wheel_extremal.py: |Phi(wheel_n)| = floor(2^n/3) exactly (ratio ->4/3),
but the wheel is NOT the irreducible minimiser for n>=6. The extremal disk
is a single MINIMAL-degree interior vertex (degree 4 or 5, both tie),
giving |Phi| = (5/4)*2^(n-2) = 5*2^(n-4). The ratio rises monotonically
with center degree, 5/4 -> 4/3, so minimal degree is extremal. Sharpens
the irreducible lemma to |Phi| >= (5/4)*2^(n-2), tight at the degree-4/5
patch.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
didericis
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