeType>() {
+            for stride in [2, 1000] {
+                let mut graph = StableGraph::<(), (), Ty>::default();
+                for _ in 0..7 * stride {
+                    graph.add_node(());
+                }
+                for i in 0..7 * stride {
+                    if i % stride != 0 {
+                        graph.remove_node(NodeIndex::new(i));
+                    }
+                }
+                graph.remove_node(NodeIndex::new(stride));
+                assert_eq!(graph.add_node(()), NodeIndex::new(stride));
+                for (a, b) in [(0, 1), (1, 2), (2, 0), (2, 3), (4, 5)] {
+                    graph.add_edge(NodeIndex::new(a * stride), NodeIndex::new(b * stride), ());
+                }
+                let result = core_number(&graph);
+                assert_eq!(
+                    result.keys().map(|node| node.index()).collect::<Vec<_>>(),
+                    (0..7).map(|i| i * stride).collect::<Vec<_>>()
+                );
+                assert_eq!(
+                    result.values().copied().collect::<Vec<_>>(),
+                    [2, 2, 2, 1, 1, 1, 0]
+                );
+            }
+        }
+        check::<Directed>();
+        check::<Undirected>();
+    }
+
+    #[test]
+    fn test_reciprocal_edges_count_one_neighbor() {
+        let graph = DiGraph::<(), ()>::from_edges([(0, 1), (1, 0)]);
+        let result = core_number(&graph);
+        assert_eq!(result.values().copied().collect::<Vec<_>>(), [1, 1]);
+    }
+
+    // Independent oracle: a node's core number is the largest minimum degree
+    // among all vertex subsets containing it. No degree buckets or peeling.
+    fn subset_core_numbers(adjacency: &[Vec<bool>]) -> Vec<usize> {
+        let n = adjacency.len();
+        let mut result = vec![0; n];
+        for subset in 1..1_usize << n {
+            let degree = (0..n)
+                .filter(|&a| subset & (1 << a) != 0)
+                .map(|a| {
+                    (0..n)
+                        .filter(|&b| subset & (1 << b) != 0 && (adjacency[a][b] || adjacency[b][a]))
+                        .count()
+                })
+                .min()
+                .unwrap();
+            for (node, core) in result.iter_mut().enumerate() {
+                if subset & (1 << node) != 0 {
+                    *core = (*core).max(degree);
+                }
+            }
+        }
+        result
+    }
+
+    #[test]
+    fn test_all_four_node_directed_graphs() {
+        let edges: Vec<_> = (0..4)
+            .flat_map(|a| (0..4).filter(move |&b| a != b).map(move |b| (a, b)))
+            .collect();
+        for mask in 0..1_usize << edges.len() {
+            let mut graph = DiGraph::<(), ()>::new();
+            let mut adjacency = vec![vec![false; 4]; 4];
+            for _ in 0..4 {
+                graph.add_node(());
+            }
+            for (bit, &(a, b)) in edges.iter().enumerate() {
+                if mask & (1 << bit) != 0 {
+                    graph.add_edge(NodeIndex::new(a), NodeIndex::new(b), ());
+                    adjacency[a][b] = true;
+                }
+            }
+            let expected = subset_core_numbers(&adjacency);
+            let actual = core_number(&graph);
+            assert_eq!(
+                actual.values().copied().collect::<Vec<_>>(),
+                expected,
+                "edge mask {mask}"
+            );
+        }
+    }
+
+    // Reuse the same graph through mixed core levels, sparse rows and
+    // empty adjacency. Working state must belong to one call, not a previous graph.
+    #[test]
+    fn test_irregular_graphs_and_repeated_calls() {
+        fn check<Ty: petgraph::EdgeType>() {
+            let mixed = [
+                (0, 1),
+                (0, 2),
+                (0, 3),
+                (1, 2),
+                (1, 3),
+                (2, 3),
+                (3, 4),
+                (4, 5),
+                (5, 3),
+                (5, 6),
+            ];
+            let star = [(0, 1), (0, 2), (0, 3), (0, 4), (0, 5), (0, 6)];
+            for stride in [1, 3] {
+                let mut graph = StableGraph::<(), (), Ty>::default();
+                for _ in 0..8 * stride {
+                    graph.add_node(());
+                }
+                for node in 0..8 * stride {
+                    if node % stride != 0 {
+                        graph.remove_node(NodeIndex::new(node));
+                    }
+                }
+                for edges in [&[][..], &mixed[..], &star[..], &mixed[..], &[][..]] {
+                    graph.clear_edges();
+                    let mut adjacency = vec![vec![false; 8]; 8];
+                    for &(source, target) in edges {
+                        graph.add_edge(
+                            NodeIndex::new(source * stride),
+                            NodeIndex::new(target * stride),
+                            (),
+                        );
+                        adjacency[source][target] = true;
+                        if Ty::is_directed() && (source + target) % 2 == 0 {
+                            graph.add_edge(
+                                NodeIndex::new(target * stride),
+                                NodeIndex::new(source * stride),
+                                (),
+                            );
+                        }
+                    }
+                    let expected = subset_core_numbers(&adjacency);
+                    for _ in 0..2 {
+                        let actual = core_number(&graph);
+                        assert_eq!(
+                            actual.keys().map(|node| node.index()).collect::<Vec<_>>(),
+                            (0..8).map(|node| node * stride).collect::<Vec<_>>()
+                        );
+                        assert_eq!(actual.values().copied().collect::<Vec<_>>(), expected);
+                    }
+                }
+            }
+        }
+        check::<Directed>();
+        check::<Undirected>();
+    }
+
+    #[test]
+    fn test_neighbor_set_compatibility() {
+        // Preserve the existing set-based behavior for unsupported loops and
+        // parallel edges. In particular, a self-neighbor in row zero occurs once.
+        let mut graph = DiGraph::<(), ()>::new();
+        for _ in 0..4 {
+            graph.add_node(());
+        }
+        graph.extend_with_edges([(0, 0), (0, 0), (0, 1), (0, 1), (1, 0), (2, 2)]);
+        for _ in 0..3 {
+            let actual = core_number(&graph);
+            assert_eq!(actual.values().copied().collect::<Vec<_>>(), [1, 1, 1, 0]);
+        }
+    }
+
     #[test]
     fn test_directed_empty() {
         let graph = DiGraph::<i32, i32>::new();
diff --git a/tests/digraph/test_core_number.py b/tests/digraph/test_core_number.py
index 1644b39cca..3f4bb21058 100644
--- a/tests/digraph/test_core_number.py
+++ b/tests/digraph/test_core_number.py
@@ -94,3 +94,68 @@ def test_directed_paper_example(self):
         res = rustworkx.core_number(digraph)
         self.assertIsInstance(res, dict)
         self.assertEqual(res, self.example_core)
+
+    # Compact working indices must not replace public node IDs.
+    def test_removed_and_reused_indices(self):
+        graph = rustworkx.PyDiGraph()
+        graph.add_nodes_from(range(10))
+        graph.remove_nodes_from([1, 3, 5, 7, 9])
+        self.assertEqual(graph.add_node("reused"), 9)
+        self.assertEqual(graph.add_node("isolated"), 7)
+        graph.add_edges_from_no_data([(0, 2), (2, 4), (4, 0), (4, 6), (8, 9)])
+        expected = {0: 2, 2: 2, 4: 2, 6: 1, 7: 0, 8: 1, 9: 1}
+        for function in (rustworkx.core_number, rustworkx.digraph_core_number):
+            with self.subTest(function=function.__name__):
+                result = function(graph)
+                self.assertEqual(result, expected)
+                self.assertEqual(list(result), list(expected))
+
+    def test_reciprocal_edges_count_one_neighbor(self):
+        graph = rustworkx.PyDiGraph()
+        graph.extend_from_edge_list([(0, 1), (1, 0)])
+        for function in (rustworkx.core_number, rustworkx.digraph_core_number):
+            with self.subTest(function=function.__name__):
+                self.assertEqual(function(graph), {0: 1, 1: 1})
+
+    # Repeated calls include reciprocal arcs and noncompact public IDs.
+    def test_repeated_calls_after_edge_changes(self):
+        graph = rustworkx.PyDiGraph()
+        graph.add_nodes_from(range(16))
+        graph.remove_nodes_from(range(1, 16, 2))
+        mixed = [
+            (0, 2),
+            (2, 0),
+            (0, 4),
+            (0, 6),
+            (2, 4),
+            (2, 6),
+            (4, 6),
+            (6, 8),
+            (8, 10),
+            (10, 6),
+            (10, 12),
+        ]
+        star = [(0, node) for node in range(2, 14, 2)]
+        for edges, values in [
+            ([], [0] * 8),
+            (mixed, [3, 3, 3, 3, 2, 2, 1, 0]),
+            (star, [1] * 7 + [0]),
+            (mixed, [3, 3, 3, 3, 2, 2, 1, 0]),
+            ([], [0] * 8),
+        ]:
+            graph.clear_edges()
+            graph.add_edges_from_no_data(edges)
+            expected = dict(zip(range(0, 16, 2), values))
+            for function in (rustworkx.core_number, rustworkx.digraph_core_number):
+                for _ in range(2):
+                    result = function(graph)
+                    self.assertEqual(result, expected)
+                    self.assertEqual(list(result), list(expected))
+
+    def test_neighbor_set_compatibility(self):
+        # Retain historical behavior without broadening the documented input contract.
+        graph = rustworkx.PyDiGraph()
+        graph.add_nodes_from(range(4))
+        graph.add_edges_from_no_data([(0, 0), (0, 0), (0, 1), (0, 1), (1, 0), (2, 2)])
+        for function in (rustworkx.core_number, rustworkx.digraph_core_number):
+            self.assertEqual(function(graph), {0: 1, 1: 1, 2: 1, 3: 0})
diff --git a/tests/graph/test_core_number.py b/tests/graph/test_core_number.py
index 8415fb478e..86005799a4 100644
--- a/tests/graph/test_core_number.py
+++ b/tests/graph/test_core_number.py
@@ -94,3 +94,41 @@ def test_undirected_paper_example(self):
         res = rustworkx.core_number(graph)
         self.assertIsInstance(res, dict)
         self.assertEqual(res, self.example_core)
+
+    # Compact working indices must not replace public node IDs.
+    def test_removed_and_reused_indices(self):
+        graph = rustworkx.PyGraph()
+        graph.add_nodes_from(range(10))
+        graph.remove_nodes_from([1, 3, 5, 7, 9])
+        self.assertEqual(graph.add_node("reused"), 9)
+        self.assertEqual(graph.add_node("isolated"), 7)
+        graph.add_edges_from_no_data([(0, 2), (2, 4), (4, 0), (4, 6), (8, 9)])
+        expected = {0: 2, 2: 2, 4: 2, 6: 1, 7: 0, 8: 1, 9: 1}
+        for function in (rustworkx.core_number, rustworkx.graph_core_number):
+            with self.subTest(function=function.__name__):
+                result = function(graph)
+                self.assertEqual(result, expected)
+                self.assertEqual(list(result), list(expected))
+
+    # Per-call work arrays must not retain earlier adjacency or core values.
+    def test_repeated_calls_after_edge_changes(self):
+        graph = rustworkx.PyGraph()
+        graph.add_nodes_from(range(16))
+        graph.remove_nodes_from(range(1, 16, 2))
+        mixed = [(0, 2), (0, 4), (0, 6), (2, 4), (2, 6), (4, 6), (6, 8), (8, 10), (10, 6), (10, 12)]
+        star = [(0, node) for node in range(2, 14, 2)]
+        for edges, values in [
+            ([], [0] * 8),
+            (mixed, [3, 3, 3, 3, 2, 2, 1, 0]),
+            (star, [1] * 7 + [0]),
+            (mixed, [3, 3, 3, 3, 2, 2, 1, 0]),
+            ([], [0] * 8),
+        ]:
+            graph.clear_edges()
+            graph.add_edges_from_no_data(edges)
+            expected = dict(zip(range(0, 16, 2), values))
+            for function in (rustworkx.core_number, rustworkx.graph_core_number):
+                for _ in range(2):
+                    result = function(graph)
+                    self.assertEqual(result, expected)
+                    self.assertEqual(list(result), list(expected))