even_level: extend conjecture test to the cyclically-5-connected case (n=24)

Add the n=24 result to the Even Level Graph Generators paper: the dual of
the unique 44-vertex non-Hamiltonian cyclically-5-connected cubic planar
graph (Holton-McKay Fig. 2.10) -- a 24-vertex 5-connected triangulation,
the first conjecture test outside the 3-cut family -- is a bridge-derived
level graph, two verified bridge switches from an Even Level Graph
(source 19).

- Generate the graph rather than transcribe it: plantri -c5 lists all 6833
  5-connected 24-vertex triangulations; exactly one has a non-Hamiltonian
  dual, which also settles the uniqueness Holton-McKay left open at 44
  vertices (cyclically-5-connected triangulation <=> dual cubic graph).
- New abstract sentence + "cyclically-5-connected case: n=24" subsection,
  noting the classic 46-vertex Tutte graph is only cyclically 3-connected.
- Figure 6 (figures/fig210_dual.png): the dual T, parity-coloured, with the
  two introduced bridge edges {6,19} and {20,22} in green (style of Fig. 5).
- Experiments: test_fig210_dual_bridge.py (generate->filter->test pipeline),
  verify_fig210_witness.py (step-verifies the witness), draw_fig210_dual.py
  (figure), fig210_dual.g6 (the unique graph). paper.pdf rebuilt (10 pages).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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"""Draw the dual of the unique 44-vertex non-Hamiltonian cyclically
5-connected cubic planar graph (Holton-McKay Fig. 2.10): a 24-vertex
5-connected triangulation T. Same style as draw_witnesses.py / Figure 5:
crossing-free planar drawing, vertices coloured by the fixed parity
labelling (blue even, orange odd), and the bridge edges introduced by the
two bridge switches from T's witness Even Level Graph drawn solid green.
Writes ../figures/fig210_dual.png.
"""
import sys
import os
sys.path.insert(0, '/Users/didericis/Code/math-research/papers/'
'level_resolutions_of_maximal_planar_graphs/experiments')
sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
import networkx as nx
import matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt
from matplotlib.lines import Line2D
from sage.all import Graph # type: ignore
from tutte_dual_treecolor import dual_triangulation
from test_tutte_bridge import valid_parity_partitions_via_coloring
from test_fig210_dual_bridge import sage_to_nx
from fast_bridge import EdgeCode, parity_bridges
from test_conjecture import is_even_level_graph
HERE = os.path.dirname(os.path.abspath(__file__))
FDIR = os.path.join(HERE, '..', 'figures')
EVEN_C = '#9ecae1'
ODD_C = '#fdae6b'
def build():
g6 = open(os.path.join(HERE, 'fig210_dual.g6')).read().strip()
T, _ = dual_triangulation(sage_to_nx(Graph(g6)))
parts, _ = valid_parity_partitions_via_coloring(T)
labels = parts[9] # the witness-bearing partition
return T, labels
def neighbors(code, labels, state):
G = code.graph_of(state)
ok, emb = nx.check_planarity(G)
ea = {v: set() for v in code.nodes if labels[v] == 0}
oa = {v: set() for v in code.nodes if labels[v] == 1}
for u, v in G.edges():
if labels[u] == labels[v]:
(ea if labels[u] == 0 else oa)[u].add(v)
(ea if labels[u] == 0 else oa)[v].add(u)
br = parity_bridges(ea) | parity_bridges(oa)
for u, v in G.edges():
f1 = emb.traverse_face(u, v)
if len(f1) != 3:
continue
f2 = emb.traverse_face(v, u)
if len(f2) != 3:
continue
w = next(a for a in f1 if a not in (u, v))
x = next(b for b in f2 if b not in (u, v))
if w == x or G.has_edge(w, x) or labels[w] != labels[x]:
continue
if labels[u] == labels[v] and frozenset((u, v)) not in br:
continue
yield (state & ~(1 << code.bit(u, v))) | (1 << code.bit(w, x))
def elg_src(code, labels, state):
G = code.graph_of(state)
for s in code.nodes:
cs = labels[s]
nb = set(G.neighbors(s))
if not nb or any(labels[w] == cs for w in nb):
continue
ok, lv = is_even_level_graph(G, frozenset({s}))
if ok and all((lv[v] % 2 == 0) == (labels[v] == cs) for v in code.nodes):
return s
return None
def witness_added_edges(T, labels):
"""Backward bridge BFS to the ELG; return (source, [added bridge edges])."""
code = EdgeCode(T.nodes())
s0 = code.state_of(T)
parent = {s0: None}
frontier = [s0]
W = None
while frontier and W is None:
nf = []
for st in frontier:
if elg_src(code, labels, st) is not None:
W = st
break
for ns in neighbors(code, labels, st):
if ns not in parent:
parent[ns] = st
nf.append(ns)
if W:
break
frontier = nf
path = []
c = W
while c is not None:
path.append(c)
c = parent[c]
# path[0]=ELG ... path[-1]=T; edges added going ELG->T are present in T
added = []
for k in range(len(path) - 1):
A = set(map(frozenset, code.graph_of(path[k]).edges()))
B = set(map(frozenset, code.graph_of(path[k + 1]).edges()))
added.append(tuple(sorted(next(iter(B - A)))))
return elg_src(code, labels, W), added
def main():
os.makedirs(FDIR, exist_ok=True)
T, labels = build()
src, added = witness_added_edges(T, labels)
print('ELG source', src, 'bridge edges introduced', added)
fig, ax = plt.subplots(figsize=(7.5, 7.5))
pos = nx.planar_layout(T)
colors = [EVEN_C if labels[v] == 0 else ODD_C for v in T.nodes()]
hl = {frozenset(e) for e in added}
plain = [e for e in T.edges() if frozenset(e) not in hl]
nx.draw_networkx_edges(T, pos, edgelist=plain, ax=ax,
edge_color='#b0b0b0', width=0.9)
nx.draw_networkx_edges(T, pos, edgelist=[tuple(e) for e in hl], ax=ax,
edge_color='#2ca02c', width=2.6)
nx.draw_networkx_nodes(T, pos, node_color=colors, node_size=240,
edgecolors='#444444', linewidths=0.6, ax=ax)
nx.draw_networkx_labels(T, pos, font_size=8, ax=ax)
ax.margins(0.12)
ax.axis('off')
handles = [
Line2D([0], [0], marker='o', color='w', markerfacecolor=EVEN_C,
markeredgecolor='#444', markersize=9, label='even parity'),
Line2D([0], [0], marker='o', color='w', markerfacecolor=ODD_C,
markeredgecolor='#444', markersize=9, label='odd parity'),
Line2D([0], [0], color='#2ca02c', lw=2.6, label='bridge edge introduced'),
]
fig.legend(handles=handles, loc='lower center', ncol=3, fontsize=9,
frameon=False)
fig.tight_layout(rect=(0, 0.05, 1, 1))
out = os.path.join(FDIR, 'fig210_dual.png')
fig.savefig(out, dpi=160)
plt.close(fig)
print('wrote', out)
if __name__ == '__main__':
main()
@@ -0,0 +1 @@
ksP@@?PE?O?`@??_?O?A@?G??OG?O??G??A@??o??A???C@??E???@????O???E????G????OG???OG???G????B?????W????@?????A@????A@?????o?????G?????@@?????CC?????GG?????E??????@K
@@ -0,0 +1,95 @@
"""Bridge-derivability test for the dual of Holton-McKay's Fig. 2.10 graph
-- the smallest known non-Hamiltonian *cyclically 5-connected* cubic
planar graph (44 vertices, attributed to Tutte).
We obtain the graph by generation rather than transcription. A 44-vertex
cubic planar graph is the dual of a 24-vertex triangulation, and a cubic
graph is cyclically 5-connected iff its dual triangulation is 5-connected
(no separating 3- or 4-cycle). So we run
plantri -c5 -d -g 24
to list every 5-connected 24-vertex triangulation's cubic dual, keep the
non-Hamiltonian ones (Hamiltonicity of the dual = intertwining-tree of the
triangulation, by the paper's equivalence), and test each resulting
triangulation T = dual(H) for bridge-derivability. This is the conjecture's
first test in the cyclically-5-connected regime -- the family the n=21 and
46-vertex-Tutte duals (all only cyclically 3-connected) never reached.
Run after /tmp/nonham_duals.g6 has been produced by the Hamiltonicity
filter (lines: "<index> <graph6>").
"""
import sys
import os
import time
sys.path.insert(0, '/Users/didericis/Code/math-research/papers/'
'level_resolutions_of_maximal_planar_graphs/experiments')
sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
import networkx as nx
from sage.all import Graph # type: ignore
from tutte_dual_treecolor import dual_triangulation
from fast_bridge import EdgeCode
from test_tutte_bridge import (
characterize, valid_parity_partitions_via_coloring, search_partition,
)
def sage_to_nx(G):
H = nx.Graph()
H.add_nodes_from(int(v) for v in G.vertices())
H.add_edges_from((int(u), int(v)) for u, v in G.edges(labels=False))
return H
def test_one(g6, cap=600_000, time_limit=180.0, log=print):
H = sage_to_nx(Graph(g6))
T, _ = dual_triangulation(H) # 24-vertex triangulation
info = characterize(T)
log('dual triangulation T: n=%(n)d m=%(m)d triangulation=%(is_triangulation)s '
'separating_triangles=%(separating_triangles)d four_connected=%(four_connected)s'
% info)
five_conn = (nx.node_connectivity(T) >= 5)
log(' vertex-connectivity(T) >= 5: %s [cyclically-5-connected cubic dual]'
% five_conn)
code = EdgeCode(T.nodes())
code.state0 = code.state_of(T)
n = info['n']
parts, n_col = valid_parity_partitions_via_coloring(T)
log(' %d valid parity partitions (from %d 4-colorings)' % (len(parts), n_col))
t0 = time.time()
for k, labels in enumerate(parts):
st, sz, depth = search_partition(code, labels, n, cap, time_limit)
if st == 'found':
log(' ==> BRIDGE-DERIVED: ELG witness at %d bridge switches '
'(partition %d, orbit>=%d, %.0fs)' % (depth, k, sz, time.time() - t0))
return 'bridge-derived', depth
if st in ('capped', 'timeout') or (k + 1) % 25 == 0:
log(' partition %d/%d: %s (orbit>=%d, %.0fs)'
% (k + 1, len(parts), st, sz, time.time() - t0))
# if every orbit fully exhausted with no witness -> conclusive NO
log(' ==> NO witness over all %d valid partitions (%.0fs)'
% (len(parts), time.time() - t0))
return 'no-witness', None
def main():
path = '/tmp/nonham_duals.g6'
rows = []
with open(path) as f:
for line in f:
line = line.strip()
if line:
idx, g6 = line.split(None, 1)
rows.append((int(idx), g6))
print('%d non-Hamiltonian cyclically-5-connected 44-vertex duals to test\n'
% len(rows), flush=True)
for idx, g6 in rows:
print('### candidate (plantri index %d): %s' % (idx, g6), flush=True)
verdict, depth = test_one(g6)
print('### verdict: %s\n' % verdict, flush=True)
if __name__ == '__main__':
main()
@@ -0,0 +1,79 @@
import sys
E='/Users/didericis/Code/math-research/papers/even_level_graph_generators/experiments'
L='/Users/didericis/Code/math-research/papers/level_resolutions_of_maximal_planar_graphs/experiments'
sys.path.insert(0,E); sys.path.insert(0,L)
import networkx as nx
from sage.all import Graph
from tutte_dual_treecolor import dual_triangulation
from test_tutte_bridge import valid_parity_partitions_via_coloring
from test_fig210_dual_bridge import sage_to_nx
from fast_bridge import EdgeCode, parity_bridges
from test_conjecture import is_even_level_graph
g6=open('/tmp/nonham_duals.g6').read().split(None,1)[1].strip()
H=sage_to_nx(Graph(g6)); T,_=dual_triangulation(H)
parts,_=valid_parity_partitions_via_coloring(T)
labels=parts[9]; code=EdgeCode(T.nodes()); s0=code.state_of(T); n=24
def neighbors(state):
G=code.graph_of(state); ok,emb=nx.check_planarity(G)
ea={v:set() for v in code.nodes if labels[v]==0}; oa={v:set() for v in code.nodes if labels[v]==1}
for u,v in G.edges():
if labels[u]==labels[v]: (ea if labels[u]==0 else oa)[u].add(v);(ea if labels[u]==0 else oa)[v].add(u)
br=parity_bridges(ea)|parity_bridges(oa); out=[]
for u,v in G.edges():
f1=emb.traverse_face(u,v)
if len(f1)!=3: continue
f2=emb.traverse_face(v,u)
if len(f2)!=3: continue
w=next(a for a in f1 if a not in(u,v)); x=next(b for b in f2 if b not in(u,v))
if w==x or G.has_edge(w,x) or labels[w]!=labels[x]: continue
if labels[u]==labels[v] and frozenset((u,v)) not in br: continue
out.append((state&~(1<<code.bit(u,v)))|(1<<code.bit(w,x)))
return out
def elg_src(state):
G=code.graph_of(state)
for s in code.nodes:
cs=labels[s]; nb=set(G.neighbors(s))
if not nb or any(labels[w]==cs for w in nb): continue
ok,lv=is_even_level_graph(G,frozenset({s}))
if ok and all((lv[v]%2==0)==(labels[v]==cs) for v in code.nodes): return s
return None
parent={s0:None}; frontier=[s0]; W=None
while frontier and W is None:
nf=[]
for st in frontier:
if elg_src(st) is not None: W=st; break
for ns in neighbors(st):
if ns not in parent: parent[ns]=st; nf.append(ns)
if W: break
frontier=nf
path=[]; c=W
while c is not None: path.append(c); c=parent[c]
# path[0]=ELG ... path[-1]=T (forward = ELG -> T)
src=elg_src(W)
print('=== Fig 2.10 dual T: bridge-derived, witness at %d bridge switches ==='%(len(path)-1))
print('ELG source s =',src)
ok,lv=is_even_level_graph(code.graph_of(W),frozenset({src}))
print('ELG verified (all level cycles even from s=%d): %s'%(src,ok))
print('max level =',max(lv.values()))
allgood=True
for k in range(len(path)-1):
A=code.graph_of(path[k]); B=code.graph_of(path[k+1])
EA=set(map(frozenset,A.edges())); EB=set(map(frozenset,B.edges()))
new=tuple(sorted(next(iter(EB-EA)))); rem=tuple(sorted(next(iter(EA-EB))))
# bridge condition on NEW edge in B's parity subgraph (forward result)
ea={v:set() for v in code.nodes if labels[v]==0}; oa={v:set() for v in code.nodes if labels[v]==1}
for u,v in B.edges():
if labels[u]==labels[v]: (ea if labels[u]==0 else oa)[u].add(v);(ea if labels[u]==0 else oa)[v].add(u)
br=parity_bridges(ea)|parity_bridges(oa)
if labels[new[0]]==labels[new[1]]:
valid=frozenset(new) in br; kind='same-parity bridge in %s subgraph'%('even' if labels[new[0]]==0 else 'odd')
else:
valid=True; kind='cross-parity (enters neither subgraph)'
allgood &= valid
print(' switch %d: remove level-edge %s, add %s [%s] valid=%s'%(k+1,rem,new,kind,valid))
print('ALL STEPS VALID BRIDGE SWITCHES:',allgood)
print('T is intertwining tree:', False, '(dual H non-Hamiltonian =', not Graph(g6).is_hamiltonian(),')')
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\@writefile{lof}{\contentsline {figure}{\numberline {5}{\ignorespaces The six Holton--McKay duals, drawn as crossing-free planar graphs and coloured by parity (blue even, orange odd, with respect to the fixed level-parity labelling). The solid green edges are the bridge edges introduced by the bridge switches from each dual's witness Even Level Graph. Each green edge is a bridge of its parity subgraph, so no new cycle -- and in particular no odd cycle -- is created; duals $1$ and $2$ coincide with their Even Level Graphs and have no added edge.}}{8}{figure.5}\protected@file@percent }
\newlabel{fig:n21-duals}{{5}{8}{The six Holton--McKay duals, drawn as crossing-free planar graphs and coloured by parity (blue even, orange odd, with respect to the fixed level-parity labelling). The solid green edges are the bridge edges introduced by the bridge switches from each dual's witness Even Level Graph. Each green edge is a bridge of its parity subgraph, so no new cycle -- and in particular no odd cycle -- is created; duals $1$ and $2$ coincide with their Even Level Graphs and have no added edge}{figure.5}{}}
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\@writefile{lot}{\contentsline {table}{\numberline {2}{\ignorespaces The six Holton--McKay duals at $n = 21$, the first triangulations that are not intertwining trees. Each is a bridge-derived level graph: duals $1$ and $2$ are Even Level Graphs outright (zero switches), and the remaining four reach an Even Level Graph in $1$--$4$ bridge switches. All witnesses are step-verified.}}{8}{table.2}\protected@file@percent }
\newlabel{tab:n21}{{2}{8}{The six Holton--McKay duals at $n = 21$, the first triangulations that are not intertwining trees. Each is a bridge-derived level graph: duals $1$ and $2$ are Even Level Graphs outright (zero switches), and the remaining four reach an Even Level Graph in $1$--$4$ bridge switches. All witnesses are step-verified}{table.2}{}}
\@writefile{lof}{\contentsline {figure}{\numberline {5}{\ignorespaces The six Holton--McKay duals, drawn as crossing-free planar graphs and coloured by parity (blue even, orange odd, with respect to the fixed level-parity labelling). The solid green edges are the bridge edges introduced by the bridge switches from each dual's witness Even Level Graph. Each green edge is a bridge of its parity subgraph, so no new cycle -- and in particular no odd cycle -- is created; duals $1$ and $2$ coincide with their Even Level Graphs and have no added edge.}}{8}{figure.5}\protected@file@percent }
\newlabel{fig:n21-duals}{{5}{8}{The six Holton--McKay duals, drawn as crossing-free planar graphs and coloured by parity (blue even, orange odd, with respect to the fixed level-parity labelling). The solid green edges are the bridge edges introduced by the bridge switches from each dual's witness Even Level Graph. Each green edge is a bridge of its parity subgraph, so no new cycle -- and in particular no odd cycle -- is created; duals $1$ and $2$ coincide with their Even Level Graphs and have no added edge}{figure.5}{}}
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\@writefile{lof}{\contentsline {figure}{\numberline {6}{\ignorespaces The $24$-vertex dual $T$ of the unique $44$-vertex non-Hamiltonian cyclically $5$-connected cubic planar graph (Holton--McKay Fig.\nonbreakingspace 2.10), drawn crossing-free and coloured by the fixed parity labelling (blue even, orange odd). $T$ is $5$-connected and not an intertwining tree, yet is a bridge-derived level graph: the two solid green edges $\{6,19\}$ and $\{20,22\}$ are the bridge edges introduced by the two bridge switches carrying its witness Even Level Graph (source $19$) to $T$. Each green edge is a bridge of its parity subgraph -- $\{6, 19\}$ in the even subgraph, $\{20,22\}$ in the odd -- so no new cycle, and in particular no odd cycle, is created.}}{10}{figure.6}\protected@file@percent }
\newlabel{fig:n24-dual}{{6}{10}{The $24$-vertex dual $T$ of the unique $44$-vertex non-Hamiltonian cyclically $5$-connected cubic planar graph (Holton--McKay Fig.~2.10), drawn crossing-free and coloured by the fixed parity labelling (blue even, orange odd). $T$ is $5$-connected and not an intertwining tree, yet is a bridge-derived level graph: the two solid green edges $\{6,19\}$ and $\{20,22\}$ are the bridge edges introduced by the two bridge switches carrying its witness Even Level Graph (source $19$) to $T$. Each green edge is a bridge of its parity subgraph -- $\{6, 19\}$ in the even subgraph, $\{20,22\}$ in the odd -- so no new cycle, and in particular no odd cycle, is created}{figure.6}{}}
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@@ -101,7 +101,12 @@ Hamiltonian, so every triangulation on at most $20$ vertices is an
intertwining tree and the first possible failures occur at $n = 21$, at
the six duals of the Holton--McKay graphs. We verify that all six are
bridge-derived level graphs, confirming the conjecture in its first
nontrivial case.
nontrivial case. Pushing further, we identify by exhaustive generation
the unique $44$-vertex non-Hamiltonian \emph{cyclically $5$-connected}
cubic planar graph -- settling a uniqueness question Holton--McKay left
open -- whose $24$-vertex $5$-connected dual is the first test of the
conjecture outside the $3$-cut family; it too is a bridge-derived level
graph, two bridge switches from an Even Level Graph.
\end{abstract}
\maketitle
@@ -528,6 +533,71 @@ with their Even Level Graphs and have no added edge.}
\label{fig:n21-duals}
\end{figure}
\subsection*{The cyclically-$5$-connected case: $n = 24$}
The six $n = 21$ duals all carry non-trivial $3$-cuts in the cubic
picture; dually, each contains a separating triangle, so each is built
from smaller pieces and lies in the most reducible part of the
non-Hamiltonian world. (The famous $46$-vertex Tutte graph is no
improvement here: it too is only cyclically $3$-connected, and its
$25$-vertex dual has separating triangles.) The genuinely new regime is
the \emph{cyclically $5$-connected} one, dual to a $5$-connected
triangulation -- no separating $3$- or $4$-cycle, hence nothing to
decompose along. By Holton--McKay, the smallest non-Hamiltonian
cyclically $5$-connected cubic planar graph has $44$ vertices (Fig.~2.10
of~\cite{holton-mckay}, attributed to Tutte; minimality due to
Faulkner--Younger), and its dual is a $24$-vertex $5$-connected
triangulation.
We obtain this graph by generation rather than transcription. A
$44$-vertex cubic planar graph is the dual of a $24$-vertex
triangulation, and a cubic graph is cyclically $5$-connected if and only
if its dual triangulation is $5$-connected. Enumerating all $5$-connected
triangulations on $24$ vertices (\texttt{plantri -c5}, $6833$ of them)
and testing each dual for Hamiltonicity, we find that \emph{exactly one}
has a non-Hamiltonian dual. This both produces the graph and, granting
the correctness of the generator and the Hamiltonicity test, settles the
uniqueness question Holton--McKay left open: there is a unique
non-Hamiltonian cyclically $5$-connected cubic planar graph on $44$
vertices.
Let $T$ be its dual: a $24$-vertex triangulation with vertex connectivity
$5$ and no separating triangle, and -- since its dual is non-Hamiltonian
-- not an intertwining tree. We find that $T$ is nonetheless a
bridge-derived level graph. Of its $333$ valid parity partitions most are
useless: their backward bridge-orbits exceed $8 \times 10^5$ states with
no Even Level Graph in sight. But one partition has a backward orbit of
only $4678$ states containing an Even Level Graph (source $s = 19$,
maximum level $4$) at depth $2$. The two bridge switches carrying that
Even Level Graph to $T$ are
\[
\text{remove } \{16,21\},\ \text{add } \{20,22\}
\quad\text{and}\quad
\text{remove } \{15,18\},\ \text{add } \{6,19\},
\]
each adding a same-parity edge that is a bridge of the (odd, resp.\ even)
parity subgraph; both steps have been verified to be valid bridge
switches. So the disjunction holds for $T$ through the bridge-derived
disjunct, and the ``one good partition suffices'' phenomenon seen at
$n = 21$ persists into the cyclically $5$-connected regime -- the first
test of the conjecture genuinely outside the $3$-cut family.
\begin{figure}[ht]
\centering
\includegraphics[width=0.7\textwidth]{figures/fig210_dual.png}
\caption{The $24$-vertex dual $T$ of the unique $44$-vertex
non-Hamiltonian cyclically $5$-connected cubic planar graph
(Holton--McKay Fig.~2.10), drawn crossing-free and coloured by the fixed
parity labelling (blue even, orange odd). $T$ is $5$-connected and not an
intertwining tree, yet is a bridge-derived level graph: the two solid
green edges $\{6,19\}$ and $\{20,22\}$ are the bridge edges introduced by
the two bridge switches carrying its witness Even Level Graph (source
$19$) to $T$. Each green edge is a bridge of its parity subgraph -- $\{6,
19\}$ in the even subgraph, $\{20,22\}$ in the odd -- so no new cycle, and
in particular no odd cycle, is created.}
\label{fig:n24-dual}
\end{figure}
\begin{thebibliography}{9}
\bibitem{holton-mckay}