math-research/papers/face_monochromatic_pairs/experiments/check_S_adjacency.py

"""For the 1,314 bad chord-apex+Kempe colourings, check whether
S-vertices form connected subgraphs (= are adjacent to each other),
which would explain why their pentagon-coverage is below 3|S|.

For each bad colouring:
  - Compute |S| and the induced subgraph H[S].
  - # connected components of H[S].
  - Edges in H[S] (= S-vertex pairs sharing an edge).
  - For each pair of adjacent S-vertices: how many G'-pentagons they
    share (= "overlap" that reduces pigeonhole bound).

Run with: sage experiments/check_S_adjacency.py
"""
import os
import sys
import time

from sage.all import Graph
from sage.graphs.graph_generators import graphs

HERE = os.path.dirname(os.path.abspath(__file__))
sys.path.insert(0, HERE)

from check_conj_3_8_scaled import (
    apply_reduction,
    proper_3_edge_colorings,
    matches_chord_apex_kempe,
    kempe_cycle_set,
    edge_idx,
)
from check_heawood_on_kempe import dual_of, vertices_of_kempe


def test_one(D):
    D.is_planar(set_embedding=True)
    bad_count = 0
    S_components_dist = {}     # (|S|, # connected components in H[S]) -> count
    S_edges_dist = {}          # (|S|, # edges in H[S]) -> count
    pentagon_overlap_dist = {} # (|S|, max overlap) -> count
    for face in D.faces():
        if len(face) != 5: continue
        for i_red in range(5):
            res = apply_reduction(D, face, i_red, 9999)
            if res is None: continue
            H = res['H']; named = res['named']
            H.is_planar(set_embedding=True)
            edges, colorings = proper_3_edge_colorings(H)
            cand = [c for c in colorings
                    if matches_chord_apex_kempe(edges, c, named)]
            v_n = 9999
            for col in cand:
                target = {named['side_0'], named['spike']}
                lower_flank = None
                for f in H.faces():
                    if target.issubset({frozenset(e) for e in f}):
                        lower_flank = f; break
                if lower_flank is None or len(lower_flank) != 5: continue
                arc_verts = [e[0] for e in lower_flank]
                if v_n not in arc_verts: continue
                k = arc_verts.index(v_n)
                cyc = arc_verts[k:] + arc_verts[:k]
                A_i = next(iter(named['side_0'] - {v_n}))
                A_ip1 = next(iter(named['spike'] - {v_n}))
                if cyc[1] == A_i and cyc[4] == A_ip1:
                    P_1, P_2 = cyc[2], cyc[3]
                elif cyc[1] == A_ip1 and cyc[4] == A_i:
                    P_2, P_1 = cyc[2], cyc[3]
                else: continue
                merged_idx = edge_idx(edges, named['merged'])
                c_col = col[merged_idx]
                c_0_col = col[edge_idx(edges, named['side_0'])]
                c_1_col = col[edge_idx(edges, named['side_1'])]
                e_AiP1 = edge_idx(edges, frozenset((A_i, P_1)))
                e_P1P2 = edge_idx(edges, frozenset((P_1, P_2)))
                if e_AiP1 is None or e_P1P2 is None: continue
                if col[e_AiP1] != c_1_col or col[e_P1P2] != c_0_col:
                    continue
                a = c_col
                other = [x for x in range(3) if x != a]
                kc_b = kempe_cycle_set(edges, col, merged_idx, (a, other[0]))
                kc_c = kempe_cycle_set(edges, col, merged_idx, (a, other[1]))
                V_b = vertices_of_kempe(edges, kc_b)
                V_c = vertices_of_kempe(edges, kc_c)
                V_union = V_b | V_c
                S = set(H.vertices()) - V_union
                if P_1 in V_union: continue
                bad_count += 1
                S_size = len(S)
                # H[S] = induced subgraph
                HS = H.subgraph(S)
                comps = HS.connected_components()
                key = (S_size, len(comps))
                S_components_dist[key] = S_components_dist.get(key, 0) + 1
                # Edges in H[S]
                n_edges = HS.size()
                key2 = (S_size, n_edges)
                S_edges_dist[key2] = S_edges_dist.get(key2, 0) + 1
                # Pentagon overlap: for each pair of adjacent S-vertices,
                # count their shared G'-pentagons.
                max_overlap = 0
                for u in S:
                    for v_other in H.neighbors(u):
                        if v_other not in S or v_other <= u:
                            continue
                        # Find G'-pentagons containing both u and v_other
                        n_shared = 0
                        for f in H.faces():
                            if len(f) != 5: continue
                            verts = {a for a, b in f} | {b for a, b in f}
                            if u in verts and v_other in verts:
                                n_shared += 1
                        max_overlap = max(max_overlap, n_shared)
                key3 = (S_size, max_overlap)
                pentagon_overlap_dist[key3] = (
                    pentagon_overlap_dist.get(key3, 0) + 1)
    return (bad_count, S_components_dist, S_edges_dist, pentagon_overlap_dist)


def main(max_n=20, time_budget_per_n=1800):
    print("Connectivity structure of S-vertices in bad colourings.\n")
    grand_bad = 0
    grand_comps = {}
    grand_edges = {}
    grand_overlap = {}
    for n in range(12, max_n + 1):
        start = time.time()
        try:
            triangulations = list(graphs.triangulations(n, minimum_degree=5))
        except Exception as ex:
            print(f"n={n}: cannot enumerate ({ex})")
            continue
        n_bad_n = 0
        for tri_idx, G in enumerate(triangulations):
            if time.time() - start > time_budget_per_n:
                print(f"  n={n}: timeout at tri {tri_idx}")
                break
            G.is_planar(set_embedding=True)
            D = dual_of(G)
            nb, sc, se, po = test_one(D)
            n_bad_n += nb
            for k, v in sc.items(): grand_comps[k] = grand_comps.get(k, 0) + v
            for k, v in se.items(): grand_edges[k] = grand_edges.get(k, 0) + v
            for k, v in po.items(): grand_overlap[k] = grand_overlap.get(k, 0) + v
        elapsed = time.time() - start
        print(f"n={n}: {n_bad_n} bad colourings [{elapsed:.0f}s]")
        sys.stdout.flush()
        grand_bad += n_bad_n
    print()
    print("=" * 70)
    print(f"Total bad colourings: {grand_bad}")
    print("\n(|S|, # connected components in H[S]) distribution:")
    for k in sorted(grand_comps):
        c = grand_comps[k]
        print(f"  |S|={k[0]}, #comps={k[1]}: {c}")
    print("\n(|S|, # edges in H[S]) distribution:")
    for k in sorted(grand_edges):
        c = grand_edges[k]
        print(f"  |S|={k[0]}, #edges={k[1]}: {c}")
    print("\n(|S|, max # shared pentagons across adjacent S-pairs):")
    for k in sorted(grand_overlap):
        c = grand_overlap[k]
        print(f"  |S|={k[0]}, max_overlap={k[1]}: {c}")


if __name__ == '__main__':
    main()